| Rfc | 4120 |
|---|
| Title | The Kerberos Network Authentication Service (V5) |
|---|
| Author | C. Neuman, T. Yu,
S. Hartman, K. Raeburn |
|---|
| Date | July 2005 |
|---|
| Format: | TXT, HTML |
|---|
| Obsoletes | RFC1510 |
|---|
| Updated by | RFC4537, RFC5021, RFC5896, RFC6111, RFC6112,
RFC6113, RFC6649, RFC6806, RFC7751, RFC8062, RFC8129, RFC8429,
RFC8553 |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Network Working Group C. Neuman
Request for Comments: 4120 USC-ISI
Obsoletes: 1510 T. Yu
Category: Standards Track S. Hartman
K. Raeburn
MIT
July 2005
The Kerberos Network Authentication Service (V5)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document provides an overview and specification of Version 5 of
the Kerberos protocol, and it obsoletes RFC 1510 to clarify aspects
of the protocol and its intended use that require more detailed or
clearer explanation than was provided in RFC 1510. This document is
intended to provide a detailed description of the protocol, suitable
for implementation, together with descriptions of the appropriate use
of protocol messages and fields within those messages.
Table of Contents
1. Introduction ....................................................5
1.1. The Kerberos Protocol ......................................6
1.2. Cross-Realm Operation ......................................8
1.3. Choosing a Principal with Which to Communicate .............9
1.4. Authorization .............................................10
1.5. Extending Kerberos without Breaking Interoperability ......11
1.5.1. Compatibility with RFC 1510 ........................11
1.5.2. Sending Extensible Messages ........................12
1.6. Environmental Assumptions .................................12
1.7. Glossary of Terms .........................................13
2. Ticket Flag Uses and Requests ..................................16
2.1. Initial, Pre-authenticated, and
Hardware-Authenticated Tickets ............................17
2.2. Invalid Tickets ...........................................17
2.3. Renewable Tickets .........................................17
2.4. Postdated Tickets .........................................18
2.5. Proxiable and Proxy Tickets ...............................19
2.6. Forwardable Tickets .......................................19
2.7. Transited Policy Checking .................................20
2.8. OK as Delegate ............................................21
2.9. Other KDC Options .........................................21
2.9.1. Renewable-OK .......................................21
2.9.2. ENC-TKT-IN-SKEY ....................................22
2.9.3. Passwordless Hardware Authentication ...............22
3. Message Exchanges ..............................................22
3.1. The Authentication Service Exchange .......................22
3.1.1. Generation of KRB_AS_REQ Message ...................24
3.1.2. Receipt of KRB_AS_REQ Message ......................24
3.1.3. Generation of KRB_AS_REP Message ...................24
3.1.4. Generation of KRB_ERROR Message ....................27
3.1.5. Receipt of KRB_AS_REP Message ......................27
3.1.6. Receipt of KRB_ERROR Message .......................28
3.2. The Client/Server Authentication Exchange .................29
3.2.1. The KRB_AP_REQ Message .............................29
3.2.2. Generation of a KRB_AP_REQ Message .................29
3.2.3. Receipt of KRB_AP_REQ Message ......................30
3.2.4. Generation of a KRB_AP_REP Message .................33
3.2.5. Receipt of KRB_AP_REP Message ......................33
3.2.6. Using the Encryption Key ...........................33
3.3. The Ticket-Granting Service (TGS) Exchange ................34
3.3.1. Generation of KRB_TGS_REQ Message ..................35
3.3.2. Receipt of KRB_TGS_REQ Message .....................37
3.3.3. Generation of KRB_TGS_REP Message ..................38
3.3.4. Receipt of KRB_TGS_REP Message .....................42
6. Naming Constraints .............................................97
6.1. Realm Names ...............................................97
6.2. Principal Names .......................................... 99
6.2.1. Name of Server Principals .........................100
7. Constants and Other Defined Values ............................101
7.1. Host Address Types .......................................101
7.2. KDC Messaging: IP Transports .............................102
7.2.1. UDP/IP transport ..................................102
7.2.2. TCP/IP Transport ..................................103
7.2.3. KDC Discovery on IP Networks ......................104
7.3. Name of the TGS ..........................................105
7.4. OID Arc for KerberosV5 ...................................106
7.5. Protocol Constants and Associated Values .................106
7.5.1. Key Usage Numbers .................................106
7.5.2. PreAuthentication Data Types ......................108
7.5.3. Address Types .....................................109
7.5.4. Authorization Data Types ..........................109
7.5.5. Transited Encoding Types ..........................109
7.5.6. Protocol Version Number ...........................109
7.5.7. Kerberos Message Types ............................110
7.5.8. Name Types ........................................110
7.5.9. Error Codes .......................................110
8. Interoperability Requirements .................................113
8.1. Specification 2 ..........................................113
8.2. Recommended KDC Values ...................................116
9. IANA Considerations ...........................................116
10. Security Considerations ......................................117
11. Acknowledgements .............................................121
A. ASN.1 Module ..................................................123
B. Changes since RFC 1510 ........................................131
Normative References .............................................134
Informative References ...........................................135
1. Introduction
This document describes the concepts and model upon which the
Kerberos network authentication system is based. It also specifies
Version 5 of the Kerberos protocol. The motivations, goals,
assumptions, and rationale behind most design decisions are treated
cursorily; they are more fully described in a paper available in IEEE
communications [NT94] and earlier in the Kerberos portion of the
Athena Technical Plan [MNSS87].
This document is not intended to describe Kerberos to the end user,
system administrator, or application developer. Higher-level papers
describing Version 5 of the Kerberos system [NT94] and documenting
version 4 [SNS88] are available elsewhere.
The Kerberos model is based in part on Needham and Schroeder's
trusted third-party authentication protocol [NS78] and on
modifications suggested by Denning and Sacco [DS81]. The original
design and implementation of Kerberos Versions 1 through 4 was the
work of two former Project Athena staff members, Steve Miller of
Digital Equipment Corporation and Clifford Neuman (now at the
Information Sciences Institute of the University of Southern
California), along with Jerome Saltzer, Technical Director of Project
Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other
members of Project Athena have also contributed to the work on
Kerberos.
Version 5 of the Kerberos protocol (described in this document) has
evolved because of new requirements and desires for features not
available in Version 4. The design of Version 5 was led by Clifford
Neuman and John Kohl with much input from the community. The
development of the MIT reference implementation was led at MIT by
John Kohl and Theodore Ts'o, with help and contributed code from many
others. Since RFC 1510 was issued, many individuals have proposed
extensions and revisions to the protocol. This document reflects
some of these proposals. Where such changes involved significant
effort, the document cites the contribution of the proposer.
Reference implementations of both Version 4 and Version 5 of Kerberos
are publicly available, and commercial implementations have been
developed and are widely used. Details on the differences between
Versions 4 and 5 can be found in [KNT94].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.1. The Kerberos Protocol
Kerberos provides a means of verifying the identities of principals,
(e.g., a workstation user or a network server) on an open
(unprotected) network. This is accomplished without relying on
assertions by the host operating system, without basing trust on host
addresses, without requiring physical security of all the hosts on
the network, and under the assumption that packets traveling along
the network can be read, modified, and inserted at will. Kerberos
performs authentication under these conditions as a trusted third-
party authentication service by using conventional (shared secret
key) cryptography. Extensions to Kerberos (outside the scope of this
document) can provide for the use of public key cryptography during
certain phases of the authentication protocol. Such extensions
support Kerberos authentication for users registered with public key
certification authorities and provide certain benefits of public key
cryptography in situations where they are needed.
The basic Kerberos authentication process proceeds as follows: A
client sends a request to the authentication server (AS) for
"credentials" for a given server. The AS responds with these
credentials, encrypted in the client's key. The credentials consist
of a "ticket" for the server and a temporary encryption key (often
called a "session key"). The client transmits the ticket (which
contains the client's identity and a copy of the session key, all
encrypted in the server's key) to the server. The session key (now
shared by the client and server) is used to authenticate the client
and may optionally be used to authenticate the server. It may also
be used to encrypt further communication between the two parties or
to exchange a separate sub-session key to be used to encrypt further
communication. Note that many applications use Kerberos' functions
only upon the initiation of a stream-based network connection.
Unless an application performs encryption or integrity protection for
the data stream, the identity verification applies only to the
initiation of the connection, and it does not guarantee that
subsequent messages on the connection originate from the same
principal.
Implementation of the basic protocol consists of one or more
authentication servers running on physically secure hosts. The
authentication servers maintain a database of principals (i.e., users
and servers) and their secret keys. Code libraries provide
encryption and implement the Kerberos protocol. In order to add
authentication to its transactions, a typical network application
adds calls to the Kerberos library directly or through the Generic
Security Services Application Programming Interface (GSS-API)
described in a separate document [RFC4121]. These calls result in
the transmission of the messages necessary to achieve authentication.
The Kerberos protocol consists of several sub-protocols (or
exchanges). There are two basic methods by which a client can ask a
Kerberos server for credentials. In the first approach, the client
sends a cleartext request for a ticket for the desired server to the
AS. The reply is sent encrypted in the client's secret key. Usually
this request is for a ticket-granting ticket (TGT), which can later
be used with the ticket-granting server (TGS). In the second method,
the client sends a request to the TGS. The client uses the TGT to
authenticate itself to the TGS in the same manner as if it were
contacting any other application server that requires Kerberos
authentication. The reply is encrypted in the session key from the
TGT. Though the protocol specification describes the AS and the TGS
as separate servers, in practice they are implemented as different
protocol entry points within a single Kerberos server.
Once obtained, credentials may be used to verify the identity of the
principals in a transaction, to ensure the integrity of messages
exchanged between them, or to preserve privacy of the messages. The
application is free to choose whatever protection may be necessary.
To verify the identities of the principals in a transaction, the
client transmits the ticket to the application server. Because the
ticket is sent "in the clear" (parts of it are encrypted, but this
encryption doesn't thwart replay) and might be intercepted and reused
by an attacker, additional information is sent to prove that the
message originated with the principal to whom the ticket was issued.
This information (called the authenticator) is encrypted in the
session key and includes a timestamp. The timestamp proves that the
message was recently generated and is not a replay. Encrypting the
authenticator in the session key proves that it was generated by a
party possessing the session key. Since no one except the requesting
principal and the server know the session key (it is never sent over
the network in the clear), this guarantees the identity of the
client.
The integrity of the messages exchanged between principals can also
be guaranteed by using the session key (passed in the ticket and
contained in the credentials). This approach provides detection of
both replay attacks and message stream modification attacks. It is
accomplished by generating and transmitting a collision-proof
checksum (elsewhere called a hash or digest function) of the client's
message, keyed with the session key. Privacy and integrity of the
messages exchanged between principals can be secured by encrypting
the data to be passed by using the session key contained in the
ticket or the sub-session key found in the authenticator.
The authentication exchanges mentioned above require read-only access
to the Kerberos database. Sometimes, however, the entries in the
database must be modified, such as when adding new principals or
changing a principal's key. This is done using a protocol between a
client and a third Kerberos server, the Kerberos Administration
Server (KADM). There is also a protocol for maintaining multiple
copies of the Kerberos database. Neither of these protocols are
described in this document.
1.2. Cross-Realm Operation
The Kerberos protocol is designed to operate across organizational
boundaries. A client in one organization can be authenticated to a
server in another. Each organization wishing to run a Kerberos
server establishes its own "realm". The name of the realm in which a
client is registered is part of the client's name and can be used by
the end-service to decide whether to honor a request.
By establishing "inter-realm" keys, the administrators of two realms
can allow a client authenticated in the local realm to prove its
identity to servers in other realms. The exchange of inter-realm
keys (a separate key may be used for each direction) registers the
ticket-granting service of each realm as a principal in the other
realm. A client is then able to obtain a TGT for the remote realm's
ticket-granting service from its local realm. When that TGT is used,
the remote ticket-granting service uses the inter-realm key (which
usually differs from its own normal TGS key) to decrypt the TGT; thus
it is certain that the ticket was issued by the client's own TGS.
Tickets issued by the remote ticket-granting service will indicate to
the end-service that the client was authenticated from another realm.
Without cross-realm operation, and with appropriate permission, the
client can arrange registration of a separately-named principal in a
remote realm and engage in normal exchanges with that realm's
services. However, for even small numbers of clients this becomes
cumbersome, and more automatic methods as described here are
necessary.
A realm is said to communicate with another realm if the two realms
share an inter-realm key, or if the local realm shares an inter-realm
key with an intermediate realm that communicates with the remote
realm. An authentication path is the sequence of intermediate realms
that are transited in communicating from one realm to another.
Realms may be organized hierarchically. Each realm shares a key with
its parent and a different key with each child. If an inter-realm
key is not directly shared by two realms, the hierarchical
organization allows an authentication path to be easily constructed.
If a hierarchical organization is not used, it may be necessary to
consult a database in order to construct an authentication path
between realms.
Although realms are typically hierarchical, intermediate realms may
be bypassed to achieve cross-realm authentication through alternate
authentication paths. (These might be established to make
communication between two realms more efficient.) It is important
for the end-service to know which realms were transited when deciding
how much faith to place in the authentication process. To facilitate
this decision, a field in each ticket contains the names of the
realms that were involved in authenticating the client.
The application server is ultimately responsible for accepting or
rejecting authentication and SHOULD check the transited field. The
application server may choose to rely on the Key Distribution Center
(KDC) for the application server's realm to check the transited
field. The application server's KDC will set the
TRANSITED-POLICY-CHECKED flag in this case. The KDCs for
intermediate realms may also check the transited field as they issue
TGTs for other realms, but they are encouraged not to do so. A
client may request that the KDCs not check the transited field by
setting the DISABLE-TRANSITED-CHECK flag. KDCs SHOULD honor this
flag.
1.3. Choosing a Principal with Which to Communicate
The Kerberos protocol provides the means for verifying (subject to
the assumptions in Section 1.6) that the entity with which one
communicates is the same entity that was registered with the KDC
using the claimed identity (principal name). It is still necessary
to determine whether that identity corresponds to the entity with
which one intends to communicate.
When appropriate data has been exchanged in advance, the application
may perform this determination syntactically based on the application
protocol specification, information provided by the user, and
configuration files. For example, the server principal name
(including realm) for a telnet server might be derived from the
user-specified host name (from the telnet command line), the "host/"
prefix specified in the application protocol specification, and a
mapping to a Kerberos realm derived syntactically from the domain
part of the specified hostname and information from the local
Kerberos realms database.
One can also rely on trusted third parties to make this
determination, but only when the data obtained from the third party
is suitably integrity-protected while resident on the third-party
server and when transmitted. Thus, for example, one should not rely
on an unprotected DNS record to map a host alias to the primary name
of a server, accepting the primary name as the party that one intends
to contact, since an attacker can modify the mapping and impersonate
the party.
Implementations of Kerberos and protocols based on Kerberos MUST NOT
use insecure DNS queries to canonicalize the hostname components of
the service principal names (i.e., they MUST NOT use insecure DNS
queries to map one name to another to determine the host part of the
principal name with which one is to communicate). In an environment
without secure name service, application authors MAY append a
statically configured domain name to unqualified hostnames before
passing the name to the security mechanisms, but they should do no
more than that. Secure name service facilities, if available, might
be trusted for hostname canonicalization, but such canonicalization
by the client SHOULD NOT be required by KDC implementations.
Implementation note: Many current implementations do some degree of
canonicalization of the provided service name, often using DNS even
though it creates security problems. However, there is no
consistency among implementations as to whether the service name is
case folded to lowercase or whether reverse resolution is used. To
maximize interoperability and security, applications SHOULD provide
security mechanisms with names that result from folding the user-
entered name to lowercase without performing any other modifications
or canonicalization.
1.4. Authorization
As an authentication service, Kerberos provides a means of verifying
the identity of principals on a network. Authentication is usually
useful primarily as a first step in the process of authorization,
determining whether a client may use a service, which objects the
client is allowed to access, and the type of access allowed for each.
Kerberos does not, by itself, provide authorization. Possession of a
client ticket for a service provides only for authentication of the
client to that service, and in the absence of a separate
authorization procedure, an application should not consider it to
authorize the use of that service.
Separate authorization methods MAY be implemented as application-
specific access control functions and may utilize files on the
application server, on separately issued authorization credentials
such as those based on proxies [Neu93], or on other authorization
services. Separately authenticated authorization credentials MAY be
embedded in a ticket's authorization data when encapsulated by the
KDC-issued authorization data element.
Applications should not accept the mere issuance of a service ticket
by the Kerberos server (even by a modified Kerberos server) as
granting authority to use the service, since such applications may
become vulnerable to the bypass of this authorization check in an
environment where other options for application authentication are
provided, or if they interoperate with other KDCs.
1.5. Extending Kerberos without Breaking Interoperability
As the deployed base of Kerberos implementations grows, extending
Kerberos becomes more important. Unfortunately, some extensions to
the existing Kerberos protocol create interoperability issues because
of uncertainty regarding the treatment of certain extensibility
options by some implementations. This section includes guidelines
that will enable future implementations to maintain interoperability.
Kerberos provides a general mechanism for protocol extensibility.
Some protocol messages contain typed holes -- sub-messages that
contain an octet-string along with an integer that defines how to
interpret the octet-string. The integer types are registered
centrally, but they can be used both for vendor extensions and for
extensions standardized through the IETF.
In this document, the word "extension" refers to extension by
defining a new type to insert into an existing typed hole in a
protocol message. It does not refer to extension by addition of new
fields to ASN.1 types, unless the text explicitly indicates
otherwise.
1.5.1. Compatibility with RFC 1510
Note that existing Kerberos message formats cannot readily be
extended by adding fields to the ASN.1 types. Sending additional
fields often results in the entire message being discarded without an
error indication. Future versions of this specification will provide
guidelines to ensure that ASN.1 fields can be added without creating
an interoperability problem.
In the meantime, all new or modified implementations of Kerberos that
receive an unknown message extension SHOULD preserve the encoding of
the extension but otherwise ignore its presence. Recipients MUST NOT
decline a request simply because an extension is present.
There is one exception to this rule. If an unknown authorization
data element type is received by a server other than the ticket-
granting service either in an AP-REQ or in a ticket contained in an
AP-REQ, then authentication MUST fail. One of the primary uses of
authorization data is to restrict the use of the ticket. If the
service cannot determine whether the restriction applies to that
service, then a security weakness may result if the ticket can be
used for that service. Authorization elements that are optional
SHOULD be enclosed in the AD-IF-RELEVANT element.
The ticket-granting service MUST ignore but propagate to derivative
tickets any unknown authorization data types, unless those data types
are embedded in a MANDATORY-FOR-KDC element, in which case the
request will be rejected. This behavior is appropriate because
requiring that the ticket-granting service understand unknown
authorization data types would require that KDC software be upgraded
to understand new application-level restrictions before applications
used these restrictions, decreasing the utility of authorization data
as a mechanism for restricting the use of tickets. No security
problem is created because services to which the tickets are issued
will verify the authorization data.
Implementation note: Many RFC 1510 implementations ignore unknown
authorization data elements. Depending on these implementations to
honor authorization data restrictions may create a security weakness.
1.5.2. Sending Extensible Messages
Care must be taken to ensure that old implementations can understand
messages sent to them, even if they do not understand an extension
that is used. Unless the sender knows that an extension is
supported, the extension cannot change the semantics of the core
message or previously defined extensions.
For example, an extension including key information necessary to
decrypt the encrypted part of a KDC-REP could only be used in
situations where the recipient was known to support the extension.
Thus when designing such extensions it is important to provide a way
for the recipient to notify the sender of support for the extension.
For example in the case of an extension that changes the KDC-REP
reply key, the client could indicate support for the extension by
including a padata element in the AS-REQ sequence. The KDC should
only use the extension if this padata element is present in the
AS-REQ. Even if policy requires the use of the extension, it is
better to return an error indicating that the extension is required
than to use the extension when the recipient may not support it.
Debugging implementations that do not interoperate is easier when
errors are returned.
1.6. Environmental Assumptions
Kerberos imposes a few assumptions on the environment in which it can
properly function, including the following:
* "Denial of service" attacks are not solved with Kerberos. There
are places in the protocols where an intruder can prevent an
application from participating in the proper authentication steps.
Detection and solution of such attacks (some of which can appear
to be not-uncommon "normal" failure modes for the system) are
usually best left to the human administrators and users.
* Principals MUST keep their secret keys secret. If an intruder
somehow steals a principal's key, it will be able to masquerade as
that principal or to impersonate any server to the legitimate
principal.
* "Password guessing" attacks are not solved by Kerberos. If a user
chooses a poor password, it is possible for an attacker to
successfully mount an offline dictionary attack by repeatedly
attempting to decrypt, with successive entries from a dictionary,
messages obtained which are encrypted under a key derived from the
user's password.
* Each host on the network MUST have a clock which is "loosely
synchronized" to the time of the other hosts; this synchronization
is used to reduce the bookkeeping needs of application servers
when they do replay detection. The degree of "looseness" can be
configured on a per-server basis, but it is typically on the order
of 5 minutes. If the clocks are synchronized over the network,
the clock synchronization protocol MUST itself be secured from
network attackers.
* Principal identifiers are not recycled on a short-term basis. A
typical mode of access control will use access control lists
(ACLs) to grant permissions to particular principals. If a stale
ACL entry remains for a deleted principal and the principal
identifier is reused, the new principal will inherit rights
specified in the stale ACL entry. By not re-using principal
identifiers, the danger of inadvertent access is removed.
1.7. Glossary of Terms
Below is a list of terms used throughout this document.
Authentication
Verifying the claimed identity of a principal.
Authentication header
A record containing a Ticket and an Authenticator to be presented
to a server as part of the authentication process.
Authentication path
A sequence of intermediate realms transited in the authentication
process when communicating from one realm to another.
Authenticator
A record containing information that can be shown to have been
recently generated using the session key known only by the client
and server.
Authorization
The process of determining whether a client may use a service,
which objects the client is allowed to access, and the type of
access allowed for each.
Capability
A token that grants the bearer permission to access an object or
service. In Kerberos, this might be a ticket whose use is
restricted by the contents of the authorization data field, but
which lists no network addresses, together with the session key
necessary to use the ticket.
Ciphertext
The output of an encryption function. Encryption transforms
plaintext into ciphertext.
Client
A process that makes use of a network service on behalf of a user.
Note that in some cases a Server may itself be a client of some
other server (e.g., a print server may be a client of a file
server).
Credentials
A ticket plus the secret session key necessary to use that ticket
successfully in an authentication exchange.
Encryption Type (etype)
When associated with encrypted data, an encryption type identifies
the algorithm used to encrypt the data and is used to select the
appropriate algorithm for decrypting the data. Encryption type
tags are communicated in other messages to enumerate algorithms
that are desired, supported, preferred, or allowed to be used for
encryption of data between parties. This preference is combined
with local information and policy to select an algorithm to be
used.
KDC
Key Distribution Center. A network service that supplies tickets
and temporary session keys; or an instance of that service or the
host on which it runs. The KDC services both initial ticket and
ticket-granting ticket requests. The initial ticket portion is
sometimes referred to as the Authentication Server (or service).
The ticket-granting ticket portion is sometimes referred to as the
ticket-granting server (or service).
Kerberos
The name given to the Project Athena's authentication service, the
protocol used by that service, or the code used to implement the
authentication service. The name is adopted from the three-headed
dog that guards Hades.
Key Version Number (kvno)
A tag associated with encrypted data identifies which key was used
for encryption when a long-lived key associated with a principal
changes over time. It is used during the transition to a new key
so that the party decrypting a message can tell whether the data
was encrypted with the old or the new key.
Plaintext
The input to an encryption function or the output of a decryption
function. Decryption transforms ciphertext into plaintext.
Principal
A named client or server entity that participates in a network
communication, with one name that is considered canonical.
Principal identifier
The canonical name used to identify each different principal
uniquely.
Seal
To encipher a record containing several fields in such a way that
the fields cannot be individually replaced without knowledge of
the encryption key or leaving evidence of tampering.
Secret key
An encryption key shared by a principal and the KDC, distributed
outside the bounds of the system, with a long lifetime. In the
case of a human user's principal, the secret key MAY be derived
from a password.
Server
A particular Principal that provides a resource to network
clients. The server is sometimes referred to as the Application
Server.
Service
A resource provided to network clients; often provided by more
than one server (for example, remote file service).
Session key
A temporary encryption key used between two principals, with a
lifetime limited to the duration of a single login "session". In
the Kerberos system, a session key is generated by the KDC. The
session key is distinct from the sub-session key, described next.
Sub-session key
A temporary encryption key used between two principals, selected
and exchanged by the principals using the session key, and with a
lifetime limited to the duration of a single association. The
sub-session key is also referred to as the subkey.
Ticket
A record that helps a client authenticate itself to a server; it
contains the client's identity, a session key, a timestamp, and
other information, all sealed using the server's secret key. It
only serves to authenticate a client when presented along with a
fresh Authenticator.
2. Ticket Flag Uses and Requests
Each Kerberos ticket contains a set of flags that are used to
indicate attributes of that ticket. Most flags may be requested by a
client when the ticket is obtained; some are automatically turned on
and off by a Kerberos server as required. The following sections
explain what the various flags mean and give examples of reasons to
use them. With the exception of the INVALID flag, clients MUST
ignore ticket flags that are not recognized. KDCs MUST ignore KDC
options that are not recognized. Some implementations of RFC 1510
are known to reject unknown KDC options, so clients may need to
resend a request without new KDC options if the request was rejected
when sent with options added since RFC 1510. Because new KDCs will
ignore unknown options, clients MUST confirm that the ticket returned
by the KDC meets their needs.
Note that it is not, in general, possible to determine whether an
option was not honored because it was not understood or because it
was rejected through either configuration or policy. When adding a
new option to the Kerberos protocol, designers should consider
whether the distinction is important for their option. If it is, a
mechanism for the KDC to return an indication that the option was
understood but rejected needs to be provided in the specification of
the option. Often in such cases, the mechanism needs to be broad
enough to permit an error or reason to be returned.
2.1. Initial, Pre-authenticated, and Hardware-Authenticated Tickets
The INITIAL flag indicates that a ticket was issued using the AS
protocol, rather than issued based on a TGT. Application servers
that want to require the demonstrated knowledge of a client's secret
key (e.g., a password-changing program) can insist that this flag be
set in any tickets they accept, and can thus be assured that the
client's key was recently presented to the authentication server.
The PRE-AUTHENT and HW-AUTHENT flags provide additional information
about the initial authentication, regardless of whether the current
ticket was issued directly (in which case INITIAL will also be set)
or issued on the basis of a TGT (in which case the INITIAL flag is
clear, but the PRE-AUTHENT and HW-AUTHENT flags are carried forward
from the TGT).
2.2. Invalid Tickets
The INVALID flag indicates that a ticket is invalid. Application
servers MUST reject tickets that have this flag set. A postdated
ticket will be issued in this form. Invalid tickets MUST be
validated by the KDC before use, by being presented to the KDC in a
TGS request with the VALIDATE option specified. The KDC will only
validate tickets after their starttime has passed. The validation is
required so that postdated tickets that have been stolen before their
starttime can be rendered permanently invalid (through a hot-list
mechanism) (see Section 3.3.3.1).
2.3. Renewable Tickets
Applications may desire to hold tickets that can be valid for long
periods of time. However, this can expose their credentials to
potential theft for equally long periods, and those stolen
credentials would be valid until the expiration time of the
ticket(s). Simply using short-lived tickets and obtaining new ones
periodically would require the client to have long-term access to its
secret key, an even greater risk. Renewable tickets can be used to
mitigate the consequences of theft. Renewable tickets have two
"expiration times": the first is when the current instance of the
ticket expires, and the second is the latest permissible value for an
individual expiration time. An application client must periodically
(i.e., before it expires) present a renewable ticket to the KDC, with
the RENEW option set in the KDC request. The KDC will issue a new
ticket with a new session key and a later expiration time. All other
fields of the ticket are left unmodified by the renewal process.
When the latest permissible expiration time arrives, the ticket
expires permanently. At each renewal, the KDC MAY consult a hot-list
to determine whether the ticket had been reported stolen since its
last renewal; it will refuse to renew stolen tickets, and thus the
usable lifetime of stolen tickets is reduced.
The RENEWABLE flag in a ticket is normally only interpreted by the
ticket-granting service (discussed below in Section 3.3). It can
usually be ignored by application servers. However, some
particularly careful application servers MAY disallow renewable
tickets.
If a renewable ticket is not renewed by its expiration time, the KDC
will not renew the ticket. The RENEWABLE flag is reset by default,
but a client MAY request it be set by setting the RENEWABLE option in
the KRB_AS_REQ message. If it is set, then the renew-till field in
the ticket contains the time after which the ticket may not be
renewed.
2.4. Postdated Tickets
Applications may occasionally need to obtain tickets for use much
later; e.g., a batch submission system would need tickets to be valid
at the time the batch job is serviced. However, it is dangerous to
hold valid tickets in a batch queue, since they will be on-line
longer and more prone to theft. Postdated tickets provide a way to
obtain these tickets from the KDC at job submission time, but to
leave them "dormant" until they are activated and validated by a
further request of the KDC. If a ticket theft were reported in the
interim, the KDC would refuse to validate the ticket, and the thief
would be foiled.
The MAY-POSTDATE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
This flag MUST be set in a TGT in order to issue a postdated ticket
based on the presented ticket. It is reset by default; a client MAY
request it by setting the ALLOW-POSTDATE option in the KRB_AS_REQ
message. This flag does not allow a client to obtain a postdated
TGT; postdated TGTs can only be obtained by requesting the postdating
in the KRB_AS_REQ message. The life (endtime-starttime) of a
postdated ticket will be the remaining life of the TGT at the time of
the request, unless the RENEWABLE option is also set, in which case
it can be the full life (endtime-starttime) of the TGT. The KDC MAY
limit how far in the future a ticket may be postdated.
The POSTDATED flag indicates that a ticket has been postdated. The
application server can check the authtime field in the ticket to see
when the original authentication occurred. Some services MAY choose
to reject postdated tickets, or they may only accept them within a
certain period after the original authentication. When the KDC
issues a POSTDATED ticket, it will also be marked as INVALID, so that
the application client MUST present the ticket to the KDC to be
validated before use.
2.5. Proxiable and Proxy Tickets
At times it may be necessary for a principal to allow a service to
perform an operation on its behalf. The service must be able to take
on the identity of the client, but only for a particular purpose. A
principal can allow a service to do this by granting it a proxy.
The process of granting a proxy by using the proxy and proxiable
flags is used to provide credentials for use with specific services.
Though conceptually also a proxy, users wishing to delegate their
identity in a form usable for all purposes MUST use the ticket
forwarding mechanism described in the next section to forward a TGT.
The PROXIABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
When set, this flag tells the ticket-granting server that it is OK to
issue a new ticket (but not a TGT) with a different network address
based on this ticket. This flag is set if requested by the client on
initial authentication. By default, the client will request that it
be set when requesting a TGT, and that it be reset when requesting
any other ticket.
This flag allows a client to pass a proxy to a server to perform a
remote request on its behalf (e.g., a print service client can give
the print server a proxy to access the client's files on a particular
file server in order to satisfy a print request).
In order to complicate the use of stolen credentials, Kerberos
tickets are often valid only from those network addresses
specifically included in the ticket, but it is permissible as a
policy option to allow requests and to issue tickets with no network
addresses specified. When granting a proxy, the client MUST specify
the new network address from which the proxy is to be used or
indicate that the proxy is to be issued for use from any address.
The PROXY flag is set in a ticket by the TGS when it issues a proxy
ticket. Application servers MAY check this flag; and at their option
they MAY require additional authentication from the agent presenting
the proxy in order to provide an audit trail.
2.6. Forwardable Tickets
Authentication forwarding is an instance of a proxy where the service
that is granted is complete use of the client's identity. An example
of where it might be used is when a user logs in to a remote system
and wants authentication to work from that system as if the login
were local.
The FORWARDABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers.
The FORWARDABLE flag has an interpretation similar to that of the
PROXIABLE flag, except TGTs may also be issued with different network
addresses. This flag is reset by default, but users MAY request that
it be set by setting the FORWARDABLE option in the AS request when
they request their initial TGT.
This flag allows for authentication forwarding without requiring the
user to enter a password again. If the flag is not set, then
authentication forwarding is not permitted, but the same result can
still be achieved if the user engages in the AS exchange, specifies
the requested network addresses, and supplies a password.
The FORWARDED flag is set by the TGS when a client presents a ticket
with the FORWARDABLE flag set and requests a forwarded ticket by
specifying the FORWARDED KDC option and supplying a set of addresses
for the new ticket. It is also set in all tickets issued based on
tickets with the FORWARDED flag set. Application servers may choose
to process FORWARDED tickets differently than non-FORWARDED tickets.
If addressless tickets are forwarded from one system to another,
clients SHOULD still use this option to obtain a new TGT in order to
have different session keys on the different systems.
2.7. Transited Policy Checking
In Kerberos, the application server is ultimately responsible for
accepting or rejecting authentication, and it SHOULD check that only
suitably trusted KDCs are relied upon to authenticate a principal.
The transited field in the ticket identifies which realms (and thus
which KDCs) were involved in the authentication process, and an
application server would normally check this field. If any of these
are untrusted to authenticate the indicated client principal
(probably determined by a realm-based policy), the authentication
attempt MUST be rejected. The presence of trusted KDCs in this list
does not provide any guarantee; an untrusted KDC may have fabricated
the list.
Although the end server ultimately decides whether authentication is
valid, the KDC for the end server's realm MAY apply a realm-specific
policy for validating the transited field and accepting credentials
for cross-realm authentication. When the KDC applies such checks and
accepts such cross-realm authentication, it will set the
TRANSITED-POLICY-CHECKED flag in the service tickets it issues based
on the cross-realm TGT. A client MAY request that the KDCs not check
the transited field by setting the DISABLE-TRANSITED-CHECK flag.
KDCs are encouraged but not required to honor this flag.
Application servers MUST either do the transited-realm checks
themselves or reject cross-realm tickets without
TRANSITED-POLICY-CHECKED set.
2.8. OK as Delegate
For some applications, a client may need to delegate authority to a
server to act on its behalf in contacting other services. This
requires that the client forward credentials to an intermediate
server. The ability for a client to obtain a service ticket to a
server conveys no information to the client about whether the server
should be trusted to accept delegated credentials. The
OK-AS-DELEGATE provides a way for a KDC to communicate local realm
policy to a client regarding whether an intermediate server is
trusted to accept such credentials.
The copy of the ticket flags in the encrypted part of the KDC reply
may have the OK-AS-DELEGATE flag set to indicate to the client that
the server specified in the ticket has been determined by the policy
of the realm to be a suitable recipient of delegation. A client can
use the presence of this flag to help it decide whether to delegate
credentials (grant either a proxy or a forwarded TGT) to this server.
It is acceptable to ignore the value of this flag. When setting this
flag, an administrator should consider the security and placement of
the server on which the service will run, as well as whether the
service requires the use of delegated credentials.
2.9. Other KDC Options
There are three additional options that MAY be set in a client's
request of the KDC.
2.9.1. Renewable-OK
The RENEWABLE-OK option indicates that the client will accept a
renewable ticket if a ticket with the requested life cannot otherwise
be provided. If a ticket with the requested life cannot be provided,
then the KDC MAY issue a renewable ticket with a renew-till equal to
the requested endtime. The value of the renew-till field MAY still
be adjusted by site-determined limits or limits imposed by the
individual principal or server.
2.9.2. ENC-TKT-IN-SKEY
In its basic form, the Kerberos protocol supports authentication in a
client-server setting and is not well suited to authentication in a
peer-to-peer environment because the long-term key of the user does
not remain on the workstation after initial login. Authentication of
such peers may be supported by Kerberos in its user-to-user variant.
The ENC-TKT-IN-SKEY option supports user-to-user authentication by
allowing the KDC to issue a service ticket encrypted using the
session key from another TGT issued to another user. The
ENC-TKT-IN-SKEY option is honored only by the ticket-granting
service. It indicates that the ticket to be issued for the end
server is to be encrypted in the session key from the additional
second TGT provided with the request. See Section 3.3.3 for specific
details.
2.9.3. Passwordless Hardware Authentication
The OPT-HARDWARE-AUTH option indicates that the client wishes to use
some form of hardware authentication instead of or in addition to the
client's password or other long-lived encryption key.
OPT-HARDWARE-AUTH is honored only by the authentication service. If
supported and allowed by policy, the KDC will return an error code of
KDC_ERR_PREAUTH_REQUIRED and include the required METHOD-DATA to
perform such authentication.
3. Message Exchanges
The following sections describe the interactions between network
clients and servers and the messages involved in those exchanges.
3.1. The Authentication Service Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_AS_REQ 5.4.1
2. Kerberos to client KRB_AS_REP or 5.4.2
KRB_ERROR 5.9.1
The Authentication Service (AS) Exchange between the client and the
Kerberos Authentication Server is initiated by a client when it
wishes to obtain authentication credentials for a given server but
currently holds no credentials. In its basic form, the client's
secret key is used for encryption and decryption. This exchange is
typically used at the initiation of a login session to obtain
credentials for a Ticket-Granting Server, which will subsequently be
used to obtain credentials for other servers (see Section 3.3)
without requiring further use of the client's secret key. This
exchange is also used to request credentials for services that must
not be mediated through the Ticket-Granting Service, but rather
require knowledge of a principal's secret key, such as the password-
changing service (the password-changing service denies requests
unless the requester can demonstrate knowledge of the user's old
password; requiring this knowledge prevents unauthorized password
changes by someone walking up to an unattended session).
This exchange does not by itself provide any assurance of the
identity of the user. To authenticate a user logging on to a local
system, the credentials obtained in the AS exchange may first be used
in a TGS exchange to obtain credentials for a local server; those
credentials must then be verified by a local server through
successful completion of the Client/Server exchange.
The AS exchange consists of two messages: KRB_AS_REQ from the client
to Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for
these messages are described in Sections 5.4.1, 5.4.2, and 5.9.1.
In the request, the client sends (in cleartext) its own identity and
the identity of the server for which it is requesting credentials,
other information about the credentials it is requesting, and a
randomly generated nonce, which can be used to detect replays and to
associate replies with the matching requests. This nonce MUST be
generated randomly by the client and remembered for checking against
the nonce in the expected reply. The response, KRB_AS_REP, contains
a ticket for the client to present to the server, and a session key
that will be shared by the client and the server. The session key
and additional information are encrypted in the client's secret key.
The encrypted part of the KRB_AS_REP message also contains the nonce
that MUST be matched with the nonce from the KRB_AS_REQ message.
Without pre-authentication, the authentication server does not know
whether the client is actually the principal named in the request.
It simply sends a reply without knowing or caring whether they are
the same. This is acceptable because nobody but the principal whose
identity was given in the request will be able to use the reply. Its
critical information is encrypted in that principal's key. However,
an attacker can send a KRB_AS_REQ message to get known plaintext in
order to attack the principal's key. Especially if the key is based
on a password, this may create a security exposure. So the initial
request supports an optional field that can be used to pass
additional information that might be needed for the initial exchange.
This field SHOULD be used for pre-authentication as described in
sections 3.1.1 and 5.2.7.
Various errors can occur; these are indicated by an error response
(KRB_ERROR) instead of the KRB_AS_REP response. The error message is
not encrypted. The KRB_ERROR message contains information that can
be used to associate it with the message to which it replies. The
contents of the KRB_ERROR message are not integrity-protected. As
such, the client cannot detect replays, fabrications, or
modifications. A solution to this problem will be included in a
future version of the protocol.
3.1.1. Generation of KRB_AS_REQ Message
The client may specify a number of options in the initial request.
Among these options are whether pre-authentication is to be
performed; whether the requested ticket is to be renewable,
proxiable, or forwardable; whether it should be postdated or allow
postdating of derivative tickets; and whether a renewable ticket will
be accepted in lieu of a non-renewable ticket if the requested ticket
expiration date cannot be satisfied by a non-renewable ticket (due to
configuration constraints).
The client prepares the KRB_AS_REQ message and sends it to the KDC.
3.1.2. Receipt of KRB_AS_REQ Message
If all goes well, processing the KRB_AS_REQ message will result in
the creation of a ticket for the client to present to the server.
The format for the ticket is described in Section 5.3.
Because Kerberos can run over unreliable transports such as UDP, the
KDC MUST be prepared to retransmit responses in case they are lost.
If a KDC receives a request identical to one it has recently
processed successfully, the KDC MUST respond with a KRB_AS_REP
message rather than a replay error. In order to reduce ciphertext
given to a potential attacker, KDCs MAY send the same response
generated when the request was first handled. KDCs MUST obey this
replay behavior even if the actual transport in use is reliable.
3.1.3. Generation of KRB_AS_REP Message
The authentication server looks up the client and server principals
named in the KRB_AS_REQ in its database, extracting their respective
keys. If the requested client principal named in the request is
unknown because it doesn't exist in the KDC's principal database,
then an error message with a KDC_ERR_C_PRINCIPAL_UNKNOWN is returned.
If required to do so, the server pre-authenticates the request, and
if the pre-authentication check fails, an error message with the code
KDC_ERR_PREAUTH_FAILED is returned. If pre-authentication is
required, but was not present in the request, an error message with
the code KDC_ERR_PREAUTH_REQUIRED is returned, and a METHOD-DATA
object will be stored in the e-data field of the KRB-ERROR message to
specify which pre-authentication mechanisms are acceptable. Usually
this will include PA-ETYPE-INFO and/or PA-ETYPE-INFO2 elements as
described below. If the server cannot accommodate any encryption
type requested by the client, an error message with code
KDC_ERR_ETYPE_NOSUPP is returned. Otherwise, the KDC generates a
'random' session key, meaning that, among other things, it should be
impossible to guess the next session key based on knowledge of past
session keys. Although this can be achieved in a pseudo-random
number generator if it is based on cryptographic principles, it is
more desirable to use a truly random number generator, such as one
based on measurements of random physical phenomena. See [RFC4086]
for an in-depth discussion of randomness.
In response to an AS request, if there are multiple encryption keys
registered for a client in the Kerberos database, then the etype
field from the AS request is used by the KDC to select the encryption
method to be used to protect the encrypted part of the KRB_AS_REP
message that is sent to the client. If there is more than one
supported strong encryption type in the etype list, the KDC SHOULD
use the first valid strong etype for which an encryption key is
available.
When the user's key is generated from a password or pass phrase, the
string-to-key function for the particular encryption key type is
used, as specified in [RFC3961]. The salt value and additional
parameters for the string-to-key function have default values
(specified by Section 4 and by the encryption mechanism
specification, respectively) that may be overridden by
pre-authentication data (PA-PW-SALT, PA-AFS3-SALT, PA-ETYPE-INFO,
PA-ETYPE-INFO2, etc). Since the KDC is presumed to store a copy of
the resulting key only, these values should not be changed for
password-based keys except when changing the principal's key.
When the AS server is to include pre-authentication data in a
KRB-ERROR or in an AS-REP, it MUST use PA-ETYPE-INFO2, not PA-ETYPE-
INFO, if the etype field of the client's AS-REQ lists at least one
"newer" encryption type. Otherwise (when the etype field of the
client's AS-REQ does not list any "newer" encryption types), it MUST
send both PA-ETYPE-INFO2 and PA-ETYPE-INFO (both with an entry for
each enctype). A "newer" enctype is any enctype first officially
specified concurrently with or subsequent to the issue of this RFC.
The enctypes DES, 3DES, or RC4 and any defined in [RFC1510] are not
"newer" enctypes.
It is not possible to generate a user's key reliably given a pass
phrase without contacting the KDC, since it will not be known whether
alternate salt or parameter values are required.
The KDC will attempt to assign the type of the random session key
from the list of methods in the etype field. The KDC will select the
appropriate type using the list of methods provided and information
from the Kerberos database indicating acceptable encryption methods
for the application server. The KDC will not issue tickets with a
weak session key encryption type.
If the requested starttime is absent, indicates a time in the past,
or is within the window of acceptable clock skew for the KDC and the
POSTDATE option has not been specified, then the starttime of the
ticket is set to the authentication server's current time. If it
indicates a time in the future beyond the acceptable clock skew, but
the POSTDATED option has not been specified, then the error
KDC_ERR_CANNOT_POSTDATE is returned. Otherwise the requested
starttime is checked against the policy of the local realm (the
administrator might decide to prohibit certain types or ranges of
postdated tickets), and if the ticket's starttime is acceptable, it
is set as requested, and the INVALID flag is set in the new ticket.
The postdated ticket MUST be validated before use by presenting it to
the KDC after the starttime has been reached.
The expiration time of the ticket will be set to the earlier of the
requested endtime and a time determined by local policy, possibly by
using realm- or principal-specific factors. For example, the
expiration time MAY be set to the earliest of the following:
* The expiration time (endtime) requested in the KRB_AS_REQ
message.
* The ticket's starttime plus the maximum allowable lifetime
associated with the client principal from the authentication
server's database.
* The ticket's starttime plus the maximum allowable lifetime
associated with the server principal.
* The ticket's starttime plus the maximum lifetime set by the
policy of the local realm.
If the requested expiration time minus the starttime (as determined
above) is less than a site-determined minimum lifetime, an error
message with code KDC_ERR_NEVER_VALID is returned. If the requested
expiration time for the ticket exceeds what was determined as above,
and if the 'RENEWABLE-OK' option was requested, then the 'RENEWABLE'
flag is set in the new ticket, and the renew-till value is set as if
the 'RENEWABLE' option were requested (the field and option names are
described fully in Section 5.4.1).
If the RENEWABLE option has been requested or if the RENEWABLE-OK
option has been set and a renewable ticket is to be issued, then the
renew-till field MAY be set to the earliest of:
* Its requested value.
* The starttime of the ticket plus the minimum of the two maximum
renewable lifetimes associated with the principals' database
entries.
* The starttime of the ticket plus the maximum renewable lifetime
set by the policy of the local realm.
The flags field of the new ticket will have the following options set
if they have been requested and if the policy of the local realm
allows: FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE.
If the new ticket is postdated (the starttime is in the future), its
INVALID flag will also be set.
If all of the above succeed, the server will encrypt the ciphertext
part of the ticket using the encryption key extracted from the server
principal's record in the Kerberos database using the encryption type
associated with the server principal's key. (This choice is NOT
affected by the etype field in the request.) It then formats a
KRB_AS_REP message (see Section 5.4.2), copying the addresses in the
request into the caddr of the response, placing any required pre-
authentication data into the padata of the response, and encrypts the
ciphertext part in the client's key using an acceptable encryption
method requested in the etype field of the request, or in some key
specified by pre-authentication mechanisms being used.
3.1.4. Generation of KRB_ERROR Message
Several errors can occur, and the Authentication Server responds by
returning an error message, KRB_ERROR, to the client, with the
error-code and e-text fields set to appropriate values. The error
message contents and details are described in Section 5.9.1.
3.1.5. Receipt of KRB_AS_REP Message
If the reply message type is KRB_AS_REP, then the client verifies
that the cname and crealm fields in the cleartext portion of the
reply match what it requested. If any padata fields are present,
they may be used to derive the proper secret key to decrypt the
message. The client decrypts the encrypted part of the response
using its secret key and verifies that the nonce in the encrypted
part matches the nonce it supplied in its request (to detect
replays). It also verifies that the sname and srealm in the response
match those in the request (or are otherwise expected values), and
that the host address field is also correct. It then stores the
ticket, session key, start and expiration times, and other
information for later use. The last-req field (and the deprecated
key-expiration field) from the encrypted part of the response MAY be
checked to notify the user of impending key expiration. This enables
the client program to suggest remedial action, such as a password
change.
Upon validation of the KRB_AS_REP message (by checking the returned
nonce against that sent in the KRB_AS_REQ message), the client knows
that the current time on the KDC is that read from the authtime field
of the encrypted part of the reply. The client can optionally use
this value for clock synchronization in subsequent messages by
recording with the ticket the difference (offset) between the
authtime value and the local clock. This offset can then be used by
the same user to adjust the time read from the system clock when
generating messages [DGT96].
This technique MUST be used when adjusting for clock skew instead of
directly changing the system clock, because the KDC reply is only
authenticated to the user whose secret key was used, but not to the
system or workstation. If the clock were adjusted, an attacker
colluding with a user logging into a workstation could agree on a
password, resulting in a KDC reply that would be correctly validated
even though it did not originate from a KDC trusted by the
workstation.
Proper decryption of the KRB_AS_REP message is not sufficient for the
host to verify the identity of the user; the user and an attacker
could cooperate to generate a KRB_AS_REP format message that decrypts
properly but is not from the proper KDC. If the host wishes to
verify the identity of the user, it MUST require the user to present
application credentials that can be verified using a securely-stored
secret key for the host. If those credentials can be verified, then
the identity of the user can be assured.
3.1.6. Receipt of KRB_ERROR Message
If the reply message type is KRB_ERROR, then the client interprets it
as an error and performs whatever application-specific tasks are
necessary for recovery.
3.2. The Client/Server Authentication Exchange
Summary
Message direction Message type Section
Client to Application server KRB_AP_REQ 5.5.1
[optional] Application server to client KRB_AP_REP or 5.5.2
KRB_ERROR 5.9.1
The client/server authentication (CS) exchange is used by network
applications to authenticate the client to the server and vice versa.
The client MUST have already acquired credentials for the server
using the AS or TGS exchange.
3.2.1. The KRB_AP_REQ Message
The KRB_AP_REQ contains authentication information that SHOULD be
part of the first message in an authenticated transaction. It
contains a ticket, an authenticator, and some additional bookkeeping
information (see Section 5.5.1 for the exact format). The ticket by
itself is insufficient to authenticate a client, since tickets are
passed across the network in cleartext (tickets contain both an
encrypted and unencrypted portion, so cleartext here refers to the
entire unit, which can be copied from one message and replayed in
another without any cryptographic skill). The authenticator is used
to prevent invalid replay of tickets by proving to the server that
the client knows the session key of the ticket and thus is entitled
to use the ticket. The KRB_AP_REQ message is referred to elsewhere
as the 'authentication header'.
3.2.2. Generation of a KRB_AP_REQ Message
When a client wishes to initiate authentication to a server, it
obtains (either through a credentials cache, the AS exchange, or the
TGS exchange) a ticket and session key for the desired service. The
client MAY re-use any tickets it holds until they expire. To use a
ticket, the client constructs a new Authenticator from the system
time and its name, and optionally from an application-specific
checksum, an initial sequence number to be used in KRB_SAFE or
KRB_PRIV messages, and/or a session subkey to be used in negotiations
for a session key unique to this particular session. Authenticators
MUST NOT be re-used and SHOULD be rejected if replayed to a server.
Note that this can make applications based on unreliable transports
difficult to code correctly. If the transport might deliver
duplicated messages, either a new authenticator MUST be generated for
each retry, or the application server MUST match requests and replies
and replay the first reply in response to a detected duplicate.
If a sequence number is to be included, it SHOULD be randomly chosen
so that even after many messages have been exchanged it is not likely
to collide with other sequence numbers in use.
The client MAY indicate a requirement of mutual authentication or the
use of a session-key based ticket (for user-to-user authentication,
see section 3.7) by setting the appropriate flag(s) in the ap-options
field of the message.
The Authenticator is encrypted in the session key and combined with
the ticket to form the KRB_AP_REQ message, which is then sent to the
end server along with any additional application-specific
information.
3.2.3. Receipt of KRB_AP_REQ Message
Authentication is based on the server's current time of day (clocks
MUST be loosely synchronized), the authenticator, and the ticket.
Several errors are possible. If an error occurs, the server is
expected to reply to the client with a KRB_ERROR message. This
message MAY be encapsulated in the application protocol if its raw
form is not acceptable to the protocol. The format of error messages
is described in Section 5.9.1.
The algorithm for verifying authentication information is as follows.
If the message type is not KRB_AP_REQ, the server returns the
KRB_AP_ERR_MSG_TYPE error. If the key version indicated by the
Ticket in the KRB_AP_REQ is not one the server can use (e.g., it
indicates an old key, and the server no longer possesses a copy of
the old key), the KRB_AP_ERR_BADKEYVER error is returned. If the
USE-SESSION-KEY flag is set in the ap-options field, it indicates to
the server that user-to-user authentication is in use, and that the
ticket is encrypted in the session key from the server's TGT rather
than in the server's secret key. See Section 3.7 for a more complete
description of the effect of user-to-user authentication on all
messages in the Kerberos protocol.
Because it is possible for the server to be registered in multiple
realms, with different keys in each, the srealm field in the
unencrypted portion of the ticket in the KRB_AP_REQ is used to
specify which secret key the server should use to decrypt that
ticket. The KRB_AP_ERR_NOKEY error code is returned if the server
doesn't have the proper key to decipher the ticket.
The ticket is decrypted using the version of the server's key
specified by the ticket. If the decryption routines detect a
modification of the ticket (each encryption system MUST provide
safeguards to detect modified ciphertext), the
KRB_AP_ERR_BAD_INTEGRITY error is returned (chances are good that
different keys were used to encrypt and decrypt).
The authenticator is decrypted using the session key extracted from
the decrypted ticket. If decryption shows that is has been modified,
the KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm
of the client from the ticket are compared against the same fields in
the authenticator. If they don't match, the KRB_AP_ERR_BADMATCH
error is returned; normally this is caused by a client error or an
attempted attack. The addresses in the ticket (if any) are then
searched for an address matching the operating-system reported
address of the client. If no match is found or the server insists on
ticket addresses but none are present in the ticket, the
KRB_AP_ERR_BADADDR error is returned. If the local (server) time and
the client time in the authenticator differ by more than the
allowable clock skew (e.g., 5 minutes), the KRB_AP_ERR_SKEW error is
returned.
Unless the application server provides its own suitable means to
protect against replay (for example, a challenge-response sequence
initiated by the server after authentication, or use of a server-
generated encryption subkey), the server MUST utilize a replay cache
to remember any authenticator presented within the allowable clock
skew. Careful analysis of the application protocol and
implementation is recommended before eliminating this cache. The
replay cache will store at least the server name, along with the
client name, time, and microsecond fields from the recently-seen
authenticators, and if a matching tuple is found, the
KRB_AP_ERR_REPEAT error is returned. Note that the rejection here is
restricted to authenticators from the same principal to the same
server. Other client principals communicating with the same server
principal should not have their authenticators rejected if the time
and microsecond fields happen to match some other client's
authenticator.
If a server loses track of authenticators presented within the
allowable clock skew, it MUST reject all requests until the clock
skew interval has passed, providing assurance that any lost or
replayed authenticators will fall outside the allowable clock skew
and can no longer be successfully replayed. If this were not done,
an attacker could subvert the authentication by recording the ticket
and authenticator sent over the network to a server and replaying
them following an event that caused the server to lose track of
recently seen authenticators.
Implementation note: If a client generates multiple requests to the
KDC with the same timestamp, including the microsecond field, all but
the first of the requests received will be rejected as replays. This
might happen, for example, if the resolution of the client's clock is
too coarse. Client implementations SHOULD ensure that the timestamps
are not reused, possibly by incrementing the microseconds field in
the time stamp when the clock returns the same time for multiple
requests.
If multiple servers (for example, different services on one machine,
or a single service implemented on multiple machines) share a service
principal (a practice that we do not recommend in general, but that
we acknowledge will be used in some cases), either they MUST share
this replay cache, or the application protocol MUST be designed so as
to eliminate the need for it. Note that this applies to all of the
services. If any of the application protocols does not have replay
protection built in, an authenticator used with such a service could
later be replayed to a different service with the same service
principal but no replay protection, if the former doesn't record the
authenticator information in the common replay cache.
If a sequence number is provided in the authenticator, the server
saves it for later use in processing KRB_SAFE and/or KRB_PRIV
messages. If a subkey is present, the server either saves it for
later use or uses it to help generate its own choice for a subkey to
be returned in a KRB_AP_REP message.
The server computes the age of the ticket: local (server) time minus
the starttime inside the Ticket. If the starttime is later than the
current time by more than the allowable clock skew, or if the INVALID
flag is set in the ticket, the KRB_AP_ERR_TKT_NYV error is returned.
Otherwise, if the current time is later than end time by more than
the allowable clock skew, the KRB_AP_ERR_TKT_EXPIRED error is
returned.
If all these checks succeed without an error, the server is assured
that the client possesses the credentials of the principal named in
the ticket, and thus, that the client has been authenticated to the
server.
Passing these checks provides only authentication of the named
principal; it does not imply authorization to use the named service.
Applications MUST make a separate authorization decision based upon
the authenticated name of the user, the requested operation, local
access control information such as that contained in a .k5login or
.k5users file, and possibly a separate distributed authorization
service.
3.2.4. Generation of a KRB_AP_REP Message
Typically, a client's request will include both the authentication
information and its initial request in the same message, and the
server need not explicitly reply to the KRB_AP_REQ. However, if
mutual authentication (authenticating not only the client to the
server, but also the server to the client) is being performed, the
KRB_AP_REQ message will have MUTUAL-REQUIRED set in its ap-options
field, and a KRB_AP_REP message is required in response. As with the
error message, this message MAY be encapsulated in the application
protocol if its "raw" form is not acceptable to the application's
protocol. The timestamp and microsecond field used in the reply MUST
be the client's timestamp and microsecond field (as provided in the
authenticator). If a sequence number is to be included, it SHOULD be
randomly chosen as described above for the authenticator. A subkey
MAY be included if the server desires to negotiate a different
subkey. The KRB_AP_REP message is encrypted in the session key
extracted from the ticket.
Note that in the Kerberos Version 4 protocol, the timestamp in the
reply was the client's timestamp plus one. This is not necessary in
Version 5 because Version 5 messages are formatted in such a way that
it is not possible to create the reply by judicious message surgery
(even in encrypted form) without knowledge of the appropriate
encryption keys.
3.2.5. Receipt of KRB_AP_REP Message
If a KRB_AP_REP message is returned, the client uses the session key
from the credentials obtained for the server to decrypt the message
and verifies that the timestamp and microsecond fields match those in
the Authenticator it sent to the server. If they match, then the
client is assured that the server is genuine. The sequence number
and subkey (if present) are retained for later use. (Note that for
encrypting the KRB_AP_REP message, the sub-session key is not used,
even if it is present in the Authentication.)
3.2.6. Using the Encryption Key
After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and
server share an encryption key that can be used by the application.
In some cases, the use of this session key will be implicit in the
protocol; in others the method of use must be chosen from several
alternatives. The application MAY choose the actual encryption key
to be used for KRB_PRIV, KRB_SAFE, or other application-specific uses
based on the session key from the ticket and subkeys in the
KRB_AP_REP message and the authenticator. Implementations of the
protocol MAY provide routines to choose subkeys based on session keys
and random numbers and to generate a negotiated key to be returned in
the KRB_AP_REP message.
To mitigate the effect of failures in random number generation on the
client, it is strongly encouraged that any key derived by an
application for subsequent use include the full key entropy derived
from the KDC-generated session key carried in the ticket. We leave
the protocol negotiations of how to use the key (e.g., for selecting
an encryption or checksum type) to the application programmer. The
Kerberos protocol does not constrain the implementation options, but
an example of how this might be done follows.
One way that an application may choose to negotiate a key to be used
for subsequent integrity and privacy protection is for the client to
propose a key in the subkey field of the authenticator. The server
can then choose a key using the key proposed by the client as input,
returning the new subkey in the subkey field of the application
reply. This key could then be used for subsequent communication.
With both the one-way and mutual authentication exchanges, the peers
should take care not to send sensitive information to each other
without proper assurances. In particular, applications that require
privacy or integrity SHOULD use the KRB_AP_REP response from the
server to the client to assure both client and server of their peer's
identity. If an application protocol requires privacy of its
messages, it can use the KRB_PRIV message (section 3.5). The
KRB_SAFE message (Section 3.4) can be used to ensure integrity.
3.3. The Ticket-Granting Service (TGS) Exchange
Summary
Message direction Message type Section
1. Client to Kerberos KRB_TGS_REQ 5.4.1
2. Kerberos to client KRB_TGS_REP or 5.4.2
KRB_ERROR 5.9.1
The TGS exchange between a client and the Kerberos TGS is initiated
by a client when it seeks to obtain authentication credentials for a
given server (which might be registered in a remote realm), when it
seeks to renew or validate an existing ticket, or when it seeks to
obtain a proxy ticket. In the first case, the client must already
have acquired a ticket for the Ticket-Granting Service using the AS
exchange (the TGT is usually obtained when a client initially
authenticates to the system, such as when a user logs in). The
message format for the TGS exchange is almost identical to that for
the AS exchange. The primary difference is that encryption and
decryption in the TGS exchange does not take place under the client's
key. Instead, the session key from the TGT or renewable ticket, or
sub-session key from an Authenticator is used. As is the case for
all application servers, expired tickets are not accepted by the TGS,
so once a renewable or TGT expires, the client must use a separate
exchange to obtain valid tickets.
The TGS exchange consists of two messages: a request (KRB_TGS_REQ)
from the client to the Kerberos Ticket-Granting Server, and a reply
(KRB_TGS_REP or KRB_ERROR). The KRB_TGS_REQ message includes
information authenticating the client plus a request for credentials.
The authentication information consists of the authentication header
(KRB_AP_REQ), which includes the client's previously obtained
ticket-granting, renewable, or invalid ticket. In the TGT and proxy
cases, the request MAY include one or more of the following: a list
of network addresses, a collection of typed authorization data to be
sealed in the ticket for authorization use by the application server,
or additional tickets (the use of which are described later). The
TGS reply (KRB_TGS_REP) contains the requested credentials, encrypted
in the session key from the TGT or renewable ticket, or, if present,
in the sub-session key from the Authenticator (part of the
authentication header). The KRB_ERROR message contains an error code
and text explaining what went wrong. The KRB_ERROR message is not
encrypted. The KRB_TGS_REP message contains information that can be
used to detect replays, and to associate it with the message to which
it replies. The KRB_ERROR message also contains information that can
be used to associate it with the message to which it replies. The
same comments about integrity protection of KRB_ERROR messages
mentioned in Section 3.1 apply to the TGS exchange.
3.3.1. Generation of KRB_TGS_REQ Message
Before sending a request to the ticket-granting service, the client
MUST determine in which realm the application server is believed to
be registered. This can be accomplished in several ways. It might
be known beforehand (since the realm is part of the principal
identifier), it might be stored in a nameserver, or it might be
obtained from a configuration file. If the realm to be used is
obtained from a nameserver, there is a danger of being spoofed if the
nameservice providing the realm name is not authenticated. This
might result in the use of a realm that has been compromised, which
would result in an attacker's ability to compromise the
authentication of the application server to the client.
If the client knows the service principal name and realm and it does
not already possess a TGT for the appropriate realm, then one must be
obtained. This is first attempted by requesting a TGT for the
destination realm from a Kerberos server for which the client
possesses a TGT (by using the KRB_TGS_REQ message recursively). The
Kerberos server MAY return a TGT for the desired realm, in which case
one can proceed. Alternatively, the Kerberos server MAY return a TGT
for a realm that is 'closer' to the desired realm (further along the
standard hierarchical path between the client's realm and the
requested realm server's realm). Note that in this case
misconfiguration of the Kerberos servers may cause loops in the
resulting authentication path, which the client should be careful to
detect and avoid.
If the Kerberos server returns a TGT for a realm 'closer' than the
desired realm, the client MAY use local policy configuration to
verify that the authentication path used is an acceptable one.
Alternatively, a client MAY choose its own authentication path,
rather than rely on the Kerberos server to select one. In either
case, any policy or configuration information used to choose or
validate authentication paths, whether by the Kerberos server or by
the client, MUST be obtained from a trusted source.
When a client obtains a TGT that is 'closer' to the destination
realm, the client MAY cache this ticket and reuse it in future
KRB-TGS exchanges with services in the 'closer' realm. However, if
the client were to obtain a TGT for the 'closer' realm by starting at
the initial KDC rather than as part of obtaining another ticket, then
a shorter path to the 'closer' realm might be used. This shorter
path may be desirable because fewer intermediate KDCs would know the
session key of the ticket involved. For this reason, clients SHOULD
evaluate whether they trust the realms transited in obtaining the
'closer' ticket when making a decision to use the ticket in future.
Once the client obtains a TGT for the appropriate realm, it
determines which Kerberos servers serve that realm and contacts one
of them. The list might be obtained through a configuration file or
network service, or it MAY be generated from the name of the realm.
As long as the secret keys exchanged by realms are kept secret, only
denial of service results from using a false Kerberos server.
As in the AS exchange, the client MAY specify a number of options in
the KRB_TGS_REQ message. One of these options is the ENC-TKT-IN-SKEY
option used for user-to-user authentication. An overview of user-
to-user authentication can be found in Section 3.7. When generating
the KRB_TGS_REQ message, this option indicates that the client is
including a TGT obtained from the application server in the
additional tickets field of the request and that the KDC SHOULD
encrypt the ticket for the application server using the session key
from this additional ticket, instead of a server key from the
principal database.
The client prepares the KRB_TGS_REQ message, providing an
authentication header as an element of the padata field, and
including the same fields as used in the KRB_AS_REQ message along
with several optional fields: the enc-authorizatfion-data field for
application server use and additional tickets required by some
options.
In preparing the authentication header, the client can select a sub-
session key under which the response from the Kerberos server will be
encrypted. If the client selects a sub-session key, care must be
taken to ensure the randomness of the selected sub-session key.
If the sub-session key is not specified, the session key from the TGT
will be used. If the enc-authorization-data is present, it MUST be
encrypted in the sub-session key, if present, from the authenticator
portion of the authentication header, or, if not present, by using
the session key from the TGT.
Once prepared, the message is sent to a Kerberos server for the
destination realm.
3.3.2. Receipt of KRB_TGS_REQ Message
The KRB_TGS_REQ message is processed in a manner similar to the
KRB_AS_REQ message, but there are many additional checks to be
performed. First, the Kerberos server MUST determine which server
the accompanying ticket is for, and it MUST select the appropriate
key to decrypt it. For a normal KRB_TGS_REQ message, it will be for
the ticket-granting service, and the TGS's key will be used. If the
TGT was issued by another realm, then the appropriate inter-realm key
MUST be used. If (a) the accompanying ticket is not a TGT for the
current realm, but is for an application server in the current realm,
(b) the RENEW, VALIDATE, or PROXY options are specified in the
request, and (c) the server for which a ticket is requested is the
server named in the accompanying ticket, then the KDC will decrypt
the ticket in the authentication header using the key of the server
for which it was issued. If no ticket can be found in the padata
field, the KDC_ERR_PADATA_TYPE_NOSUPP error is returned.
Once the accompanying ticket has been decrypted, the user-supplied
checksum in the Authenticator MUST be verified against the contents
of the request, and the message MUST be rejected if the checksums do
not match (with an error code of KRB_AP_ERR_MODIFIED) or if the
checksum is not collision-proof (with an error code of
KRB_AP_ERR_INAPP_CKSUM). If the checksum type is not supported, the
KDC_ERR_SUMTYPE_NOSUPP error is returned. If the authorization-data
are present, they are decrypted using the sub-session key from the
Authenticator.
If any of the decryptions indicate failed integrity checks, the
KRB_AP_ERR_BAD_INTEGRITY error is returned.
As discussed in Section 3.1.2, the KDC MUST send a valid KRB_TGS_REP
message if it receives a KRB_TGS_REQ message identical to one it has
recently processed. However, if the authenticator is a replay, but
the rest of the request is not identical, then the KDC SHOULD return
KRB_AP_ERR_REPEAT.
3.3.3. Generation of KRB_TGS_REP Message
The KRB_TGS_REP message shares its format with the KRB_AS_REP
(KRB_KDC_REP), but with its type field set to KRB_TGS_REP. The
detailed specification is in Section 5.4.2.
The response will include a ticket for the requested server or for a
ticket granting server of an intermediate KDC to be contacted to
obtain the requested ticket. The Kerberos database is queried to
retrieve the record for the appropriate server (including the key
with which the ticket will be encrypted). If the request is for a
TGT for a remote realm, and if no key is shared with the requested
realm, then the Kerberos server will select the realm 'closest' to
the requested realm with which it does share a key and use that realm
instead. This is the only case where the response for the KDC will
be for a different server than that requested by the client.
By default, the address field, the client's name and realm, the list
of transited realms, the time of initial authentication, the
expiration time, and the authorization data of the newly-issued
ticket will be copied from the TGT or renewable ticket. If the
transited field needs to be updated, but the transited type is not
supported, the KDC_ERR_TRTYPE_NOSUPP error is returned.
If the request specifies an endtime, then the endtime of the new
ticket is set to the minimum of (a) that request, (b) the endtime
from the TGT, and (c) the starttime of the TGT plus the minimum of
the maximum life for the application server and the maximum life for
the local realm (the maximum life for the requesting principal was
already applied when the TGT was issued). If the new ticket is to be
a renewal, then the endtime above is replaced by the minimum of (a)
the value of the renew_till field of the ticket and (b) the starttime
for the new ticket plus the life (endtime-starttime) of the old
ticket.
If the FORWARDED option has been requested, then the resulting ticket
will contain the addresses specified by the client. This option will
only be honored if the FORWARDABLE flag is set in the TGT. The PROXY
option is similar; the resulting ticket will contain the addresses
specified by the client. It will be honored only if the PROXIABLE
flag in the TGT is set. The PROXY option will not be honored on
requests for additional TGTs.
If the requested starttime is absent, indicates a time in the past,
or is within the window of acceptable clock skew for the KDC and the
POSTDATE option has not been specified, then the starttime of the
ticket is set to the authentication server's current time. If it
indicates a time in the future beyond the acceptable clock skew, but
the POSTDATED option has not been specified or the MAY-POSTDATE flag
is not set in the TGT, then the error KDC_ERR_CANNOT_POSTDATE is
returned. Otherwise, if the TGT has the MAY-POSTDATE flag set, then
the resulting ticket will be postdated, and the requested starttime
is checked against the policy of the local realm. If acceptable, the
ticket's starttime is set as requested, and the INVALID flag is set.
The postdated ticket MUST be validated before use by presenting it to
the KDC after the starttime has been reached. However, in no case
may the starttime, endtime, or renew-till time of a newly-issued
postdated ticket extend beyond the renew-till time of the TGT.
If the ENC-TKT-IN-SKEY option has been specified and an additional
ticket has been included in the request, it indicates that the client
is using user-to-user authentication to prove its identity to a
server that does not have access to a persistent key. Section 3.7
describes the effect of this option on the entire Kerberos protocol.
When generating the KRB_TGS_REP message, this option in the
KRB_TGS_REQ message tells the KDC to decrypt the additional ticket
using the key for the server to which the additional ticket was
issued and to verify that it is a TGT. If the name of the requested
server is missing from the request, the name of the client in the
additional ticket will be used. Otherwise, the name of the requested
server will be compared to the name of the client in the additional
ticket. If it is different, the request will be rejected. If the
request succeeds, the session key from the additional ticket will be
used to encrypt the new ticket that is issued instead of using the
key of the server for which the new ticket will be used.
If (a) the name of the server in the ticket that is presented to the
KDC as part of the authentication header is not that of the TGS
itself, (b) the server is registered in the realm of the KDC, and (c)
the RENEW option is requested, then the KDC will verify that the
RENEWABLE flag is set in the ticket, that the INVALID flag is not set
in the ticket, and that the renew_till time is still in the future.
If the VALIDATE option is requested, the KDC will check that the
starttime has passed and that the INVALID flag is set. If the PROXY
option is requested, then the KDC will check that the PROXIABLE flag
is set in the ticket. If the tests succeed and the ticket passes the
hotlist check described in the next section, the KDC will issue the
appropriate new ticket.
The ciphertext part of the response in the KRB_TGS_REP message is
encrypted in the sub-session key from the Authenticator, if present,
or in the session key from the TGT. It is not encrypted using the
client's secret key. Furthermore, the client's key's expiration date
and the key version number fields are left out since these values are
stored along with the client's database record, and that record is
not needed to satisfy a request based on a TGT.
3.3.3.1. Checking for Revoked Tickets
Whenever a request is made to the ticket-granting server, the
presented ticket(s) is (are) checked against a hot-list of tickets
that have been canceled. This hot-list might be implemented by
storing a range of issue timestamps for 'suspect tickets'; if a
presented ticket had an authtime in that range, it would be rejected.
In this way, a stolen TGT or renewable ticket cannot be used to gain
additional tickets (renewals or otherwise) once the theft has been
reported to the KDC for the realm in which the server resides. Any
normal ticket obtained before it was reported stolen will still be
valid (because tickets require no interaction with the KDC), but only
until its normal expiration time. If TGTs have been issued for
cross-realm authentication, use of the cross-realm TGT will not be
affected unless the hot-list is propagated to the KDCs for the realms
for which such cross-realm tickets were issued.
3.3.3.2. Encoding the Transited Field
If the identity of the server in the TGT that is presented to the KDC
as part of the authentication header is that of the ticket-granting
service, but the TGT was issued from another realm, the KDC will look
up the inter-realm key shared with that realm and use that key to
decrypt the ticket. If the ticket is valid, then the KDC will honor
the request, subject to the constraints outlined above in the section
describing the AS exchange. The realm part of the client's identity
will be taken from the TGT. The name of the realm that issued the
TGT, if it is not the realm of the client principal, will be added to
the transited field of the ticket to be issued. This is accomplished
by reading the transited field from the TGT (which is treated as an
unordered set of realm names), adding the new realm to the set, and
then constructing and writing out its encoded (shorthand) form (this
may involve a rearrangement of the existing encoding).
Note that the ticket-granting service does not add the name of its
own realm. Instead, its responsibility is to add the name of the
previous realm. This prevents a malicious Kerberos server from
intentionally leaving out its own name (it could, however, omit other
realms' names).
The names of neither the local realm nor the principal's realm are to
be included in the transited field. They appear elsewhere in the
ticket and both are known to have taken part in authenticating the
principal. Because the endpoints are not included, both local and
single-hop inter-realm authentication result in a transited field
that is empty.
Because this field has the name of each transited realm added to it,
it might potentially be very long. To decrease the length of this
field, its contents are encoded. The initially supported encoding is
optimized for the normal case of inter-realm communication: a
hierarchical arrangement of realms using either domain or X.500 style
realm names. This encoding (called DOMAIN-X500-COMPRESS) is now
described.
Realm names in the transited field are separated by a ",". The ",",
"\", trailing "."s, and leading spaces (" ") are special characters,
and if they are part of a realm name, they MUST be quoted in the
transited field by preceding them with a "\".
A realm name ending with a "." is interpreted as being prepended to
the previous realm. For example, we can encode traversal of EDU,
MIT.EDU, ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:
"EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".
Note that if either ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were
endpoints, they would not be included in this field, and we would
have:
"EDU,MIT.,WASHINGTON.EDU"
A realm name beginning with a "/" is interpreted as being appended to
the previous realm. For the purpose of appending, the realm
preceding the first listed realm is considered the null realm ("").
If a realm name beginning with a "/" is to stand by itself, then it
SHOULD be preceded by a space (" "). For example, we can encode
traversal of /COM/HP/APOLLO, /COM/HP, /COM, and /COM/DEC as:
"/COM,/HP,/APOLLO, /COM/DEC".
As in the example above, if /COM/HP/APOLLO and /COM/DEC were
endpoints, they would not be included in this field, and we would
have:
"/COM,/HP"
A null subfield preceding or following a "," indicates that all
realms between the previous realm and the next realm have been
traversed. For the purpose of interpreting null subfields, the
client's realm is considered to precede those in the transited field,
and the server's realm is considered to follow them. Thus, "," means
that all realms along the path between the client and the server have
been traversed. ",EDU, /COM," means that all realms from the
client's realm up to EDU (in a domain style hierarchy) have been
traversed, and that everything from /COM down to the server's realm
in an X.500 style has also been traversed. This could occur if the
EDU realm in one hierarchy shares an inter-realm key directly with
the /COM realm in another hierarchy.
3.3.4. Receipt of KRB_TGS_REP Message
When the KRB_TGS_REP is received by the client, it is processed in
the same manner as the KRB_AS_REP processing described above. The
primary difference is that the ciphertext part of the response must
be decrypted using the sub-session key from the Authenticator, if it
was specified in the request, or the session key from the TGT, rather
than the client's secret key. The server name returned in the reply
is the true principal name of the service.
3.4. The KRB_SAFE Exchange
The KRB_SAFE message MAY be used by clients requiring the ability to
detect modifications of messages they exchange. It achieves this by
including a keyed collision-proof checksum of the user data and some
control information. The checksum is keyed with an encryption key
(usually the last key negotiated via subkeys, or the session key if
no negotiation has occurred).
3.4.1. Generation of a KRB_SAFE Message
When an application wishes to send a KRB_SAFE message, it collects
its data and the appropriate control information and computes a
checksum over them. The checksum algorithm should be the keyed
checksum mandated to be implemented along with the crypto system used
for the sub-session or session key. The checksum is generated using
the sub-session key, if present, or the session key. Some
implementations use a different checksum algorithm for the KRB_SAFE
messages, but doing so in an interoperable manner is not always
possible.
The control information for the KRB_SAFE message includes both a
timestamp and a sequence number. The designer of an application
using the KRB_SAFE message MUST choose at least one of the two
mechanisms. This choice SHOULD be based on the needs of the
application protocol.
Sequence numbers are useful when all messages sent will be received
by one's peer. Connection state is presently required to maintain
the session key, so maintaining the next sequence number should not
present an additional problem.
If the application protocol is expected to tolerate lost messages
without their being resent, the use of the timestamp is the
appropriate replay detection mechanism. Using timestamps is also the
appropriate mechanism for multi-cast protocols in which all of one's
peers share a common sub-session key, but some messages will be sent
to a subset of one's peers.
After computing the checksum, the client then transmits the
information and checksum to the recipient in the message format
specified in Section 5.6.1.
3.4.2. Receipt of KRB_SAFE Message
When an application receives a KRB_SAFE message, it verifies it as
follows. If any error occurs, an error code is reported for use by
the application.
The message is first checked by verifying that the protocol version
and type fields match the current version and KRB_SAFE, respectively.
A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
error. The application verifies that the checksum used is a
collision-proof keyed checksum that uses keys compatible with the
sub-session or session key as appropriate (or with the application
key derived from the session or sub-session keys). If it is not, a
KRB_AP_ERR_INAPP_CKSUM error is generated. The sender's address MUST
be included in the control information; the recipient verifies that
the operating system's report of the sender's address matches the
sender's address in the message, and (if a recipient address is
specified or the recipient requires an address) that one of the
recipient's addresses appears as the recipient's address in the
message. To work with network address translation, senders MAY use
the directional address type specified in Section 8.1 for the sender
address and not include recipient addresses. A failed match for
either case generates a KRB_AP_ERR_BADADDR error. Then the timestamp
and usec and/or the sequence number fields are checked. If timestamp
and usec are expected and not present, or if they are present but not
current, the KRB_AP_ERR_SKEW error is generated. Timestamps are not
required to be strictly ordered; they are only required to be in the
skew window. If the server name, along with the client name, time,
and microsecond fields from the Authenticator match any recently-seen
(sent or received) such tuples, the KRB_AP_ERR_REPEAT error is
generated. If an incorrect sequence number is included, or if a
sequence number is expected but not present, the KRB_AP_ERR_BADORDER
error is generated. If neither a time-stamp and usec nor a sequence
number is present, a KRB_AP_ERR_MODIFIED error is generated.
Finally, the checksum is computed over the data and control
information, and if it doesn't match the received checksum, a
KRB_AP_ERR_MODIFIED error is generated.
If all the checks succeed, the application is assured that the
message was generated by its peer and was not modified in transit.
Implementations SHOULD accept any checksum algorithm they implement
that has both adequate security and keys compatible with the sub-
session or session key. Unkeyed or non-collision-proof checksums are
not suitable for this use.
3.5. The KRB_PRIV Exchange
The KRB_PRIV message MAY be used by clients requiring confidentiality
and the ability to detect modifications of exchanged messages. It
achieves this by encrypting the messages and adding control
information.
3.5.1. Generation of a KRB_PRIV Message
When an application wishes to send a KRB_PRIV message, it collects
its data and the appropriate control information (specified in
Section 5.7.1) and encrypts them under an encryption key (usually the
last key negotiated via subkeys, or the session key if no negotiation
has occurred). As part of the control information, the client MUST
choose to use either a timestamp or a sequence number (or both); see
the discussion in Section 3.4.1 for guidelines on which to use.
After the user data and control information are encrypted, the client
transmits the ciphertext and some 'envelope' information to the
recipient.
3.5.2. Receipt of KRB_PRIV Message
When an application receives a KRB_PRIV message, it verifies it as
follows. If any error occurs, an error code is reported for use by
the application.
The message is first checked by verifying that the protocol version
and type fields match the current version and KRB_PRIV, respectively.
A mismatch generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE
error. The application then decrypts the ciphertext and processes
the resultant plaintext. If decryption shows that the data has been
modified, a KRB_AP_ERR_BAD_INTEGRITY error is generated.
The sender's address MUST be included in the control information; the
recipient verifies that the operating system's report of the sender's
address matches the sender's address in the message. If a recipient
address is specified or the recipient requires an address, then one
of the recipient's addresses MUST also appear as the recipient's
address in the message. Where a sender's or receiver's address might
not otherwise match the address in a message because of network
address translation, an application MAY be written to use addresses
of the directional address type in place of the actual network
address.
A failed match for either case generates a KRB_AP_ERR_BADADDR error.
To work with network address translation, implementations MAY use the
directional address type defined in Section 7.1 for the sender
address and include no recipient address.
Next the timestamp and usec and/or the sequence number fields are
checked. If timestamp and usec are expected and not present, or if
they are present but not current, the KRB_AP_ERR_SKEW error is
generated. If the server name, along with the client name, time, and
microsecond fields from the Authenticator match any such recently-
seen tuples, the KRB_AP_ERR_REPEAT error is generated. If an
incorrect sequence number is included, or if a sequence number is
expected but not present, the KRB_AP_ERR_BADORDER error is generated.
If neither a time-stamp and usec nor a sequence number is present, a
KRB_AP_ERR_MODIFIED error is generated.
If all the checks succeed, the application can assume the message was
generated by its peer and was securely transmitted (without intruders
seeing the unencrypted contents).
3.6. The KRB_CRED Exchange
The KRB_CRED message MAY be used by clients requiring the ability to
send Kerberos credentials from one host to another. It achieves this
by sending the tickets together with encrypted data containing the
session keys and other information associated with the tickets.
3.6.1. Generation of a KRB_CRED Message
When an application wishes to send a KRB_CRED message, it first
(using the KRB_TGS exchange) obtains credentials to be sent to the
remote host. It then constructs a KRB_CRED message using the ticket
or tickets so obtained, placing the session key needed to use each
ticket in the key field of the corresponding KrbCredInfo sequence of
the encrypted part of the KRB_CRED message.
Other information associated with each ticket and obtained during the
KRB_TGS exchange is also placed in the corresponding KrbCredInfo
sequence in the encrypted part of the KRB_CRED message. The current
time and, if they are specifically required by the application, the
nonce, s-address, and r-address fields are placed in the encrypted
part of the KRB_CRED message, which is then encrypted under an
encryption key previously exchanged in the KRB_AP exchange (usually
the last key negotiated via subkeys, or the session key if no
negotiation has occurred).
Implementation note: When constructing a KRB_CRED message for
inclusion in a GSSAPI initial context token, the MIT implementation
of Kerberos will not encrypt the KRB_CRED message if the session key
is a DES or triple DES key. For interoperability with MIT, the
Microsoft implementation will not encrypt the KRB_CRED in a GSSAPI
token if it is using a DES session key. Starting at version 1.2.5,
MIT Kerberos can receive and decode either encrypted or unencrypted
KRB_CRED tokens in the GSSAPI exchange. The Heimdal implementation
of Kerberos can also accept either encrypted or unencrypted KRB_CRED
messages. Since the KRB_CRED message in a GSSAPI token is encrypted
in the authenticator, the MIT behavior does not present a security
problem, although it is a violation of the Kerberos specification.
3.6.2. Receipt of KRB_CRED Message
When an application receives a KRB_CRED message, it verifies it. If
any error occurs, an error code is reported for use by the
application. The message is verified by checking that the protocol
version and type fields match the current version and KRB_CRED,
respectively. A mismatch generates a KRB_AP_ERR_BADVERSION or
KRB_AP_ERR_MSG_TYPE error. The application then decrypts the
ciphertext and processes the resultant plaintext. If decryption
shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY
error is generated.
If present or required, the recipient MAY verify that the operating
system's report of the sender's address matches the sender's address
in the message, and that one of the recipient's addresses appears as
the recipient's address in the message. The address check does not
provide any added security, since the address, if present, has
already been checked in the KRB_AP_REQ message and there is not any
benefit to be gained by an attacker in reflecting a KRB_CRED message
back to its originator. Thus, the recipient MAY ignore the address
even if it is present in order to work better in Network Address
Translation (NAT) environments. A failed match for either case
generates a KRB_AP_ERR_BADADDR error. Recipients MAY skip the
address check, as the KRB_CRED message cannot generally be reflected
back to the originator. The timestamp and usec fields (and the nonce
field, if required) are checked next. If the timestamp and usec are
not present, or if they are present but not current, the
KRB_AP_ERR_SKEW error is generated.
If all the checks succeed, the application stores each of the new
tickets in its credentials cache together with the session key and
other information in the corresponding KrbCredInfo sequence from the
encrypted part of the KRB_CRED message.
3.7. User-to-User Authentication Exchanges
User-to-User authentication provides a method to perform
authentication when the verifier does not have a access to long-term
service key. This might be the case when running a server (for
example, a window server) as a user on a workstation. In such cases,
the server may have access to the TGT obtained when the user logged
in to the workstation, but because the server is running as an
unprivileged user, it might not have access to system keys. Similar
situations may arise when running peer-to-peer applications.
Summary
Message direction Message type Sections
0. Message from application server Not specified
1. Client to Kerberos KRB_TGS_REQ 3.3 & 5.4.1
2. Kerberos to client KRB_TGS_REP or 3.3 & 5.4.2
KRB_ERROR 5.9.1
3. Client to application server KRB_AP_REQ 3.2 & 5.5.1
To address this problem, the Kerberos protocol allows the client to
request that the ticket issued by the KDC be encrypted using a
session key from a TGT issued to the party that will verify the
authentication. This TGT must be obtained from the verifier by means
of an exchange external to the Kerberos protocol, usually as part of
the application protocol. This message is shown in the summary above
as message 0. Note that because the TGT is encrypted in the KDC's
secret key, it cannot be used for authentication without possession
of the corresponding secret key. Furthermore, because the verifier
does not reveal the corresponding secret key, providing a copy of the
verifier's TGT does not allow impersonation of the verifier.
Message 0 in the table above represents an application-specific
negotiation between the client and server, at the end of which both
have determined that they will use user-to-user authentication, and
the client has obtained the server's TGT.
Next, the client includes the server's TGT as an additional ticket in
its KRB_TGS_REQ request to the KDC (message 1 in the table above) and
specifies the ENC-TKT-IN-SKEY option in its request.
If validated according to the instructions in Section 3.3.3, the
application ticket returned to the client (message 2 in the table
above) will be encrypted using the session key from the additional
ticket and the client will note this when it uses or stores the
application ticket.
When contacting the server using a ticket obtained for user-to-user
authentication (message 3 in the table above), the client MUST
specify the USE-SESSION-KEY flag in the ap-options field. This tells
the application server to use the session key associated with its TGT
to decrypt the server ticket provided in the application request.
4. Encryption and Checksum Specifications
The Kerberos protocols described in this document are designed to
encrypt messages of arbitrary sizes, using stream or block encryption
ciphers. Encryption is used to prove the identities of the network
entities participating in message exchanges. The Key Distribution
Center for each realm is trusted by all principals registered in that
realm to store a secret key in confidence. Proof of knowledge of
this secret key is used to verify the authenticity of a principal.
The KDC uses the principal's secret key (in the AS exchange) or a
shared session key (in the TGS exchange) to encrypt responses to
ticket requests; the ability to obtain the secret key or session key
implies the knowledge of the appropriate keys and the identity of the
KDC. The ability of a principal to decrypt the KDC response and to
present a Ticket and a properly formed Authenticator (generated with
the session key from the KDC response) to a service verifies the
identity of the principal; likewise the ability of the service to
extract the session key from the Ticket and to prove its knowledge
thereof in a response verifies the identity of the service.
[RFC3961] defines a framework for defining encryption and checksum
mechanisms for use with Kerberos. It also defines several such
mechanisms, and more may be added in future updates to that document.
The string-to-key operation provided by [RFC3961] is used to produce
a long-term key for a principal (generally for a user). The default
salt string, if none is provided via pre-authentication data, is the
concatenation of the principal's realm and name components, in order,
with no separators. Unless it is indicated otherwise, the default
string-to-key opaque parameter set as defined in [RFC3961] is used.
Encrypted data, keys, and checksums are transmitted using the
EncryptedData, EncryptionKey, and Checksum data objects defined in
Section 5.2.9. The encryption, decryption, and checksum operations
described in this document use the corresponding encryption,
decryption, and get_mic operations described in [RFC3961], with
implicit "specific key" generation using the "key usage" values
specified in the description of each EncryptedData or Checksum object
to vary the key for each operation. Note that in some cases, the
value to be used is dependent on the method of choosing the key or
the context of the message.
Key usages are unsigned 32-bit integers; zero is not permitted. The
key usage values for encrypting or checksumming Kerberos messages are
indicated in Section 5 along with the message definitions. The key
usage values 512-1023 are reserved for uses internal to a Kerberos
implementation. (For example, seeding a pseudo-random number
generator with a value produced by encrypting something with a
session key and a key usage value not used for any other purpose.)
Key usage values between 1024 and 2047 (inclusive) are reserved for
application use; applications SHOULD use even values for encryption
and odd values for checksums within this range. Key usage values are
also summarized in a table in Section 7.5.1.
There might exist other documents that define protocols in terms of
the RFC 1510 encryption types or checksum types. These documents
would not know about key usages. In order that these specifications
continue to be meaningful until they are updated, if no key usage
values are specified, then key usages 1024 and 1025 must be used to
derive keys for encryption and checksums, respectively. (This does
not apply to protocols that do their own encryption independent of
this framework, by directly using the key resulting from the Kerberos
authentication exchange.) New protocols defined in terms of the
Kerberos encryption and checksum types SHOULD use their own key usage
values.
Unless it is indicated otherwise, no cipher state chaining is done
from one encryption operation to another.
Implementation note: Although it is not recommended, some application
protocols will continue to use the key data directly, even if only in
currently existing protocol specifications. An implementation
intended to support general Kerberos applications may therefore need
to make key data available, as well as the attributes and operations
described in [RFC3961]. One of the more common reasons for directly
performing encryption is direct control over negotiation and
selection of a "sufficiently strong" encryption algorithm (in the
context of a given application). Although Kerberos does not directly
provide a facility for negotiating encryption types between the
application client and server, there are approaches for using
Kerberos to facilitate this negotiation. For example, a client may
request only "sufficiently strong" session key types from the KDC and
expect that any type returned by the KDC will be understood and
supported by the application server.
5. Message Specifications
The ASN.1 collected here should be identical to the contents of
Appendix A. In the case of a conflict, the contents of Appendix A
shall take precedence.
The Kerberos protocol is defined here in terms of Abstract Syntax
Notation One (ASN.1) [X680], which provides a syntax for specifying
both the abstract layout of protocol messages as well as their
encodings. Implementors not utilizing an existing ASN.1 compiler or
support library are cautioned to understand the actual ASN.1
specification thoroughly in order to ensure correct implementation
behavior. There is more complexity in the notation than is
immediately obvious, and some tutorials and guides to ASN.1 are
misleading or erroneous.
Note that in several places, changes to abstract types from RFC 1510
have been made. This is in part to address widespread assumptions
that various implementors have made, in some cases resulting in
unintentional violations of the ASN.1 standard. These are clearly
flagged where they occur. The differences between the abstract types
in RFC 1510 and abstract types in this document can cause
incompatible encodings to be emitted when certain encoding rules,
e.g., the Packed Encoding Rules (PER), are used. This theoretical
incompatibility should not be relevant for Kerberos, since Kerberos
explicitly specifies the use of the Distinguished Encoding Rules
(DER). It might be an issue for protocols seeking to use Kerberos
types with other encoding rules. (This practice is not recommended.)
With very few exceptions (most notably the usages of BIT STRING), the
encodings resulting from using the DER remain identical between the
types defined in RFC 1510 and the types defined in this document.
The type definitions in this section assume an ASN.1 module
definition of the following form:
KerberosV5Spec2 {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) krb5spec2(2)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
-- rest of definitions here
END
This specifies that the tagging context for the module will be
explicit and non-automatic.
Note that in some other publications (such as [RFC1510] and
[RFC1964]), the "dod" portion of the object identifier is erroneously
specified as having the value "5". In the case of RFC 1964, use of
the "correct" OID value would result in a change in the wire
protocol; therefore, it remains unchanged for now.
Note that elsewhere in this document, nomenclature for various
message types is inconsistent, but it largely follows C language
conventions, including use of underscore (_) characters and all-caps
spelling of names intended to be numeric constants. Also, in some
places, identifiers (especially those referring to constants) are
written in all-caps in order to distinguish them from surrounding
explanatory text.
The ASN.1 notation does not permit underscores in identifiers, so in
actual ASN.1 definitions, underscores are replaced with hyphens (-).
Additionally, structure member names and defined values in ASN.1 MUST
begin with a lowercase letter, whereas type names MUST begin with an
uppercase letter.
5.1. Specific Compatibility Notes on ASN.1
For compatibility purposes, implementors should heed the following
specific notes regarding the use of ASN.1 in Kerberos. These notes
do not describe deviations from standard usage of ASN.1. The purpose
of these notes is instead to describe some historical quirks and
non-compliance of various implementations, as well as historical
ambiguities, which, although they are valid ASN.1, can lead to
confusion during implementation.
5.1.1. ASN.1 Distinguished Encoding Rules
The encoding of Kerberos protocol messages shall obey the
Distinguished Encoding Rules (DER) of ASN.1 as described in [X690].
Some implementations (believed primarily to be those derived from DCE
1.1 and earlier) are known to use the more general Basic Encoding
Rules (BER); in particular, these implementations send indefinite
encodings of lengths. Implementations MAY accept such encodings in
the interest of backward compatibility, though implementors are
warned that decoding fully-general BER is fraught with peril.
5.1.2. Optional Integer Fields
Some implementations do not internally distinguish between an omitted
optional integer value and a transmitted value of zero. The places
in the protocol where this is relevant include various microseconds
fields, nonces, and sequence numbers. Implementations SHOULD treat
omitted optional integer values as having been transmitted with a
value of zero, if the application is expecting this.
5.1.3. Empty SEQUENCE OF Types
There are places in the protocol where a message contains a SEQUENCE
OF type as an optional member. This can result in an encoding that
contains an empty SEQUENCE OF encoding. The Kerberos protocol does
not semantically distinguish between an absent optional SEQUENCE OF
type and a present optional but empty SEQUENCE OF type.
Implementations SHOULD NOT send empty SEQUENCE OF encodings that are
marked OPTIONAL, but SHOULD accept them as being equivalent to an
omitted OPTIONAL type. In the ASN.1 syntax describing Kerberos
messages, instances of these problematic optional SEQUENCE OF types
are indicated with a comment.
5.1.4. Unrecognized Tag Numbers
Future revisions to this protocol may include new message types with
different APPLICATION class tag numbers. Such revisions should
protect older implementations by only sending the message types to
parties that are known to understand them; e.g., by means of a flag
bit set by the receiver in a preceding request. In the interest of
robust error handling, implementations SHOULD gracefully handle
receiving a message with an unrecognized tag anyway, and return an
error message, if appropriate.
In particular, KDCs SHOULD return KRB_AP_ERR_MSG_TYPE if the
incorrect tag is sent over a TCP transport. The KDCs SHOULD NOT
respond to messages received with an unknown tag over UDP transport
in order to avoid denial of service attacks. For non-KDC
applications, the Kerberos implementation typically indicates an
error to the application which takes appropriate steps based on the
application protocol.
5.1.5. Tag Numbers Greater Than 30
A naive implementation of a DER ASN.1 decoder may experience problems
with ASN.1 tag numbers greater than 30, due to such tag numbers being
encoded using more than one byte. Future revisions of this protocol
may utilize tag numbers greater than 30, and implementations SHOULD
be prepared to gracefully return an error, if appropriate, when they
do not recognize the tag.
5.2. Basic Kerberos Types
This section defines a number of basic types that are potentially
used in multiple Kerberos protocol messages.
5.2.1. KerberosString
The original specification of the Kerberos protocol in RFC 1510 uses
GeneralString in numerous places for human-readable string data.
Historical implementations of Kerberos cannot utilize the full power
of GeneralString. This ASN.1 type requires the use of designation
and invocation escape sequences as specified in ISO-2022/ECMA-35
[ISO-2022/ECMA-35] to switch character sets, and the default
character set that is designated as G0 is the ISO-646/ECMA-6
[ISO-646/ECMA-6] International Reference Version (IRV) (a.k.a. U.S.
ASCII), which mostly works.
ISO-2022/ECMA-35 defines four character-set code elements (G0..G3)
and two Control-function code elements (C0..C1). DER prohibits the
designation of character sets as any but the G0 and C0 sets.
Unfortunately, this seems to have the side effect of prohibiting the
use of ISO-8859 (ISO Latin) [ISO-8859] character sets or any other
character sets that utilize a 96-character set, as ISO-2022/ECMA-35
prohibits designating them as the G0 code element. This side effect
is being investigated in the ASN.1 standards community.
In practice, many implementations treat GeneralStrings as if they
were 8-bit strings of whichever character set the implementation
defaults to, without regard to correct usage of character-set
designation escape sequences. The default character set is often
determined by the current user's operating system-dependent locale.
At least one major implementation places unescaped UTF-8 encoded
Unicode characters in the GeneralString. This failure to adhere to
the GeneralString specifications results in interoperability issues
when conflicting character encodings are utilized by the Kerberos
clients, services, and KDC.
This unfortunate situation is the result of improper documentation of
the restrictions of the ASN.1 GeneralString type in prior Kerberos
specifications.
The new (post-RFC 1510) type KerberosString, defined below, is a
GeneralString that is constrained to contain only characters in
IA5String.
KerberosString ::= GeneralString (IA5String)
In general, US-ASCII control characters should not be used in
KerberosString. Control characters SHOULD NOT be used in principal
names or realm names.
For compatibility, implementations MAY choose to accept GeneralString
values that contain characters other than those permitted by
IA5String, but they should be aware that character set designation
codes will likely be absent, and that the encoding should probably be
treated as locale-specific in almost every way. Implementations MAY
also choose to emit GeneralString values that are beyond those
permitted by IA5String, but they should be aware that doing so is
extraordinarily risky from an interoperability perspective.
Some existing implementations use GeneralString to encode unescaped
locale-specific characters. This is a violation of the ASN.1
standard. Most of these implementations encode US-ASCII in the
left-hand half, so as long as the implementation transmits only
US-ASCII, the ASN.1 standard is not violated in this regard. As soon
as such an implementation encodes unescaped locale-specific
characters with the high bit set, it violates the ASN.1 standard.
Other implementations have been known to use GeneralString to contain
a UTF-8 encoding. This also violates the ASN.1 standard, since UTF-8
is a different encoding, not a 94 or 96 character "G" set as defined
by ISO 2022. It is believed that these implementations do not even
use the ISO 2022 escape sequence to change the character encoding.
Even if implementations were to announce the encoding change by using
that escape sequence, the ASN.1 standard prohibits the use of any
escape sequences other than those used to designate/invoke "G" or "C"
sets allowed by GeneralString.
Future revisions to this protocol will almost certainly allow for a
more interoperable representation of principal names, probably
including UTF8String.
Note that applying a new constraint to a previously unconstrained
type constitutes creation of a new ASN.1 type. In this particular
case, the change does not result in a changed encoding under DER.
5.2.2. Realm and PrincipalName
Realm ::= KerberosString
PrincipalName ::= SEQUENCE {
name-type [0] Int32,
name-string [1] SEQUENCE OF KerberosString
}
Kerberos realm names are encoded as KerberosStrings. Realms shall
not contain a character with the code 0 (the US-ASCII NUL). Most
realms will usually consist of several components separated by
periods (.), in the style of Internet Domain Names, or separated by
slashes (/), in the style of X.500 names. Acceptable forms for realm
names are specified in Section 6.1. A PrincipalName is a typed
sequence of components consisting of the following subfields:
name-type
This field specifies the type of name that follows. Pre-defined
values for this field are specified in Section 6.2. The name-type
SHOULD be treated as a hint. Ignoring the name type, no two names
can be the same (i.e., at least one of the components, or the
realm, must be different).
name-string
This field encodes a sequence of components that form a name, each
component encoded as a KerberosString. Taken together, a
PrincipalName and a Realm form a principal identifier. Most
PrincipalNames will have only a few components (typically one or
two).
5.2.3. KerberosTime
KerberosTime ::= GeneralizedTime -- with no fractional seconds
The timestamps used in Kerberos are encoded as GeneralizedTimes. A
KerberosTime value shall not include any fractional portions of the
seconds. As required by the DER, it further shall not include any
separators, and it shall specify the UTC time zone (Z). Example: The
only valid format for UTC time 6 minutes, 27 seconds after 9 pm on 6
November 1985 is 19851106210627Z.
5.2.4. Constrained Integer Types
Some integer members of types SHOULD be constrained to values
representable in 32 bits, for compatibility with reasonable
implementation limits.
Int32 ::= INTEGER (-2147483648..2147483647)
-- signed values representable in 32 bits
UInt32 ::= INTEGER (0..4294967295)
-- unsigned 32 bit values
Microseconds ::= INTEGER (0..999999)
-- microseconds
Although this results in changes to the abstract types from the RFC
1510 version, the encoding in DER should be unaltered. Historical
implementations were typically limited to 32-bit integer values
anyway, and assigned numbers SHOULD fall in the space of integer
values representable in 32 bits in order to promote interoperability
anyway.
Several integer fields in messages are constrained to fixed values.
pvno
also TKT-VNO or AUTHENTICATOR-VNO, this recurring field is always
the constant integer 5. There is no easy way to make this field
into a useful protocol version number, so its value is fixed.
msg-type
this integer field is usually identical to the application tag
number of the containing message type.
5.2.5. HostAddress and HostAddresses
HostAddress ::= SEQUENCE {
addr-type [0] Int32,
address [1] OCTET STRING
}
-- NOTE: HostAddresses is always used as an OPTIONAL field and
-- should not be empty.
HostAddresses -- NOTE: subtly different from rfc1510,
-- but has a value mapping and encodes the same
::= SEQUENCE OF HostAddress
The host address encodings consist of two fields:
addr-type
This field specifies the type of address that follows. Pre-
defined values for this field are specified in Section 7.5.3.
address
This field encodes a single address of type addr-type.
5.2.6. AuthorizationData
-- NOTE: AuthorizationData is always used as an OPTIONAL field and
-- should not be empty.
AuthorizationData ::= SEQUENCE OF SEQUENCE {
ad-type [0] Int32,
ad-data [1] OCTET STRING
}
ad-data
This field contains authorization data to be interpreted according
to the value of the corresponding ad-type field.
ad-type
This field specifies the format for the ad-data subfield. All
negative values are reserved for local use. Non-negative values
are reserved for registered use.
Each sequence of type and data is referred to as an authorization
element. Elements MAY be application specific; however, there is a
common set of recursive elements that should be understood by all
implementations. These elements contain other elements embedded
within them, and the interpretation of the encapsulating element
determines which of the embedded elements must be interpreted, and
which may be ignored.
These common authorization data elements are recursively defined,
meaning that the ad-data for these types will itself contain a
sequence of authorization data whose interpretation is affected by
the encapsulating element. Depending on the meaning of the
encapsulating element, the encapsulated elements may be ignored,
might be interpreted as issued directly by the KDC, or might be
stored in a separate plaintext part of the ticket. The types of the
encapsulating elements are specified as part of the Kerberos
specification because the behavior based on these values should be
understood across implementations, whereas other elements need only
be understood by the applications that they affect.
Authorization data elements are considered critical if present in a
ticket or authenticator. If an unknown authorization data element
type is received by a server either in an AP-REQ or in a ticket
contained in an AP-REQ, then, unless it is encapsulated in a known
authorization data element amending the criticality of the elements
it contains, authentication MUST fail. Authorization data is
intended to restrict the use of a ticket. If the service cannot
determine whether the restriction applies to that service, then a
security weakness may result if the ticket can be used for that
service. Authorization elements that are optional can be enclosed in
an AD-IF-RELEVANT element.
In the definitions that follow, the value of the ad-type for the
element will be specified as the least significant part of the
subsection number, and the value of the ad-data will be as shown in
the ASN.1 structure that follows the subsection heading.
Contents of ad-data ad-type
DER encoding of AD-IF-RELEVANT 1
DER encoding of AD-KDCIssued 4
DER encoding of AD-AND-OR 5
DER encoding of AD-MANDATORY-FOR-KDC 8
5.2.6.1. IF-RELEVANT
AD-IF-RELEVANT ::= AuthorizationData
AD elements encapsulated within the if-relevant element are intended
for interpretation only by application servers that understand the
particular ad-type of the embedded element. Application servers that
do not understand the type of an element embedded within the
if-relevant element MAY ignore the uninterpretable element. This
element promotes interoperability across implementations that may
have local extensions for authorization. The ad-type for
AD-IF-RELEVANT is (1).
5.2.6.2. KDCIssued
AD-KDCIssued ::= SEQUENCE {
ad-checksum [0] Checksum,
i-realm [1] Realm OPTIONAL,
i-sname [2] PrincipalName OPTIONAL,
elements [3] AuthorizationData
}
ad-checksum
A cryptographic checksum computed over the DER encoding of the
AuthorizationData in the "elements" field, keyed with the session
key. Its checksumtype is the mandatory checksum type for the
encryption type of the session key, and its key usage value is 19.
i-realm, i-sname
The name of the issuing principal if different from that of the
KDC itself. This field would be used when the KDC can verify the
authenticity of elements signed by the issuing principal, and it
allows this KDC to notify the application server of the validity
of those elements.
elements
A sequence of authorization data elements issued by the KDC.
The KDC-issued ad-data field is intended to provide a means for
Kerberos principal credentials to embed within themselves privilege
attributes and other mechanisms for positive authorization,
amplifying the privileges of the principal beyond what can be done
using credentials without such an a-data element.
The above means cannot be provided without this element because the
definition of the authorization-data field allows elements to be
added at will by the bearer of a TGT at the time when they request
service tickets, and elements may also be added to a delegated ticket
by inclusion in the authenticator.
For KDC-issued elements, this is prevented because the elements are
signed by the KDC by including a checksum encrypted using the
server's key (the same key used to encrypt the ticket or a key
derived from that key). Elements encapsulated with in the KDC-issued
element MUST be ignored by the application server if this "signature"
is not present. Further, elements encapsulated within this element
from a TGT MAY be interpreted by the KDC, and used as a basis
according to policy for including new signed elements within
derivative tickets, but they will not be copied to a derivative
ticket directly. If they are copied directly to a derivative ticket
by a KDC that is not aware of this element, the signature will not be
correct for the application ticket elements, and the field will be
ignored by the application server.
This element and the elements it encapsulates MAY safely be ignored
by applications, application servers, and KDCs that do not implement
this element.
The ad-type for AD-KDC-ISSUED is (4).
5.2.6.3. AND-OR
AD-AND-OR ::= SEQUENCE {
condition-count [0] Int32,
elements [1] AuthorizationData
}
When restrictive AD elements are encapsulated within the and-or
element, the and-or element is considered satisfied if and only if at
least the number of encapsulated elements specified in condition-
count are satisfied. Therefore, this element MAY be used to
implement an "or" operation by setting the condition-count field to
1, and it MAY specify an "and" operation by setting the condition
count to the number of embedded elements. Application servers that
do not implement this element MUST reject tickets that contain
authorization data elements of this type.
The ad-type for AD-AND-OR is (5).
5.2.6.4. MANDATORY-FOR-KDC
AD-MANDATORY-FOR-KDC ::= AuthorizationData
AD elements encapsulated within the mandatory-for-kdc element are to
be interpreted by the KDC. KDCs that do not understand the type of
an element embedded within the mandatory-for-kdc element MUST reject
the request.
The ad-type for AD-MANDATORY-FOR-KDC is (8).
5.2.7. PA-DATA
Historically, PA-DATA have been known as "pre-authentication data",
meaning that they were used to augment the initial authentication
with the KDC. Since that time, they have also been used as a typed
hole with which to extend protocol exchanges with the KDC.
PA-DATA ::= SEQUENCE {
-- NOTE: first tag is [1], not [0]
padata-type [1] Int32,
padata-value [2] OCTET STRING -- might be encoded AP-REQ
}
padata-type
Indicates the way that the padata-value element is to be
interpreted. Negative values of padata-type are reserved for
unregistered use; non-negative values are used for a registered
interpretation of the element type.
padata-value
Usually contains the DER encoding of another type; the padata-type
field identifies which type is encoded here.
padata-type Name Contents of padata-value
1 pa-tgs-req DER encoding of AP-REQ
2 pa-enc-timestamp DER encoding of PA-ENC-TIMESTAMP
3 pa-pw-salt salt (not ASN.1 encoded)
11 pa-etype-info DER encoding of ETYPE-INFO
19 pa-etype-info2 DER encoding of ETYPE-INFO2
This field MAY also contain information needed by certain
extensions to the Kerberos protocol. For example, it might be
used to verify the identity of a client initially before any
response is returned.
The padata field can also contain information needed to help the
KDC or the client select the key needed for generating or
decrypting the response. This form of the padata is useful for
supporting the use of certain token cards with Kerberos. The
details of such extensions are specified in separate documents.
See [Pat92] for additional uses of this field.
5.2.7.1. PA-TGS-REQ
In the case of requests for additional tickets (KRB_TGS_REQ),
padata-value will contain an encoded AP-REQ. The checksum in the
authenticator (which MUST be collision-proof) is to be computed over
the KDC-REQ-BODY encoding.
5.2.7.2. Encrypted Timestamp Pre-authentication
There are pre-authentication types that may be used to pre-
authenticate a client by means of an encrypted timestamp.
PA-ENC-TIMESTAMP ::= EncryptedData -- PA-ENC-TS-ENC
PA-ENC-TS-ENC ::= SEQUENCE {
patimestamp [0] KerberosTime -- client's time --,
pausec [1] Microseconds OPTIONAL
}
Patimestamp contains the client's time, and pausec contains the
microseconds, which MAY be omitted if a client will not generate more
than one request per second. The ciphertext (padata-value) consists
of the PA-ENC-TS-ENC encoding, encrypted using the client's secret
key and a key usage value of 1.
This pre-authentication type was not present in RFC 1510, but many
implementations support it.
5.2.7.3. PA-PW-SALT
The padata-value for this pre-authentication type contains the salt
for the string-to-key to be used by the client to obtain the key for
decrypting the encrypted part of an AS-REP message. Unfortunately,
for historical reasons, the character set to be used is unspecified
and probably locale-specific.
This pre-authentication type was not present in RFC 1510, but many
implementations support it. It is necessary in any case where the
salt for the string-to-key algorithm is not the default.
In the trivial example, a zero-length salt string is very commonplace
for realms that have converted their principal databases from
Kerberos Version 4.
A KDC SHOULD NOT send PA-PW-SALT when issuing a KRB-ERROR message
that requests additional pre-authentication. Implementation note:
Some KDC implementations issue an erroneous PA-PW-SALT when issuing a
KRB-ERROR message that requests additional pre-authentication.
Therefore, clients SHOULD ignore a PA-PW-SALT accompanying a
KRB-ERROR message that requests additional pre-authentication. As
noted in section 3.1.3, a KDC MUST NOT send PA-PW-SALT when the
client's AS-REQ includes at least one "newer" etype.
5.2.7.4. PA-ETYPE-INFO
The ETYPE-INFO pre-authentication type is sent by the KDC in a
KRB-ERROR indicating a requirement for additional pre-authentication.
It is usually used to notify a client of which key to use for the
encryption of an encrypted timestamp for the purposes of sending a
PA-ENC-TIMESTAMP pre-authentication value. It MAY also be sent in an
AS-REP to provide information to the client about which key salt to
use for the string-to-key to be used by the client to obtain the key
for decrypting the encrypted part the AS-REP.
ETYPE-INFO-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] OCTET STRING OPTIONAL
}
ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY
The salt, like that of PA-PW-SALT, is also completely unspecified
with respect to character set and is probably locale-specific.
If ETYPE-INFO is sent in an AS-REP, there shall be exactly one
ETYPE-INFO-ENTRY, and its etype shall match that of the enc-part in
the AS-REP.
This pre-authentication type was not present in RFC 1510, but many
implementations that support encrypted timestamps for pre-
authentication need to support ETYPE-INFO as well. As noted in
Section 3.1.3, a KDC MUST NOT send PA-ETYPE-INFO when the client's
AS-REQ includes at least one "newer" etype.
5.2.7.5. PA-ETYPE-INFO2
The ETYPE-INFO2 pre-authentication type is sent by the KDC in a
KRB-ERROR indicating a requirement for additional pre-authentication.
It is usually used to notify a client of which key to use for the
encryption of an encrypted timestamp for the purposes of sending a
PA-ENC-TIMESTAMP pre-authentication value. It MAY also be sent in an
AS-REP to provide information to the client about which key salt to
use for the string-to-key to be used by the client to obtain the key
for decrypting the encrypted part the AS-REP.
ETYPE-INFO2-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] KerberosString OPTIONAL,
s2kparams [2] OCTET STRING OPTIONAL
}
ETYPE-INFO2 ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY
The type of the salt is KerberosString, but existing installations
might have locale-specific characters stored in salt strings, and
implementors MAY choose to handle them.
The interpretation of s2kparams is specified in the cryptosystem
description associated with the etype. Each cryptosystem has a
default interpretation of s2kparams that will hold if that element is
omitted from the encoding of ETYPE-INFO2-ENTRY.
If ETYPE-INFO2 is sent in an AS-REP, there shall be exactly one
ETYPE-INFO2-ENTRY, and its etype shall match that of the enc-part in
the AS-REP.
The preferred ordering of the "hint" pre-authentication data that
affect client key selection is: ETYPE-INFO2, followed by ETYPE-INFO,
followed by PW-SALT. As noted in Section 3.1.3, a KDC MUST NOT send
ETYPE-INFO or PW-SALT when the client's AS-REQ includes at least one
"newer" etype.
The ETYPE-INFO2 pre-authentication type was not present in RFC 1510.
5.2.8. KerberosFlags
For several message types, a specific constrained bit string type,
KerberosFlags, is used.
KerberosFlags ::= BIT STRING (SIZE (32..MAX))
-- minimum number of bits shall be sent,
-- but no fewer than 32
Compatibility note: The following paragraphs describe a change from
the RFC 1510 description of bit strings that would result in
incompatility in the case of an implementation that strictly
conformed to ASN.1 DER and RFC 1510.
ASN.1 bit strings have multiple uses. The simplest use of a bit
string is to contain a vector of bits, with no particular meaning
attached to individual bits. This vector of bits is not necessarily
a multiple of eight bits long. The use in Kerberos of a bit string
as a compact boolean vector wherein each element has a distinct
meaning poses some problems. The natural notation for a compact
boolean vector is the ASN.1 "NamedBit" notation, and the DER require
that encodings of a bit string using "NamedBit" notation exclude any
trailing zero bits. This truncation is easy to neglect, especially
given C language implementations that naturally choose to store
boolean vectors as 32-bit integers.
For example, if the notation for KDCOptions were to include the
"NamedBit" notation, as in RFC 1510, and a KDCOptions value to be
encoded had only the "forwardable" (bit number one) bit set, the DER
encoding MUST include only two bits: the first reserved bit
("reserved", bit number zero, value zero) and the one-valued bit (bit
number one) for "forwardable".
Most existing implementations of Kerberos unconditionally send 32
bits on the wire when encoding bit strings used as boolean vectors.
This behavior violates the ASN.1 syntax used for flag values in RFC
1510, but it occurs on such a widely installed base that the protocol
description is being modified to accommodate it.
Consequently, this document removes the "NamedBit" notations for
individual bits, relegating them to comments. The size constraint on
the KerberosFlags type requires that at least 32 bits be encoded at
all times, though a lenient implementation MAY choose to accept fewer
than 32 bits and to treat the missing bits as set to zero.
Currently, no uses of KerberosFlags specify more than 32 bits' worth
of flags, although future revisions of this document may do so. When
more than 32 bits are to be transmitted in a KerberosFlags value,
future revisions to this document will likely specify that the
smallest number of bits needed to encode the highest-numbered one-
valued bit should be sent. This is somewhat similar to the DER
encoding of a bit string that is declared with the "NamedBit"
notation.
5.2.9. Cryptosystem-Related Types
Many Kerberos protocol messages contain an EncryptedData as a
container for arbitrary encrypted data, which is often the encrypted
encoding of another data type. Fields within EncryptedData assist
the recipient in selecting a key with which to decrypt the enclosed
data.
EncryptedData ::= SEQUENCE {
etype [0] Int32 -- EncryptionType --,
kvno [1] UInt32 OPTIONAL,
cipher [2] OCTET STRING -- ciphertext
}
etype
This field identifies which encryption algorithm was used to
encipher the cipher.
kvno
This field contains the version number of the key under which data
is encrypted. It is only present in messages encrypted under long
lasting keys, such as principals' secret keys.
cipher
This field contains the enciphered text, encoded as an OCTET
STRING. (Note that the encryption mechanisms defined in [RFC3961]
MUST incorporate integrity protection as well, so no additional
checksum is required.)
The EncryptionKey type is the means by which cryptographic keys used
for encryption are transferred.
EncryptionKey ::= SEQUENCE {
keytype [0] Int32 -- actually encryption type --,
keyvalue [1] OCTET STRING
}
keytype
This field specifies the encryption type of the encryption key
that follows in the keyvalue field. Although its name is
"keytype", it actually specifies an encryption type. Previously,
multiple cryptosystems that performed encryption differently but
were capable of using keys with the same characteristics were
permitted to share an assigned number to designate the type of
key; this usage is now deprecated.
keyvalue
This field contains the key itself, encoded as an octet string.
Messages containing cleartext data to be authenticated will usually
do so by using a member of type Checksum. Most instances of Checksum
use a keyed hash, though exceptions will be noted.
Checksum ::= SEQUENCE {
cksumtype [0] Int32,
checksum [1] OCTET STRING
}
cksumtype
This field indicates the algorithm used to generate the
accompanying checksum.
checksum
This field contains the checksum itself, encoded as an octet
string.
See Section 4 for a brief description of the use of encryption and
checksums in Kerberos.
5.3. Tickets
This section describes the format and encryption parameters for
tickets and authenticators. When a ticket or authenticator is
included in a protocol message, it is treated as an opaque object. A
ticket is a record that helps a client authenticate to a service. A
Ticket contains the following information:
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno [0] INTEGER (5),
realm [1] Realm,
sname [2] PrincipalName,
enc-part [3] EncryptedData -- EncTicketPart
}
-- Encrypted part of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags [0] TicketFlags,
key [1] EncryptionKey,
crealm [2] Realm,
cname [3] PrincipalName,
transited [4] TransitedEncoding,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
caddr [9] HostAddresses OPTIONAL,
authorization-data [10] AuthorizationData OPTIONAL
}
-- encoded Transited field
TransitedEncoding ::= SEQUENCE {
tr-type [0] Int32 -- must be registered --,
contents [1] OCTET STRING
}
TicketFlags ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- may-postdate(5),
-- postdated(6),
-- invalid(7),
-- renewable(8),
-- initial(9),
-- pre-authent(10),
-- hw-authent(11),
-- the following are new since 1510
-- transited-policy-checked(12),
-- ok-as-delegate(13)
tkt-vno
This field specifies the version number for the ticket format.
This document describes version number 5.
realm
This field specifies the realm that issued a ticket. It also
serves to identify the realm part of the server's principal
identifier. Since a Kerberos server can only issue tickets for
servers within its realm, the two will always be identical.
sname
This field specifies all components of the name part of the
server's identity, including those parts that identify a specific
instance of a service.
enc-part
This field holds the encrypted encoding of the EncTicketPart
sequence. It is encrypted in the key shared by Kerberos and the
end server (the server's secret key), using a key usage value of
2.
flags
This field indicates which of various options were used or
requested when the ticket was issued. The meanings of the flags
are as follows:
Bit(s) Name Description
0 reserved Reserved for future expansion of this field.
1 forwardable The FORWARDABLE flag is normally only
interpreted by the TGS, and can be ignored
by end servers. When set, this flag tells
the ticket-granting server that it is OK to
issue a new TGT with a different network
address based on the presented ticket.
2 forwarded When set, this flag indicates that the
ticket has either been forwarded or was
issued based on authentication involving a
forwarded TGT.
3 proxiable The PROXIABLE flag is normally only
interpreted by the TGS, and can be ignored
by end servers. The PROXIABLE flag has an
interpretation identical to that of the
FORWARDABLE flag, except that the PROXIABLE
flag tells the ticket-granting server that
only non-TGTs may be issued with different
network addresses.
4 proxy When set, this flag indicates that a ticket
is a proxy.
5 may-postdate The MAY-POSTDATE flag is normally only
interpreted by the TGS, and can be ignored
by end servers. This flag tells the
ticket-granting server that a post-dated
ticket MAY be issued based on this TGT.
6 postdated This flag indicates that this ticket has
been postdated. The end-service can check
the authtime field to see when the original
authentication occurred.
7 invalid This flag indicates that a ticket is
invalid, and it must be validated by the KDC
before use. Application servers must reject
tickets which have this flag set.
8 renewable The RENEWABLE flag is normally only
interpreted by the TGS, and can usually be
ignored by end servers (some particularly
careful servers MAY disallow renewable
tickets). A renewable ticket can be used to
obtain a replacement ticket that expires at
a later date.
9 initial This flag indicates that this ticket was
issued using the AS protocol, and not issued
based on a TGT.
10 pre-authent This flag indicates that during initial
authentication, the client was authenticated
by the KDC before a ticket was issued. The
strength of the pre-authentication method is
not indicated, but is acceptable to the KDC.
11 hw-authent This flag indicates that the protocol
employed for initial authentication required
the use of hardware expected to be possessed
solely by the named client. The hardware
authentication method is selected by the KDC
and the strength of the method is not
indicated.
12 transited- This flag indicates that the KDC for
policy-checked the realm has checked the transited field
against a realm-defined policy for trusted
certifiers. If this flag is reset (0), then
the application server must check the
transited field itself, and if unable to do
so, it must reject the authentication. If
the flag is set (1), then the application
server MAY skip its own validation of the
transited field, relying on the validation
performed by the KDC. At its option the
application server MAY still apply its own
validation based on a separate policy for
acceptance.
This flag is new since RFC 1510.
13 ok-as-delegate This flag indicates that the server (not the
client) specified in the ticket has been
determined by policy of the realm to be a
suitable recipient of delegation. A client
can use the presence of this flag to help it
decide whether to delegate credentials
(either grant a proxy or a forwarded TGT) to
this server. The client is free to ignore
the value of this flag. When setting this
flag, an administrator should consider the
security and placement of the server on
which the service will run, as well as
whether the service requires the use of
delegated credentials.
This flag is new since RFC 1510.
14-31 reserved Reserved for future use.
key
This field exists in the ticket and the KDC response and is used
to pass the session key from Kerberos to the application server
and the client.
crealm
This field contains the name of the realm in which the client is
registered and in which initial authentication took place.
cname
This field contains the name part of the client's principal
identifier.
transited
This field lists the names of the Kerberos realms that took part
in authenticating the user to whom this ticket was issued. It
does not specify the order in which the realms were transited.
See Section 3.3.3.2 for details on how this field encodes the
traversed realms. When the names of CAs are to be embedded in the
transited field (as specified for some extensions to the
protocol), the X.500 names of the CAs SHOULD be mapped into items
in the transited field using the mapping defined by RFC 2253.
authtime
This field indicates the time of initial authentication for the
named principal. It is the time of issue for the original ticket
on which this ticket is based. It is included in the ticket to
provide additional information to the end service, and to provide
the necessary information for implementation of a "hot list"
service at the KDC. An end service that is particularly paranoid
could refuse to accept tickets for which the initial
authentication occurred "too far" in the past. This field is also
returned as part of the response from the KDC. When it is
returned as part of the response to initial authentication
(KRB_AS_REP), this is the current time on the Kerberos server. It
is NOT recommended that this time value be used to adjust the
workstation's clock, as the workstation cannot reliably determine
that such a KRB_AS_REP actually came from the proper KDC in a
timely manner.
starttime
This field in the ticket specifies the time after which the ticket
is valid. Together with endtime, this field specifies the life of
the ticket. If the starttime field is absent from the ticket,
then the authtime field SHOULD be used in its place to determine
the life of the ticket.
endtime
This field contains the time after which the ticket will not be
honored (its expiration time). Note that individual services MAY
place their own limits on the life of a ticket and MAY reject
tickets which have not yet expired. As such, this is really an
upper bound on the expiration time for the ticket.
renew-till
This field is only present in tickets that have the RENEWABLE flag
set in the flags field. It indicates the maximum endtime that may
be included in a renewal. It can be thought of as the absolute
expiration time for the ticket, including all renewals.
caddr
This field in a ticket contains zero (if omitted) or more (if
present) host addresses. These are the addresses from which the
ticket can be used. If there are no addresses, the ticket can be
used from any location. The decision by the KDC to issue or by
the end server to accept addressless tickets is a policy decision
and is left to the Kerberos and end-service administrators; they
MAY refuse to issue or accept such tickets. Because of the wide
deployment of network address translation, it is recommended that
policy allow the issue and acceptance of such tickets.
Network addresses are included in the ticket to make it harder for
an attacker to use stolen credentials. Because the session key is
not sent over the network in cleartext, credentials can't be
stolen simply by listening to the network; an attacker has to gain
access to the session key (perhaps through operating system
security breaches or a careless user's unattended session) to make
use of stolen tickets.
Note that the network address from which a connection is received
cannot be reliably determined. Even if it could be, an attacker
who has compromised the client's workstation could use the
credentials from there. Including the network addresses only
makes it more difficult, not impossible, for an attacker to walk
off with stolen credentials and then to use them from a "safe"
location.
authorization-data
The authorization-data field is used to pass authorization data
from the principal on whose behalf a ticket was issued to the
application service. If no authorization data is included, this
field will be left out. Experience has shown that the name of
this field is confusing, and that a better name would be
"restrictions". Unfortunately, it is not possible to change the
name at this time.
This field contains restrictions on any authority obtained on the
basis of authentication using the ticket. It is possible for any
principal in possession of credentials to add entries to the
authorization data field since these entries further restrict what
can be done with the ticket. Such additions can be made by
specifying the additional entries when a new ticket is obtained
during the TGS exchange, or they MAY be added during chained
delegation using the authorization data field of the
authenticator.
Because entries may be added to this field by the holder of
credentials, except when an entry is separately authenticated by
encapsulation in the KDC-issued element, it is not allowable for
the presence of an entry in the authorization data field of a
ticket to amplify the privileges one would obtain from using a
ticket.
The data in this field may be specific to the end service; the
field will contain the names of service specific objects, and the
rights to those objects. The format for this field is described
in Section 5.2.6. Although Kerberos is not concerned with the
format of the contents of the subfields, it does carry type
information (ad-type).
By using the authorization_data field, a principal is able to
issue a proxy that is valid for a specific purpose. For example,
a client wishing to print a file can obtain a file server proxy to
be passed to the print server. By specifying the name of the file
in the authorization_data field, the file server knows that the
print server can only use the client's rights when accessing the
particular file to be printed.
A separate service providing authorization or certifying group
membership may be built using the authorization-data field. In
this case, the entity granting authorization (not the authorized
entity) may obtain a ticket in its own name (e.g., the ticket is
issued in the name of a privilege server), and this entity adds
restrictions on its own authority and delegates the restricted
authority through a proxy to the client. The client would then
present this authorization credential to the application server
separately from the authentication exchange. Alternatively, such
authorization credentials MAY be embedded in the ticket
authenticating the authorized entity, when the authorization is
separately authenticated using the KDC-issued authorization data
element (see 5.2.6.2).
Similarly, if one specifies the authorization-data field of a
proxy and leaves the host addresses blank, the resulting ticket
and session key can be treated as a capability. See [Neu93] for
some suggested uses of this field.
The authorization-data field is optional and does not have to be
included in a ticket.
5.4. Specifications for the AS and TGS Exchanges
This section specifies the format of the messages used in the
exchange between the client and the Kerberos server. The format of
possible error messages appears in Section 5.9.1.
5.4.1. KRB_KDC_REQ Definition
The KRB_KDC_REQ message has no application tag number of its own.
Instead, it is incorporated into either KRB_AS_REQ or KRB_TGS_REQ,
each of which has an application tag, depending on whether the
request is for an initial ticket or an additional ticket. In either
case, the message is sent from the client to the KDC to request
credentials for a service.
The message fields are as follows:
AS-REQ ::= [APPLICATION 10] KDC-REQ
TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ ::= SEQUENCE {
-- NOTE: first tag is [1], not [0]
pvno [1] INTEGER (5) ,
msg-type [2] INTEGER (10 -- AS -- | 12 -- TGS --),
padata [3] SEQUENCE OF PA-DATA OPTIONAL
-- NOTE: not empty --,
req-body [4] KDC-REQ-BODY
}
KDC-REQ-BODY ::= SEQUENCE {
kdc-options [0] KDCOptions,
cname [1] PrincipalName OPTIONAL
-- Used only in AS-REQ --,
realm [2] Realm
-- Server's realm
-- Also client's in AS-REQ --,
sname [3] PrincipalName OPTIONAL,
from [4] KerberosTime OPTIONAL,
till [5] KerberosTime,
rtime [6] KerberosTime OPTIONAL,
nonce [7] UInt32,
etype [8] SEQUENCE OF Int32 -- EncryptionType
-- in preference order --,
addresses [9] HostAddresses OPTIONAL,
enc-authorization-data [10] EncryptedData OPTIONAL
-- AuthorizationData --,
additional-tickets [11] SEQUENCE OF Ticket OPTIONAL
-- NOTE: not empty
}
KDCOptions ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- allow-postdate(5),
-- postdated(6),
-- unused7(7),
-- renewable(8),
-- unused9(9),
-- unused10(10),
-- opt-hardware-auth(11),
-- unused12(12),
-- unused13(13),
-- 15 is reserved for canonicalize
-- unused15(15),
-- 26 was unused in 1510
-- disable-transited-check(26),
--
-- renewable-ok(27),
-- enc-tkt-in-skey(28),
-- renew(30),
-- validate(31)
The fields in this message are as follows:
pvno
This field is included in each message, and specifies the protocol
version number. This document specifies protocol version 5.
msg-type
This field indicates the type of a protocol message. It will
almost always be the same as the application identifier associated
with a message. It is included to make the identifier more
readily accessible to the application. For the KDC-REQ message,
this type will be KRB_AS_REQ or KRB_TGS_REQ.
padata
Contains pre-authentication data. Requests for additional tickets
(KRB_TGS_REQ) MUST contain a padata of PA-TGS-REQ.
The padata (pre-authentication data) field contains a sequence of
authentication information that may be needed before credentials
can be issued or decrypted.
req-body
This field is a placeholder delimiting the extent of the remaining
fields. If a checksum is to be calculated over the request, it is
calculated over an encoding of the KDC-REQ-BODY sequence which is
enclosed within the req-body field.
kdc-options
This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to
the KDC and indicates the flags that the client wants set on the
tickets as well as other information that is to modify the
behavior of the KDC. Where appropriate, the name of an option may
be the same as the flag that is set by that option. Although in
most cases, the bit in the options field will be the same as that
in the flags field, this is not guaranteed, so it is not
acceptable simply to copy the options field to the flags field.
There are various checks that must be made before an option is
honored anyway.
The kdc_options field is a bit-field, where the selected options
are indicated by the bit being set (1), and the unselected options
and reserved fields being reset (0). The encoding of the bits is
specified in Section 5.2. The options are described in more
detail above in Section 2. The meanings of the options are as
follows:
Bits Name Description
0 RESERVED Reserved for future expansion of
this field.
1 FORWARDABLE The FORWARDABLE option indicates
that the ticket to be issued is to
have its forwardable flag set. It
may only be set on the initial
request, or in a subsequent request
if the TGT on which it is based is
also forwardable.
2 FORWARDED The FORWARDED option is only
specified in a request to the
ticket-granting server and will only
be honored if the TGT in the request
has its FORWARDABLE bit set. This
option indicates that this is a
request for forwarding. The
address(es) of the host from which
the resulting ticket is to be valid
are included in the addresses field
of the request.
3 PROXIABLE The PROXIABLE option indicates that
the ticket to be issued is to have
its proxiable flag set. It may only
be set on the initial request, or a
subsequent request if the TGT on
which it is based is also proxiable.
4 PROXY The PROXY option indicates that this
is a request for a proxy. This
option will only be honored if the
TGT in the request has its PROXIABLE
bit set. The address(es) of the
host from which the resulting ticket
is to be valid are included in the
addresses field of the request.
5 ALLOW-POSTDATE The ALLOW-POSTDATE option indicates
that the ticket to be issued is to
have its MAY-POSTDATE flag set. It
may only be set on the initial
request, or in a subsequent request
if the TGT on which it is based also
has its MAY-POSTDATE flag set.
6 POSTDATED The POSTDATED option indicates that
this is a request for a postdated
ticket. This option will only be
honored if the TGT on which it is
based has its MAY-POSTDATE flag set.
The resulting ticket will also have
its INVALID flag set, and that flag
may be reset by a subsequent request
to the KDC after the starttime in
the ticket has been reached.
7 RESERVED This option is presently unused.
8 RENEWABLE The RENEWABLE option indicates that
the ticket to be issued is to have
its RENEWABLE flag set. It may only
be set on the initial request, or
when the TGT on which the request is
based is also renewable. If this
option is requested, then the rtime
field in the request contains the
desired absolute expiration time for
the ticket.
9 RESERVED Reserved for PK-Cross.
10 RESERVED Reserved for future use.
11 RESERVED Reserved for opt-hardware-auth.
12-25 RESERVED Reserved for future use.
26 DISABLE-TRANSITED-CHECK By default the KDC will check the
transited field of a TGT against the
policy of the local realm before it
will issue derivative tickets based
on the TGT. If this flag is set in
the request, checking of the
transited field is disabled.
Tickets issued without the
performance of this check will be
noted by the reset (0) value of the
TRANSITED-POLICY-CHECKED flag,
indicating to the application server
that the transited field must be
checked locally. KDCs are
encouraged but not required to honor
the DISABLE-TRANSITED-CHECK option.
This flag is new since RFC 1510.
27 RENEWABLE-OK The RENEWABLE-OK option indicates
that a renewable ticket will be
acceptable if a ticket with the
requested life cannot otherwise be
provided, in which case a renewable
ticket may be issued with a renew-
till equal to the requested endtime.
The value of the renew-till field
may still be limited by local
limits, or limits selected by the
individual principal or server.
28 ENC-TKT-IN-SKEY This option is used only by the
ticket-granting service. The ENC-
TKT-IN-SKEY option indicates that
the ticket for the end server is to
be encrypted in the session key from
the additional TGT provided.
29 RESERVED Reserved for future use.
30 RENEW This option is used only by the
ticket-granting service. The RENEW
option indicates that the present
request is for a renewal. The
ticket provided is encrypted in the
secret key for the server on which
it is valid. This option will only
be honored if the ticket to be
renewed has its RENEWABLE flag set
and if the time in its renew-till
field has not passed. The ticket to
be renewed is passed in the padata
field as part of the authentication
header.
31 VALIDATE This option is used only by the
ticket-granting service. The
VALIDATE option indicates that the
request is to validate a postdated
ticket. It will only be honored if
the ticket presented is postdated,
presently has its INVALID flag set,
and would otherwise be usable at
this time. A ticket cannot be
validated before its starttime. The
ticket presented for validation is
encrypted in the key of the server
for which it is valid and is passed
in the padata field as part of the
authentication header.
cname and sname
These fields are the same as those described for the ticket in
section 5.3. The sname may only be absent when the ENC-TKT-IN-
SKEY option is specified. If the sname is absent, the name of the
server is taken from the name of the client in the ticket passed
as additional-tickets.
enc-authorization-data
The enc-authorization-data, if present (and it can only be present
in the TGS_REQ form), is an encoding of the desired
authorization-data encrypted under the sub-session key if present
in the Authenticator, or alternatively from the session key in the
TGT (both the Authenticator and TGT come from the padata field in
the KRB_TGS_REQ). The key usage value used when encrypting is 5
if a sub-session key is used, or 4 if the session key is used.
realm
This field specifies the realm part of the server's principal
identifier. In the AS exchange, this is also the realm part of
the client's principal identifier.
from
This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket
requests when the requested ticket is to be postdated. It
specifies the desired starttime for the requested ticket. If this
field is omitted, then the KDC SHOULD use the current time
instead.
till
This field contains the expiration date requested by the client in
a ticket request. It is not optional, but if the requested
endtime is "19700101000000Z", the requested ticket is to have the
maximum endtime permitted according to KDC policy. Implementation
note: This special timestamp corresponds to a UNIX time_t value of
zero on most systems.
rtime
This field is the requested renew-till time sent from a client to
the KDC in a ticket request. It is optional.
nonce
This field is part of the KDC request and response. It is
intended to hold a random number generated by the client. If the
same number is included in the encrypted response from the KDC, it
provides evidence that the response is fresh and has not been
replayed by an attacker. Nonces MUST NEVER be reused.
etype
This field specifies the desired encryption algorithm to be used
in the response.
addresses
This field is included in the initial request for tickets, and it
is optionally included in requests for additional tickets from the
ticket-granting server. It specifies the addresses from which the
requested ticket is to be valid. Normally it includes the
addresses for the client's host. If a proxy is requested, this
field will contain other addresses. The contents of this field
are usually copied by the KDC into the caddr field of the
resulting ticket.
additional-tickets
Additional tickets MAY be optionally included in a request to the
ticket-granting server. If the ENC-TKT-IN-SKEY option has been
specified, then the session key from the additional ticket will be
used in place of the server's key to encrypt the new ticket. When
the ENC-TKT-IN-SKEY option is used for user-to-user
authentication, this additional ticket MAY be a TGT issued by the
local realm or an inter-realm TGT issued for the current KDC's
realm by a remote KDC. If more than one option that requires
additional tickets has been specified, then the additional tickets
are used in the order specified by the ordering of the options
bits (see kdc-options, above).
The application tag number will be either ten (10) or twelve (12)
depending on whether the request is for an initial ticket (AS-REQ) or
for an additional ticket (TGS-REQ).
The optional fields (addresses, authorization-data, and additional-
tickets) are only included if necessary to perform the operation
specified in the kdc-options field.
Note that in KRB_TGS_REQ, the protocol version number appears twice
and two different message types appear: the KRB_TGS_REQ message
contains these fields as does the authentication header (KRB_AP_REQ)
that is passed in the padata field.
5.4.2. KRB_KDC_REP Definition
The KRB_KDC_REP message format is used for the reply from the KDC for
either an initial (AS) request or a subsequent (TGS) request. There
is no message type for KRB_KDC_REP. Instead, the type will be either
KRB_AS_REP or KRB_TGS_REP. The key used to encrypt the ciphertext
part of the reply depends on the message type. For KRB_AS_REP, the
ciphertext is encrypted in the client's secret key, and the client's
key version number is included in the key version number for the
encrypted data. For KRB_TGS_REP, the ciphertext is encrypted in the
sub-session key from the Authenticator; if it is absent, the
ciphertext is encrypted in the session key from the TGT used in the
request. In that case, no version number will be present in the
EncryptedData sequence.
The KRB_KDC_REP message contains the following fields:
AS-REP ::= [APPLICATION 11] KDC-REP
TGS-REP ::= [APPLICATION 13] KDC-REP
KDC-REP ::= SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (11 -- AS -- | 13 -- TGS --),
padata [2] SEQUENCE OF PA-DATA OPTIONAL
-- NOTE: not empty --,
crealm [3] Realm,
cname [4] PrincipalName,
ticket [5] Ticket,
enc-part [6] EncryptedData
-- EncASRepPart or EncTGSRepPart,
-- as appropriate
}
EncASRepPart ::= [APPLICATION 25] EncKDCRepPart
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
EncKDCRepPart ::= SEQUENCE {
key [0] EncryptionKey,
last-req [1] LastReq,
nonce [2] UInt32,
key-expiration [3] KerberosTime OPTIONAL,
flags [4] TicketFlags,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
srealm [9] Realm,
sname [10] PrincipalName,
caddr [11] HostAddresses OPTIONAL
}
LastReq ::= SEQUENCE OF SEQUENCE {
lr-type [0] Int32,
lr-value [1] KerberosTime
}
pvno and msg-type
These fields are described above in Section 5.4.1. msg-type is
either KRB_AS_REP or KRB_TGS_REP.
padata
This field is described in detail in Section 5.4.1. One possible
use for it is to encode an alternate "salt" string to be used with
a string-to-key algorithm. This ability is useful for easing
transitions if a realm name needs to change (e.g., when a company
is acquired); in such a case all existing password-derived entries
in the KDC database would be flagged as needing a special salt
string until the next password change.
crealm, cname, srealm, and sname
These fields are the same as those described for the ticket in
section 5.3.
ticket
The newly-issued ticket, from Section 5.3.
enc-part
This field is a place holder for the ciphertext and related
information that forms the encrypted part of a message. The
description of the encrypted part of the message follows each
appearance of this field.
The key usage value for encrypting this field is 3 in an AS-REP
message, using the client's long-term key or another key selected
via pre-authentication mechanisms. In a TGS-REP message, the key
usage value is 8 if the TGS session key is used, or 9 if a TGS
authenticator subkey is used.
Compatibility note: Some implementations unconditionally send an
encrypted EncTGSRepPart (application tag number 26) in this field
regardless of whether the reply is a AS-REP or a TGS-REP. In the
interest of compatibility, implementors MAY relax the check on the
tag number of the decrypted ENC-PART.
key
This field is the same as described for the ticket in Section 5.3.
last-req
This field is returned by the KDC and specifies the time(s) of the
last request by a principal. Depending on what information is
available, this might be the last time that a request for a TGT
was made, or the last time that a request based on a TGT was
successful. It also might cover all servers for a realm, or just
the particular server. Some implementations MAY display this
information to the user to aid in discovering unauthorized use of
one's identity. It is similar in spirit to the last login time
displayed when logging in to timesharing systems.
lr-type
This field indicates how the following lr-value field is to be
interpreted. Negative values indicate that the information
pertains only to the responding server. Non-negative values
pertain to all servers for the realm.
If the lr-type field is zero (0), then no information is conveyed
by the lr-value subfield. If the absolute value of the lr-type
field is one (1), then the lr-value subfield is the time of last
initial request for a TGT. If it is two (2), then the lr-value
subfield is the time of last initial request. If it is three (3),
then the lr-value subfield is the time of issue for the newest TGT
used. If it is four (4), then the lr-value subfield is the time
of the last renewal. If it is five (5), then the lr-value
subfield is the time of last request (of any type). If it is (6),
then the lr-value subfield is the time when the password will
expire. If it is (7), then the lr-value subfield is the time when
the account will expire.
lr-value
This field contains the time of the last request. The time MUST
be interpreted according to the contents of the accompanying lr-
type subfield.
nonce
This field is described above in Section 5.4.1.
key-expiration
The key-expiration field is part of the response from the KDC and
specifies the time that the client's secret key is due to expire.
The expiration might be the result of password aging or an account
expiration. If present, it SHOULD be set to the earlier of the
user's key expiration and account expiration. The use of this
field is deprecated, and the last-req field SHOULD be used to
convey this information instead. This field will usually be left
out of the TGS reply since the response to the TGS request is
encrypted in a session key and no client information has to be
retrieved from the KDC database. It is up to the application
client (usually the login program) to take appropriate action
(such as notifying the user) if the expiration time is imminent.
flags, authtime, starttime, endtime, renew-till and caddr
These fields are duplicates of those found in the encrypted
portion of the attached ticket (see Section 5.3), provided so the
client MAY verify that they match the intended request and in
order to assist in proper ticket caching. If the message is of
type KRB_TGS_REP, the caddr field will only be filled in if the
request was for a proxy or forwarded ticket, or if the user is
substituting a subset of the addresses from the TGT. If the
client-requested addresses are not present or not used, then the
addresses contained in the ticket will be the same as those
included in the TGT.
5.5. Client/Server (CS) Message Specifications
This section specifies the format of the messages used for the
authentication of the client to the application server.
5.5.1. KRB_AP_REQ Definition
The KRB_AP_REQ message contains the Kerberos protocol version number,
the message type KRB_AP_REQ, an options field to indicate any options
in use, and the ticket and authenticator themselves. The KRB_AP_REQ
message is often referred to as the "authentication header".
AP-REQ ::= [APPLICATION 14] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (14),
ap-options [2] APOptions,
ticket [3] Ticket,
authenticator [4] EncryptedData -- Authenticator
}
APOptions ::= KerberosFlags
-- reserved(0),
-- use-session-key(1),
-- mutual-required(2)
pvno and msg-type
These fields are described above in Section 5.4.1. msg-type is
KRB_AP_REQ.
ap-options
This field appears in the application request (KRB_AP_REQ) and
affects the way the request is processed. It is a bit-field,
where the selected options are indicated by the bit being set (1),
and the unselected options and reserved fields by being reset (0).
The encoding of the bits is specified in Section 5.2. The
meanings of the options are as follows:
Bit(s) Name Description
0 reserved Reserved for future expansion of this field.
1 use-session-key The USE-SESSION-KEY option indicates that
the ticket the client is presenting to a
server is encrypted in the session key from
the server's TGT. When this option is not
specified, the ticket is encrypted in the
server's secret key.
2 mutual-required The MUTUAL-REQUIRED option tells the server
that the client requires mutual
authentication, and that it must respond
with a KRB_AP_REP message.
3-31 reserved Reserved for future use.
ticket
This field is a ticket authenticating the client to the server.
authenticator
This contains the encrypted authenticator, which includes the
client's choice of a subkey.
The encrypted authenticator is included in the AP-REQ; it certifies
to a server that the sender has recent knowledge of the encryption
key in the accompanying ticket, to help the server detect replays.
It also assists in the selection of a "true session key" to use with
the particular session. The DER encoding of the following is
encrypted in the ticket's session key, with a key usage value of 11
in normal application exchanges, or 7 when used as the PA-TGS-REQ
PA-DATA field of a TGS-REQ exchange (see Section 5.4.1):
-- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE {
authenticator-vno [0] INTEGER (5),
crealm [1] Realm,
cname [2] PrincipalName,
cksum [3] Checksum OPTIONAL,
cusec [4] Microseconds,
ctime [5] KerberosTime,
subkey [6] EncryptionKey OPTIONAL,
seq-number [7] UInt32 OPTIONAL,
authorization-data [8] AuthorizationData OPTIONAL
}
authenticator-vno
This field specifies the version number for the format of the
authenticator. This document specifies version 5.
crealm and cname
These fields are the same as those described for the ticket in
section 5.3.
cksum
This field contains a checksum of the application data that
accompanies the KRB_AP_REQ, computed using a key usage value of 10
in normal application exchanges, or 6 when used in the TGS-REQ
PA-TGS-REQ AP-DATA field.
cusec
This field contains the microsecond part of the client's
timestamp. Its value (before encryption) ranges from 0 to 999999.
It often appears along with ctime. The two fields are used
together to specify a reasonably accurate timestamp.
ctime
This field contains the current time on the client's host.
subkey
This field contains the client's choice for an encryption key to
be used to protect this specific application session. Unless an
application specifies otherwise, if this field is left out, the
session key from the ticket will be used.
seq-number
This optional field includes the initial sequence number to be
used by the KRB_PRIV or KRB_SAFE messages when sequence numbers
are used to detect replays. (It may also be used by application
specific messages.) When included in the authenticator, this
field specifies the initial sequence number for messages from the
client to the server. When included in the AP-REP message, the
initial sequence number is that for messages from the server to
the client. When used in KRB_PRIV or KRB_SAFE messages, it is
incremented by one after each message is sent. Sequence numbers
fall in the range 0 through 2^32 - 1 and wrap to zero following
the value 2^32 - 1.
For sequence numbers to support the detection of replays
adequately, they SHOULD be non-repeating, even across connection
boundaries. The initial sequence number SHOULD be random and
uniformly distributed across the full space of possible sequence
numbers, so that it cannot be guessed by an attacker and so that
it and the successive sequence numbers do not repeat other
sequences. In the event that more than 2^32 messages are to be
generated in a series of KRB_PRIV or KRB_SAFE messages, rekeying
SHOULD be performed before sequence numbers are reused with the
same encryption key.
Implmentation note: Historically, some implementations transmit
signed twos-complement numbers for sequence numbers. In the
interests of compatibility, implementations MAY accept the
equivalent negative number where a positive number greater than
2^31 - 1 is expected.
Implementation note: As noted before, some implementations omit
the optional sequence number when its value would be zero.
Implementations MAY accept an omitted sequence number when
expecting a value of zero, and SHOULD NOT transmit an
Authenticator with a initial sequence number of zero.
authorization-data
This field is the same as described for the ticket in Section 5.3.
It is optional and will only appear when additional restrictions
are to be placed on the use of a ticket, beyond those carried in
the ticket itself.
5.5.2. KRB_AP_REP Definition
The KRB_AP_REP message contains the Kerberos protocol version number,
the message type, and an encrypted time-stamp. The message is sent
in response to an application request (KRB_AP_REQ) for which the
mutual authentication option has been selected in the ap-options
field.
AP-REP ::= [APPLICATION 15] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (15),
enc-part [2] EncryptedData -- EncAPRepPart
}
EncAPRepPart ::= [APPLICATION 27] SEQUENCE {
ctime [0] KerberosTime,
cusec [1] Microseconds,
subkey [2] EncryptionKey OPTIONAL,
seq-number [3] UInt32 OPTIONAL
}
The encoded EncAPRepPart is encrypted in the shared session key of
the ticket. The optional subkey field can be used in an
application-arranged negotiation to choose a per association session
key.
pvno and msg-type
These fields are described above in Section 5.4.1. msg-type is
KRB_AP_REP.
enc-part
This field is described above in Section 5.4.2. It is computed
with a key usage value of 12.
ctime
This field contains the current time on the client's host.
cusec
This field contains the microsecond part of the client's
timestamp.
subkey
This field contains an encryption key that is to be used to
protect this specific application session. See Section 3.2.6 for
specifics on how this field is used to negotiate a key. Unless an
application specifies otherwise, if this field is left out, the
sub-session key from the authenticator or if the latter is also
left out, the session key from the ticket will be used.
seq-number
This field is described above in Section 5.3.2.
5.5.3. Error Message Reply
If an error occurs while processing the application request, the
KRB_ERROR message will be sent in response. See Section 5.9.1 for
the format of the error message. The cname and crealm fields MAY be
left out if the server cannot determine their appropriate values from
the corresponding KRB_AP_REQ message. If the authenticator was
decipherable, the ctime and cusec fields will contain the values from
it.
5.6. KRB_SAFE Message Specification
This section specifies the format of a message that can be used by
either side (client or server) of an application to send a tamper-
proof message to its peer. It presumes that a session key has
previously been exchanged (for example, by using the
KRB_AP_REQ/KRB_AP_REP messages).
5.6.1. KRB_SAFE definition
The KRB_SAFE message contains user data along with a collision-proof
checksum keyed with the last encryption key negotiated via subkeys,
or with the session key if no negotiation has occurred. The message
fields are as follows:
KRB-SAFE ::= [APPLICATION 20] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (20),
safe-body [2] KRB-SAFE-BODY,
cksum [3] Checksum
}
KRB-SAFE-BODY ::= SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress,
r-address [5] HostAddress OPTIONAL
}
pvno and msg-type
These fields are described above in Section 5.4.1. msg-type is
KRB_SAFE.
safe-body
This field is a placeholder for the body of the KRB-SAFE message.
cksum
This field contains the checksum of the application data, computed
with a key usage value of 15.
The checksum is computed over the encoding of the KRB-SAFE
sequence. First, the cksum is set to a type zero, zero-length
value, and the checksum is computed over the encoding of the KRB-
SAFE sequence. Then the checksum is set to the result of that
computation. Finally, the KRB-SAFE sequence is encoded again.
This method, although different than the one specified in RFC
1510, corresponds to existing practice.
user-data
This field is part of the KRB_SAFE and KRB_PRIV messages, and
contains the application-specific data that is being passed from
the sender to the recipient.
timestamp
This field is part of the KRB_SAFE and KRB_PRIV messages. Its
contents are the current time as known by the sender of the
message. By checking the timestamp, the recipient of the message
is able to make sure that it was recently generated, and is not a
replay.
usec
This field is part of the KRB_SAFE and KRB_PRIV headers. It
contains the microsecond part of the timestamp.
seq-number
This field is described above in Section 5.3.2.
s-address
Sender's address.
This field specifies the address in use by the sender of the
message.
r-address
This field specifies the address in use by the recipient of the
message. It MAY be omitted for some uses (such as broadcast
protocols), but the recipient MAY arbitrarily reject such
messages. This field, along with s-address, can be used to help
detect messages that have been incorrectly or maliciously
delivered to the wrong recipient.
5.7. KRB_PRIV Message Specification
This section specifies the format of a message that can be used by
either side (client or server) of an application to send a message to
its peer securely and privately. It presumes that a session key has
previously been exchanged (for example, by using the
KRB_AP_REQ/KRB_AP_REP messages).
5.7.1. KRB_PRIV Definition
The KRB_PRIV message contains user data encrypted in the Session Key.
The message fields are as follows:
KRB-PRIV ::= [APPLICATION 21] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (21),
-- NOTE: there is no [2] tag
enc-part [3] EncryptedData -- EncKrbPrivPart
}
EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress -- sender's addr --,
r-address [5] HostAddress OPTIONAL -- recip's addr
}
pvno and msg-type
These fields are described above in Section 5.4.1. msg-type is
KRB_PRIV.
enc-part
This field holds an encoding of the EncKrbPrivPart sequence
encrypted under the session key, with a key usage value of 13.
This encrypted encoding is used for the enc-part field of the
KRB-PRIV message.
user-data, timestamp, usec, s-address, and r-address
These fields are described above in Section 5.6.1.
seq-number
This field is described above in Section 5.3.2.
5.8. KRB_CRED Message Specification
This section specifies the format of a message that can be used to
send Kerberos credentials from one principal to another. It is
presented here to encourage a common mechanism to be used by
applications when forwarding tickets or providing proxies to
subordinate servers. It presumes that a session key has already been
exchanged, perhaps by using the KRB_AP_REQ/KRB_AP_REP messages.
5.8.1. KRB_CRED Definition
The KRB_CRED message contains a sequence of tickets to be sent and
information needed to use the tickets, including the session key from
each. The information needed to use the tickets is encrypted under
an encryption key previously exchanged or transferred alongside the
KRB_CRED message. The message fields are as follows:
KRB-CRED ::= [APPLICATION 22] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (22),
tickets [2] SEQUENCE OF Ticket,
enc-part [3] EncryptedData -- EncKrbCredPart
}
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {
ticket-info [0] SEQUENCE OF KrbCredInfo,
nonce [1] UInt32 OPTIONAL,
timestamp [2] KerberosTime OPTIONAL,
usec [3] Microseconds OPTIONAL,
s-address [4] HostAddress OPTIONAL,
r-address [5] HostAddress OPTIONAL
}
KrbCredInfo ::= SEQUENCE {
key [0] EncryptionKey,
prealm [1] Realm OPTIONAL,
pname [2] PrincipalName OPTIONAL,
flags [3] TicketFlags OPTIONAL,
authtime [4] KerberosTime OPTIONAL,
starttime [5] KerberosTime OPTIONAL,
endtime [6] KerberosTime OPTIONAL,
renew-till [7] KerberosTime OPTIONAL,
srealm [8] Realm OPTIONAL,
sname [9] PrincipalName OPTIONAL,
caddr [10] HostAddresses OPTIONAL
}
pvno and msg-type
These fields are described above in Section 5.4.1. msg-type is
KRB_CRED.
tickets
These are the tickets obtained from the KDC specifically for use
by the intended recipient. Successive tickets are paired with the
corresponding KrbCredInfo sequence from the enc-part of the KRB-
CRED message.
enc-part
This field holds an encoding of the EncKrbCredPart sequence
encrypted under the session key shared by the sender and the
intended recipient, with a key usage value of 14. This encrypted
encoding is used for the enc-part field of the KRB-CRED message.
Implementation note: Implementations of certain applications, most
notably certain implementations of the Kerberos GSS-API mechanism,
do not separately encrypt the contents of the EncKrbCredPart of
the KRB-CRED message when sending it. In the case of those GSS-
API mechanisms, this is not a security vulnerability, as the
entire KRB-CRED message is itself embedded in an encrypted
message.
nonce
If practical, an application MAY require the inclusion of a nonce
generated by the recipient of the message. If the same value is
included as the nonce in the message, it provides evidence that
the message is fresh and has not been replayed by an attacker. A
nonce MUST NEVER be reused.
timestamp and usec
These fields specify the time that the KRB-CRED message was
generated. The time is used to provide assurance that the message
is fresh.
s-address and r-address
These fields are described above in Section 5.6.1. They are used
optionally to provide additional assurance of the integrity of the
KRB-CRED message.
key
This field exists in the corresponding ticket passed by the KRB-
CRED message and is used to pass the session key from the sender
to the intended recipient. The field's encoding is described in
Section 5.2.9.
The following fields are optional. If present, they can be
associated with the credentials in the remote ticket file. If left
out, then it is assumed that the recipient of the credentials already
knows their values.
prealm and pname
The name and realm of the delegated principal identity.
flags, authtime, starttime, endtime, renew-till, srealm, sname,
and caddr
These fields contain the values of the corresponding fields from
the ticket found in the ticket field. Descriptions of the fields
are identical to the descriptions in the KDC-REP message.
5.9. Error Message Specification
This section specifies the format for the KRB_ERROR message. The
fields included in the message are intended to return as much
information as possible about an error. It is not expected that all
the information required by the fields will be available for all
types of errors. If the appropriate information is not available
when the message is composed, the corresponding field will be left
out of the message.
Note that because the KRB_ERROR message is not integrity protected,
it is quite possible for an intruder to synthesize or modify it. In
particular, this means that the client SHOULD NOT use any fields in
this message for security-critical purposes, such as setting a system
clock or generating a fresh authenticator. The message can be
useful, however, for advising a user on the reason for some failure.
5.9.1. KRB_ERROR Definition
The KRB_ERROR message consists of the following fields:
KRB-ERROR ::= [APPLICATION 30] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (30),
ctime [2] KerberosTime OPTIONAL,
cusec [3] Microseconds OPTIONAL,
stime [4] KerberosTime,
susec [5] Microseconds,
error-code [6] Int32,
crealm [7] Realm OPTIONAL,
cname [8] PrincipalName OPTIONAL,
realm [9] Realm -- service realm --,
sname [10] PrincipalName -- service name --,
e-text [11] KerberosString OPTIONAL,
e-data [12] OCTET STRING OPTIONAL
}
pvno and msg-type
These fields are described above in Section 5.4.1. msg-type is
KRB_ERROR.
ctime and cusec
These fields are described above in Section 5.5.2. If the values
for these fields are known to the entity generating the error (as
they would be if the KRB-ERROR is generated in reply to, e.g., a
failed authentication service request), they should be populated
in the KRB-ERROR. If the values are not available, these fields
can be omitted.
stime
This field contains the current time on the server. It is of type
KerberosTime.
susec
This field contains the microsecond part of the server's
timestamp. Its value ranges from 0 to 999999. It appears along
with stime. The two fields are used in conjunction to specify a
reasonably accurate timestamp.
error-code
This field contains the error code returned by Kerberos or the
server when a request fails. To interpret the value of this field
see the list of error codes in Section 7.5.9. Implementations are
encouraged to provide for national language support in the display
of error messages.
crealm, and cname
These fields are described above in Section 5.3. When the entity
generating the error knows these values, they should be populated
in the KRB-ERROR. If the values are not known, the crealm and
cname fields SHOULD be omitted.
realm and sname
These fields are described above in Section 5.3.
e-text
This field contains additional text to help explain the error code
associated with the failed request (for example, it might include
a principal name which was unknown).
e-data
This field contains additional data about the error for use by the
application to help it recover from or handle the error. If the
errorcode is KDC_ERR_PREAUTH_REQUIRED, then the e-data field will
contain an encoding of a sequence of padata fields, each
corresponding to an acceptable pre-authentication method and
optionally containing data for the method:
METHOD-DATA ::= SEQUENCE OF PA-DATA
For error codes defined in this document other than
KDC_ERR_PREAUTH_REQUIRED, the format and contents of the e-data field
are implementation-defined. Similarly, for future error codes, the
format and contents of the e-data field are implementation-defined
unless specified otherwise. Whether defined by the implementation or
in a future document, the e-data field MAY take the form of TYPED-
DATA:
TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
data-type [0] Int32,
data-value [1] OCTET STRING OPTIONAL
}
5.10. Application Tag Numbers
The following table lists the application class tag numbers used by
various data types defined in this section.
Tag Number(s) Type Name Comments
0 unused
1 Ticket PDU
2 Authenticator non-PDU
3 EncTicketPart non-PDU
4-9 unused
10 AS-REQ PDU
11 AS-REP PDU
12 TGS-REQ PDU
13 TGS-REP PDU
14 AP-REQ PDU
15 AP-REP PDU
16 RESERVED16 TGT-REQ (for user-to-user)
17 RESERVED17 TGT-REP (for user-to-user)
18-19 unused
20 KRB-SAFE PDU
21 KRB-PRIV PDU
22 KRB-CRED PDU
23-24 unused
25 EncASRepPart non-PDU
26 EncTGSRepPart non-PDU
27 EncApRepPart non-PDU
28 EncKrbPrivPart non-PDU
29 EncKrbCredPart non-PDU
30 KRB-ERROR PDU
The ASN.1 types marked above as "PDU" (Protocol Data Unit) are the
only ASN.1 types intended as top-level types of the Kerberos
protocol, and are the only types that may be used as elements in
another protocol that makes use of Kerberos.
6. Naming Constraints
6.1. Realm Names
Although realm names are encoded as GeneralStrings and technically a
realm can select any name it chooses, interoperability across realm
boundaries requires agreement on how realm names are to be assigned,
and what information they imply.
To enforce these conventions, each realm MUST conform to the
conventions itself, and it MUST require that any realms with which
inter-realm keys are shared also conform to the conventions and
require the same from its neighbors.
Kerberos realm names are case sensitive. Realm names that differ
only in the case of the characters are not equivalent. There are
presently three styles of realm names: domain, X500, and other.
Examples of each style follow:
domain: ATHENA.MIT.EDU
X500: C=US/O=OSF
other: NAMETYPE:rest/of.name=without-restrictions
Domain style realm names MUST look like domain names: they consist of
components separated by periods (.) and they contain neither colons
(:) nor slashes (/). Though domain names themselves are case
insensitive, in order for realms to match, the case must match as
well. When establishing a new realm name based on an internet domain
name it is recommended by convention that the characters be converted
to uppercase.
X.500 names contain an equals sign (=) and cannot contain a colon (:)
before the equals sign. The realm names for X.500 names will be
string representations of the names with components separated by
slashes. Leading and trailing slashes will not be included. Note
that the slash separator is consistent with Kerberos implementations
based on RFC 1510, but it is different from the separator recommended
in RFC 2253.
Names that fall into the other category MUST begin with a prefix that
contains no equals sign (=) or period (.), and the prefix MUST be
followed by a colon (:) and the rest of the name. All prefixes
expect those beginning with used. Presently none are assigned.
The reserved category includes strings that do not fall into the
first three categories. All names in this category are reserved. It
is unlikely that names will be assigned to this category unless there
is a very strong argument for not using the 'other' category.
These rules guarantee that there will be no conflicts between the
various name styles. The following additional constraints apply to
the assignment of realm names in the domain and X.500 categories:
either the name of a realm for the domain or X.500 formats must be
used by the organization owning (to whom it was assigned) an Internet
domain name or X.500 name, or, in the case that no such names are
registered, authority to use a realm name MAY be derived from the
authority of the parent realm. For example, if there is no domain
name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can
authorize the creation of a realm with that name.
This is acceptable because the organization to which the parent is
assigned is presumably the organization authorized to assign names to
its children in the X.500 and domain name systems as well. If the
parent assigns a realm name without also registering it in the domain
name or X.500 hierarchy, it is the parent's responsibility to make
sure that in the future there will not exist a name identical to the
realm name of the child unless it is assigned to the same entity as
the realm name.
6.2. Principal Names
As was the case for realm names, conventions are needed to ensure
that all agree on what information is implied by a principal name.
The name-type field that is part of the principal name indicates the
kind of information implied by the name. The name-type SHOULD be
treated only as a hint to interpreting the meaning of a name. It is
not significant when checking for equivalence. Principal names that
differ only in the name-type identify the same principal. The name
type does not partition the name space. Ignoring the name type, no
two names can be the same (i.e., at least one of the components, or
the realm, MUST be different). The following name types are defined:
Name Type Value Meaning
NT-UNKNOWN 0 Name type not known
NT-PRINCIPAL 1 Just the name of the principal as in DCE,
or for users
NT-SRV-INST 2 Service and other unique instance (krbtgt)
NT-SRV-HST 3 Service with host name as instance
(telnet, rcommands)
NT-SRV-XHST 4 Service with host as remaining components
NT-UID 5 Unique ID
NT-X500-PRINCIPAL 6 Encoded X.509 Distinguished name [RFC2253]
NT-SMTP-NAME 7 Name in form of SMTP email name
(e.g., user@example.com)
NT-ENTERPRISE 10 Enterprise name - may be mapped to principal
name
When a name implies no information other than its uniqueness at a
particular time, the name type PRINCIPAL SHOULD be used. The
principal name type SHOULD be used for users, and it might also be
used for a unique server. If the name is a unique machine-generated
ID that is guaranteed never to be reassigned, then the name type of
UID SHOULD be used. (Note that it is generally a bad idea to
reassign names of any type since stale entries might remain in access
control lists.)
If the first component of a name identifies a service and the
remaining components identify an instance of the service in a
server-specified manner, then the name type of SRV-INST SHOULD be
used. An example of this name type is the Kerberos ticket-granting
service whose name has a first component of krbtgt and a second
component identifying the realm for which the ticket is valid.
If the first component of a name identifies a service and there is a
single component following the service name identifying the instance
as the host on which the server is running, then the name type
SRV-HST SHOULD be used. This type is typically used for Internet
services such as telnet and the Berkeley R commands. If the separate
components of the host name appear as successive components following
the name of the service, then the name type SRV-XHST SHOULD be used.
This type might be used to identify servers on hosts with X.500
names, where the slash (/) might otherwise be ambiguous.
A name type of NT-X500-PRINCIPAL SHOULD be used when a name from an
X.509 certificate is translated into a Kerberos name. The encoding
of the X.509 name as a Kerberos principal shall conform to the
encoding rules specified in RFC 2253.
A name type of SMTP allows a name to be of a form that resembles an
SMTP email name. This name, including an "@" and a domain name, is
used as the one component of the principal name.
A name type of UNKNOWN SHOULD be used when the form of the name is
not known. When comparing names, a name of type UNKNOWN will match
principals authenticated with names of any type. A principal
authenticated with a name of type UNKNOWN, however, will only match
other names of type UNKNOWN.
Names of any type with an initial component of 'krbtgt' are reserved
for the Kerberos ticket-granting service. See Section 7.3 for the
form of such names.
6.2.1. Name of Server Principals
The principal identifier for a server on a host will generally be
composed of two parts: (1) the realm of the KDC with which the server
is registered, and (2) a two-component name of type NT-SRV-HST, if
the host name is an Internet domain name, or a multi-component name
of type NT-SRV-XHST, if the name of the host is of a form (such as
X.500) that allows slash (/) separators. The first component of the
two- or multi-component name will identify the service, and the
latter components will identify the host. Where the name of the host
is not case sensitive (for example, with Internet domain names) the
name of the host MUST be lowercase. If specified by the application
protocol for services such as telnet and the Berkeley R commands that
run with system privileges, the first component MAY be the string
'host' instead of a service-specific identifier.
7. Constants and Other Defined Values
7.1. Host Address Types
All negative values for the host address type are reserved for local
use. All non-negative values are reserved for officially assigned
type fields and interpretations.
Internet (IPv4) Addresses
Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded
in MSB order (most significant byte first). The IPv4 loopback
address SHOULD NOT appear in a Kerberos PDU. The type of IPv4
addresses is two (2).
Internet (IPv6) Addresses
IPv6 addresses [RFC3513] are 128-bit (16-octet) quantities,
encoded in MSB order (most significant byte first). The type of
IPv6 addresses is twenty-four (24). The following addresses MUST
NOT appear in any Kerberos PDU:
* the Unspecified Address
* the Loopback Address
* Link-Local addresses
This restriction applies to the inclusion in the address fields of
Kerberos PDUs, but not to the address fields of packets that might
carry such PDUs. The restriction is necessary because the use of
an address with non-global scope could allow the acceptance of a
message sent from a node that may have the same address, but which
is not the host intended by the entity that added the restriction.
If the link-local address type needs to be used for communication,
then the address restriction in tickets must not be used (i.e.,
addressless tickets must be used).
IPv4-mapped IPv6 addresses MUST be represented as addresses of
type 2.
DECnet Phase IV Addresses
DECnet Phase IV addresses are 16-bit addresses, encoded in LSB
order. The type of DECnet Phase IV addresses is twelve (12).
Netbios Addresses
Netbios addresses are 16-octet addresses typically composed of 1
to 15 alphanumeric characters and padded with the US-ASCII SPC
character (code 32). The 16th octet MUST be the US-ASCII NUL
character (code 0). The type of Netbios addresses is twenty (20).
Directional Addresses
Including the sender address in KRB_SAFE and KRB_PRIV messages is
undesirable in many environments because the addresses may be
changed in transport by network address translators. However, if
these addresses are removed, the messages may be subject to a
reflection attack in which a message is reflected back to its
originator. The directional address type provides a way to avoid
transport addresses and reflection attacks. Directional addresses
are encoded as four-byte unsigned integers in network byte order.
If the message is originated by the party sending the original
KRB_AP_REQ message, then an address of 0 SHOULD be used. If the
message is originated by the party to whom that KRB_AP_REQ was
sent, then the address 1 SHOULD be used. Applications involving
multiple parties can specify the use of other addresses.
Directional addresses MUST only be used for the sender address
field in the KRB_SAFE or KRB_PRIV messages. They MUST NOT be used
as a ticket address or in a KRB_AP_REQ message. This address type
SHOULD only be used in situations where the sending party knows
that the receiving party supports the address type. This
generally means that directional addresses may only be used when
the application protocol requires their support. Directional
addresses are type (3).
7.2. KDC Messaging: IP Transports
Kerberos defines two IP transport mechanisms for communication
between clients and servers: UDP/IP and TCP/IP.
7.2.1. UDP/IP transport
Kerberos servers (KDCs) supporting IP transports MUST accept UDP
requests and SHOULD listen for them on port 88 (decimal) unless
specifically configured to listen on an alternative UDP port.
Alternate ports MAY be used when running multiple KDCs for multiple
realms on the same host.
Kerberos clients supporting IP transports SHOULD support the sending
of UDP requests. Clients SHOULD use KDC discovery [7.2.3] to
identify the IP address and port to which they will send their
request.
When contacting a KDC for a KRB_KDC_REQ request using UDP/IP
transport, the client shall send a UDP datagram containing only an
encoding of the request to the KDC. The KDC will respond with a
reply datagram containing only an encoding of the reply message
(either a KRB_ERROR or a KRB_KDC_REP) to the sending port at the
sender's IP address. The response to a request made through UDP/IP
transport MUST also use UDP/IP transport. If the response cannot be
handled using UDP (for example, because it is too large), the KDC
MUST return KRB_ERR_RESPONSE_TOO_BIG, forcing the client to retry the
request using the TCP transport.
7.2.2. TCP/IP Transport
Kerberos servers (KDCs) supporting IP transports MUST accept TCP
requests and SHOULD listen for them on port 88 (decimal) unless
specifically configured to listen on an alternate TCP port.
Alternate ports MAY be used when running multiple KDCs for multiple
realms on the same host.
Clients MUST support the sending of TCP requests, but MAY choose to
try a request initially using the UDP transport. Clients SHOULD use
KDC discovery [7.2.3] to identify the IP address and port to which
they will send their request.
Implementation note: Some extensions to the Kerberos protocol will
not succeed if any client or KDC not supporting the TCP transport is
involved. Implementations of RFC 1510 were not required to support
TCP/IP transports.
When the KRB_KDC_REQ message is sent to the KDC over a TCP stream,
the response (KRB_KDC_REP or KRB_ERROR message) MUST be returned to
the client on the same TCP stream that was established for the
request. The KDC MAY close the TCP stream after sending a response,
but MAY leave the stream open for a reasonable period of time if it
expects a follow-up. Care must be taken in managing TCP/IP
connections on the KDC to prevent denial of service attacks based on
the number of open TCP/IP connections.
The client MUST be prepared to have the stream closed by the KDC at
any time after the receipt of a response. A stream closure SHOULD
NOT be treated as a fatal error. Instead, if multiple exchanges are
required (e.g., certain forms of pre-authentication), the client may
need to establish a new connection when it is ready to send
subsequent messages. A client MAY close the stream after receiving a
response, and SHOULD close the stream if it does not expect to send
follow-up messages.
A client MAY send multiple requests before receiving responses,
though it must be prepared to handle the connection being closed
after the first response.
Each request (KRB_KDC_REQ) and response (KRB_KDC_REP or KRB_ERROR)
sent over the TCP stream is preceded by the length of the request as
4 octets in network byte order. The high bit of the length is
reserved for future expansion and MUST currently be set to zero. If
a KDC that does not understand how to interpret a set high bit of the
length encoding receives a request with the high order bit of the
length set, it MUST return a KRB-ERROR message with the error
KRB_ERR_FIELD_TOOLONG and MUST close the TCP stream.
If multiple requests are sent over a single TCP connection and the
KDC sends multiple responses, the KDC is not required to send the
responses in the order of the corresponding requests. This may
permit some implementations to send each response as soon as it is
ready, even if earlier requests are still being processed (for
example, waiting for a response from an external device or database).
7.2.3. KDC Discovery on IP Networks
Kerberos client implementations MUST provide a means for the client
to determine the location of the Kerberos Key Distribution Centers
(KDCs). Traditionally, Kerberos implementations have stored such
configuration information in a file on each client machine.
Experience has shown that this method of storing configuration
information presents problems with out-of-date information and
scaling, especially when using cross-realm authentication. This
section describes a method for using the Domain Name System [RFC1035]
for storing KDC location information.
7.2.3.1. DNS vs. Kerberos: Case Sensitivity of Realm Names
In Kerberos, realm names are case sensitive. Although it is strongly
encouraged that all realm names be all uppercase, this recommendation
has not been adopted by all sites. Some sites use all lowercase
names and other use mixed case. DNS, on the other hand, is case
insensitive for queries. Because the realm names "MYREALM",
"myrealm", and "MyRealm" are all different, but resolve the same in
the domain name system, it is necessary that only one of the possible
combinations of upper- and lowercase characters be used in realm
names.
7.2.3.2. Specifying KDC Location Information with DNS SRV records
KDC location information is to be stored using the DNS SRV RR
[RFC2782]. The format of this RR is as follows:
_Service._Proto.Realm TTL Class SRV Priority Weight Port Target
The Service name for Kerberos is always "kerberos".
The Proto can be either "udp" or "tcp". If these SRV records are to
be used, both "udp" and "tcp" records MUST be specified for all KDC
deployments.
The Realm is the Kerberos realm that this record corresponds to. The
realm MUST be a domain-style realm name.
TTL, Class, SRV, Priority, Weight, and Target have the standard
meaning as defined in RFC 2782.
As per RFC 2782, the Port number used for "_udp" and "_tcp" SRV
records SHOULD be the value assigned to "kerberos" by the Internet
Assigned Number Authority: 88 (decimal), unless the KDC is configured
to listen on an alternate TCP port.
Implementation note: Many existing client implementations do not
support KDC Discovery and are configured to send requests to the IANA
assigned port (88 decimal), so it is strongly recommended that KDCs
be configured to listen on that port.
7.2.3.3. KDC Discovery for Domain Style Realm Names on IP Networks
These are DNS records for a Kerberos realm EXAMPLE.COM. It has two
Kerberos servers, kdc1.example.com and kdc2.example.com. Queries
should be directed to kdc1.example.com first as per the specified
priority. Weights are not used in these sample records.
_kerberos._udp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
_kerberos._udp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.
_kerberos._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
_kerberos._tcp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.
7.3. Name of the TGS
The principal identifier of the ticket-granting service shall be
composed of three parts: the realm of the KDC issuing the TGS ticket,
and a two-part name of type NT-SRV-INST, with the first part "krbtgt"
and the second part the name of the realm that will accept the TGT.
For example, a TGT issued by the ATHENA.MIT.EDU realm to be used to
get tickets from the ATHENA.MIT.EDU KDC has a principal identifier of
"ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A TGT
issued by the ATHENA.MIT.EDU realm to be used to get tickets from the
MIT.EDU realm has a principal identifier of "ATHENA.MIT.EDU" (realm),
("krbtgt", "MIT.EDU") (name).
7.4. OID Arc for KerberosV5
This OID MAY be used to identify Kerberos protocol messages
encapsulated in other protocols. It also designates the OID arc for
KerberosV5-related OIDs assigned by future IETF action.
Implementation note: RFC 1510 had an incorrect value (5) for "dod" in
its OID.
id-krb5 OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2)
}
Assignment of OIDs beneath the id-krb5 arc must be obtained by
contacting the registrar for the id-krb5 arc, or its designee. At
the time of the issuance of this RFC, such registrations can be
obtained by contacting krb5-oid-registrar@mit.edu.
7.5. Protocol Constants and Associated Values
The following tables list constants used in the protocol and define
their meanings. In the "specification" section, ranges are specified
that limit the values of constants for which values are defined here.
This allows implementations to make assumptions about the maximum
values that will be received for these constants. Implementations
receiving values outside the range specified in the "specification"
section MAY reject the request, but they MUST recover cleanly.
7.5.1. Key Usage Numbers
The encryption and checksum specifications in [RFC3961] require as
input a "key usage number", to alter the encryption key used in any
specific message in order to make certain types of cryptographic
attack more difficult. These are the key usage values assigned in
this document:
1. AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with
the client key (Section 5.2.7.2)
2. AS-REP Ticket and TGS-REP Ticket (includes TGS session
key or application session key), encrypted with the
service key (Section 5.3)
3. AS-REP encrypted part (includes TGS session key or
application session key), encrypted with the client key
(Section 5.4.2)
4. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
the TGS session key (Section 5.4.1)
5. TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with
the TGS authenticator subkey (Section 5.4.1)
6. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum,
keyed with the TGS session key (Section 5.5.1)
7. TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes
TGS authenticator subkey), encrypted with the TGS session
key (Section 5.5.1)
8. TGS-REP encrypted part (includes application session
key), encrypted with the TGS session key (Section 5.4.2)
9. TGS-REP encrypted part (includes application session
key), encrypted with the TGS authenticator subkey
(Section 5.4.2)
10. AP-REQ Authenticator cksum, keyed with the application
session key (Section 5.5.1)
11. AP-REQ Authenticator (includes application authenticator
subkey), encrypted with the application session key
(Section 5.5.1)
12. AP-REP encrypted part (includes application session
subkey), encrypted with the application session key
(Section 5.5.2)
13. KRB-PRIV encrypted part, encrypted with a key chosen by
the application (Section 5.7.1)
14. KRB-CRED encrypted part, encrypted with a key chosen by
the application (Section 5.8.1)
15. KRB-SAFE cksum, keyed with a key chosen by the
application (Section 5.6.1)
16-18. Reserved for future use in Kerberos and related
protocols.
19. AD-KDC-ISSUED checksum (ad-checksum in 5.2.6.4)
20-21. Reserved for future use in Kerberos and related
protocols.
22-25. Reserved for use in the Kerberos Version 5 GSS-API
mechanisms [RFC4121].
26-511. Reserved for future use in Kerberos and related
protocols.
512-1023. Reserved for uses internal to a Kerberos implementation.
1024. Encryption for application use in protocols that do not
specify key usage values
1025. Checksums for application use in protocols that do not
specify key usage values
1026-2047. Reserved for application use.
7.5.2. PreAuthentication Data Types
Padata and Data Type Padata-type Comment
Value
PA-TGS-REQ 1
PA-ENC-TIMESTAMP 2
PA-PW-SALT 3
[reserved] 4
PA-ENC-UNIX-TIME 5 (deprecated)
PA-SANDIA-SECUREID 6
PA-SESAME 7
PA-OSF-DCE 8
PA-CYBERSAFE-SECUREID 9
PA-AFS3-SALT 10
PA-ETYPE-INFO 11
PA-SAM-CHALLENGE 12 (sam/otp)
PA-SAM-RESPONSE 13 (sam/otp)
PA-PK-AS-REQ_OLD 14 (pkinit)
PA-PK-AS-REP_OLD 15 (pkinit)
PA-PK-AS-REQ 16 (pkinit)
PA-PK-AS-REP 17 (pkinit)
PA-ETYPE-INFO2 19 (replaces pa-etype-info)
PA-USE-SPECIFIED-KVNO 20
PA-SAM-REDIRECT 21 (sam/otp)
PA-GET-FROM-TYPED-DATA 22 (embedded in typed data)
TD-PADATA 22 (embeds padata)
PA-SAM-ETYPE-INFO 23 (sam/otp)
PA-ALT-PRINC 24 (crawdad@fnal.gov)
PA-SAM-CHALLENGE2 30 (kenh@pobox.com)
PA-SAM-RESPONSE2 31 (kenh@pobox.com)
PA-EXTRA-TGT 41 Reserved extra TGT
TD-PKINIT-CMS-CERTIFICATES 101 CertificateSet from CMS
TD-KRB-PRINCIPAL 102 PrincipalName
TD-KRB-REALM 103 Realm
TD-TRUSTED-CERTIFIERS 104 from PKINIT
TD-CERTIFICATE-INDEX 105 from PKINIT
TD-APP-DEFINED-ERROR 106 application specific
TD-REQ-NONCE 107 INTEGER
TD-REQ-SEQ 108 INTEGER
PA-PAC-REQUEST 128 (jbrezak@exchange.microsoft.com)
7.5.3. Address Types
Address Type Value
IPv4 2
Directional 3
ChaosNet 5
XNS 6
ISO 7
DECNET Phase IV 12
AppleTalk DDP 16
NetBios 20
IPv6 24
7.5.4. Authorization Data Types
Authorization Data Type Ad-type Value
AD-IF-RELEVANT 1
AD-INTENDED-FOR-SERVER 2
AD-INTENDED-FOR-APPLICATION-CLASS 3
AD-KDC-ISSUED 4
AD-AND-OR 5
AD-MANDATORY-TICKET-EXTENSIONS 6
AD-IN-TICKET-EXTENSIONS 7
AD-MANDATORY-FOR-KDC 8
Reserved values 9-63
OSF-DCE 64
SESAME 65
AD-OSF-DCE-PKI-CERTID 66 (hemsath@us.ibm.com)
AD-WIN2K-PAC 128 (jbrezak@exchange.microsoft.com)
AD-ETYPE-NEGOTIATION 129 (lzhu@windows.microsoft.com)
7.5.5. Transited Encoding Types
Transited Encoding Type Tr-type Value
DOMAIN-X500-COMPRESS 1
Reserved values All others
7.5.6. Protocol Version Number
Label Value Meaning or MIT Code
pvno 5 Current Kerberos protocol version number
7.5.7. Kerberos Message Types
Message Type Value Meaning
KRB_AS_REQ 10 Request for initial authentication
KRB_AS_REP 11 Response to KRB_AS_REQ request
KRB_TGS_REQ 12 Request for authentication based on TGT
KRB_TGS_REP 13 Response to KRB_TGS_REQ request
KRB_AP_REQ 14 Application request to server
KRB_AP_REP 15 Response to KRB_AP_REQ_MUTUAL
KRB_RESERVED16 16 Reserved for user-to-user krb_tgt_request
KRB_RESERVED17 17 Reserved for user-to-user krb_tgt_reply
KRB_SAFE 20 Safe (checksummed) application message
KRB_PRIV 21 Private (encrypted) application message
KRB_CRED 22 Private (encrypted) message to forward
credentials
KRB_ERROR 30 Error response
7.5.8. Name Types
Name Type Value Meaning
KRB_NT_UNKNOWN 0 Name type not known
KRB_NT_PRINCIPAL 1 Just the name of the principal as in DCE,
or for users
KRB_NT_SRV_INST 2 Service and other unique instance (krbtgt)
KRB_NT_SRV_HST 3 Service with host name as instance
(telnet, rcommands)
KRB_NT_SRV_XHST 4 Service with host as remaining components
KRB_NT_UID 5 Unique ID
KRB_NT_X500_PRINCIPAL 6 Encoded X.509 Distinguished name [RFC2253]
KRB_NT_SMTP_NAME 7 Name in form of SMTP email name
(e.g., user@example.com)
KRB_NT_ENTERPRISE 10 Enterprise name; may be mapped to
principal name
7.5.9. Error Codes
Error Code Value Meaning
KDC_ERR_NONE 0 No error
KDC_ERR_NAME_EXP 1 Client's entry in database
has expired
KDC_ERR_SERVICE_EXP 2 Server's entry in database
has expired
KDC_ERR_BAD_PVNO 3 Requested protocol version
number not supported
KDC_ERR_C_OLD_MAST_KVNO 4 Client's key encrypted in
old master key
KDC_ERR_S_OLD_MAST_KVNO 5 Server's key encrypted in
old master key
KDC_ERR_C_PRINCIPAL_UNKNOWN 6 Client not found in
Kerberos database
KDC_ERR_S_PRINCIPAL_UNKNOWN 7 Server not found in
Kerberos database
KDC_ERR_PRINCIPAL_NOT_UNIQUE 8 Multiple principal entries
in database
KDC_ERR_NULL_KEY 9 The client or server has a
null key
KDC_ERR_CANNOT_POSTDATE 10 Ticket not eligible for
postdating
KDC_ERR_NEVER_VALID 11 Requested starttime is
later than end time
KDC_ERR_POLICY 12 KDC policy rejects request
KDC_ERR_BADOPTION 13 KDC cannot accommodate
requested option
KDC_ERR_ETYPE_NOSUPP 14 KDC has no support for
encryption type
KDC_ERR_SUMTYPE_NOSUPP 15 KDC has no support for
checksum type
KDC_ERR_PADATA_TYPE_NOSUPP 16 KDC has no support for
padata type
KDC_ERR_TRTYPE_NOSUPP 17 KDC has no support for
transited type
KDC_ERR_CLIENT_REVOKED 18 Clients credentials have
been revoked
KDC_ERR_SERVICE_REVOKED 19 Credentials for server have
been revoked
KDC_ERR_TGT_REVOKED 20 TGT has been revoked
KDC_ERR_CLIENT_NOTYET 21 Client not yet valid; try
again later
KDC_ERR_SERVICE_NOTYET 22 Server not yet valid; try
again later
KDC_ERR_KEY_EXPIRED 23 Password has expired;
change password to reset
KDC_ERR_PREAUTH_FAILED 24 Pre-authentication
information was invalid
KDC_ERR_PREAUTH_REQUIRED 25 Additional pre-
authentication required
KDC_ERR_SERVER_NOMATCH 26 Requested server and ticket
don't match
KDC_ERR_MUST_USE_USER2USER 27 Server principal valid for
user2user only
KDC_ERR_PATH_NOT_ACCEPTED 28 KDC Policy rejects
transited path
KDC_ERR_SVC_UNAVAILABLE 29 A service is not available
KRB_AP_ERR_BAD_INTEGRITY 31 Integrity check on
decrypted field failed
KRB_AP_ERR_TKT_EXPIRED 32 Ticket expired
KRB_AP_ERR_TKT_NYV 33 Ticket not yet valid
KRB_AP_ERR_REPEAT 34 Request is a replay
KRB_AP_ERR_NOT_US 35 The ticket isn't for us
KRB_AP_ERR_BADMATCH 36 Ticket and authenticator
don't match
KRB_AP_ERR_SKEW 37 Clock skew too great
KRB_AP_ERR_BADADDR 38 Incorrect net address
KRB_AP_ERR_BADVERSION 39 Protocol version mismatch
KRB_AP_ERR_MSG_TYPE 40 Invalid msg type
KRB_AP_ERR_MODIFIED 41 Message stream modified
KRB_AP_ERR_BADORDER 42 Message out of order
KRB_AP_ERR_BADKEYVER 44 Specified version of key is
not available
KRB_AP_ERR_NOKEY 45 Service key not available
KRB_AP_ERR_MUT_FAIL 46 Mutual authentication
failed
KRB_AP_ERR_BADDIRECTION 47 Incorrect message direction
KRB_AP_ERR_METHOD 48 Alternative authentication
method required
KRB_AP_ERR_BADSEQ 49 Incorrect sequence number
in message
KRB_AP_ERR_INAPP_CKSUM 50 Inappropriate type of
checksum in message
KRB_AP_PATH_NOT_ACCEPTED 51 Policy rejects transited
path
KRB_ERR_RESPONSE_TOO_BIG 52 Response too big for UDP;
retry with TCP
KRB_ERR_GENERIC 60 Generic error (description
in e-text)
KRB_ERR_FIELD_TOOLONG 61 Field is too long for this
implementation
KDC_ERROR_CLIENT_NOT_TRUSTED 62 Reserved for PKINIT
KDC_ERROR_KDC_NOT_TRUSTED 63 Reserved for PKINIT
KDC_ERROR_INVALID_SIG 64 Reserved for PKINIT
KDC_ERR_KEY_TOO_WEAK 65 Reserved for PKINIT
KDC_ERR_CERTIFICATE_MISMATCH 66 Reserved for PKINIT
KRB_AP_ERR_NO_TGT 67 No TGT available to
validate USER-TO-USER
KDC_ERR_WRONG_REALM 68 Reserved for future use
KRB_AP_ERR_USER_TO_USER_REQUIRED 69 Ticket must be for
USER-TO-USER
KDC_ERR_CANT_VERIFY_CERTIFICATE 70 Reserved for PKINIT
KDC_ERR_INVALID_CERTIFICATE 71 Reserved for PKINIT
KDC_ERR_REVOKED_CERTIFICATE 72 Reserved for PKINIT
KDC_ERR_REVOCATION_STATUS_UNKNOWN 73 Reserved for PKINIT
KDC_ERR_REVOCATION_STATUS_UNAVAILABLE 74 Reserved for PKINIT
KDC_ERR_CLIENT_NAME_MISMATCH 75 Reserved for PKINIT
KDC_ERR_KDC_NAME_MISMATCH 76 Reserved for PKINIT
8. Interoperability Requirements
Version 5 of the Kerberos protocol supports a myriad of options.
Among these are multiple encryption and checksum types; alternative
encoding schemes for the transited field; optional mechanisms for
pre-authentication; the handling of tickets with no addresses;
options for mutual authentication; user-to-user authentication;
support for proxies; the format of realm names; the handling of
authorization data; and forwarding, postdating, and renewing tickets.
In order to ensure the interoperability of realms, it is necessary to
define a minimal configuration that must be supported by all
implementations. This minimal configuration is subject to change as
technology does. For example, if at some later date it is discovered
that one of the required encryption or checksum algorithms is not
secure, it will be replaced.
8.1. Specification 2
This section defines the second specification of these options.
Implementations which are configured in this way can be said to
support Kerberos Version 5 Specification 2 (5.2). Specification 1
(deprecated) may be found in RFC 1510.
Transport
TCP/IP and UDP/IP transport MUST be supported by clients and KDCs
claiming conformance to specification 2.
Encryption and Checksum Methods
The following encryption and checksum mechanisms MUST be
supported:
Encryption: AES256-CTS-HMAC-SHA1-96 [RFC3962]
Checksums: HMAC-SHA1-96-AES256 [RFC3962]
Implementations SHOULD support other mechanisms as well, but the
additional mechanisms may only be used when communicating with
principals known to also support them. The following mechanisms
from [RFC3961] and [RFC3962] SHOULD be supported:
Encryption: AES128-CTS-HMAC-SHA1-96, DES-CBC-MD5, DES3-CBC-SHA1-KD
Checksums: DES-MD5, HMAC-SHA1-DES3-KD, HMAC-SHA1-96-AES128
Implementations MAY support other mechanisms as well, but the
additional mechanisms may only be used when communicating with
principals known to support them also.
Implementation note: Earlier implementations of Kerberos generate
messages using the CRC-32 and RSA-MD5 checksum methods. For
interoperability with these earlier releases, implementors MAY
consider supporting these checksum methods but should carefully
analyze the security implications to limit the situations within
which these methods are accepted.
Realm Names
All implementations MUST understand hierarchical realms in both
the Internet Domain and the X.500 style. When a TGT for an
unknown realm is requested, the KDC MUST be able to determine the
names of the intermediate realms between the KDCs realm and the
requested realm.
Transited Field Encoding
DOMAIN-X500-COMPRESS (described in Section 3.3.3.2) MUST be
supported. Alternative encodings MAY be supported, but they may
only be used when that encoding is supported by ALL intermediate
realms.
Pre-authentication Methods
The TGS-REQ method MUST be supported. It is not used on the
initial request. The PA-ENC-TIMESTAMP method MUST be supported by
clients, but whether it is enabled by default MAY be determined on
a realm-by-realm basis. If the method is not used in the initial
request and the error KDC_ERR_PREAUTH_REQUIRED is returned
specifying PA-ENC-TIMESTAMP as an acceptable method, the client
SHOULD retry the initial request using the PA-ENC-TIMESTAMP pre-
authentication method. Servers need not support the PA-ENC-
TIMESTAMP method, but if it is not supported the server SHOULD
ignore the presence of PA-ENC-TIMESTAMP pre-authentication in a
request.
The ETYPE-INFO2 method MUST be supported; this method is used to
communicate the set of supported encryption types, and
corresponding salt and string to key parameters. The ETYPE-INFO
method SHOULD be supported for interoperability with older
implementation.
Mutual Authentication
Mutual authentication (via the KRB_AP_REP message) MUST be
supported.
Ticket Addresses and Flags
All KDCs MUST pass through tickets that carry no addresses (i.e.,
if a TGT contains no addresses, the KDC will return derivative
tickets). Implementations SHOULD default to requesting
addressless tickets, as this significantly increases
interoperability with network address translation. In some cases,
realms or application servers MAY require that tickets have an
address.
Implementations SHOULD accept directional address type for the
KRB_SAFE and KRB_PRIV message and SHOULD include directional
addresses in these messages when other address types are not
available.
Proxies and forwarded tickets MUST be supported. Individual
realms and application servers can set their own policy on when
such tickets will be accepted.
All implementations MUST recognize renewable and postdated
tickets, but they need not actually implement them. If these
options are not supported, the starttime and endtime in the ticket
SHALL specify a ticket's entire useful life. When a postdated
ticket is decoded by a server, all implementations SHALL make the
presence of the postdated flag visible to the calling server.
User-to-User Authentication
Support for user-to-user authentication (via the ENC-TKT-IN-SKEY
KDC option) MUST be provided by implementations, but individual
realms MAY decide as a matter of policy to reject such requests on
a per-principal or realm-wide basis.
Authorization Data
Implementations MUST pass all authorization data subfields from
TGTs to any derivative tickets unless they are directed to
suppress a subfield as part of the definition of that registered
subfield type. (It is never incorrect to pass on a subfield, and
no registered subfield types presently specify suppression at the
KDC.)
Implementations MUST make the contents of any authorization data
subfields available to the server when a ticket is used.
Implementations are not required to allow clients to specify the
contents of the authorization data fields.
Constant Ranges
All protocol constants are constrained to 32-bit (signed) values
unless further constrained by the protocol definition. This limit
is provided to allow implementations to make assumptions about the
maximum values that will be received for these constants.
Implementations receiving values outside this range MAY reject the
request, but they MUST recover cleanly.
8.2. Recommended KDC Values
Following is a list of recommended values for a KDC configuration.
Minimum lifetime 5 minutes
Maximum renewable lifetime 1 week
Maximum ticket lifetime 1 day
Acceptable clock skew 5 minutes
Empty addresses Allowed
Proxiable, etc. Allowed
9. IANA Considerations
Section 7 of this document specifies protocol constants and other
defined values required for the interoperability of multiple
implementations. Until a subsequent RFC specifies otherwise, or the
Kerberos working group is shut down, allocations of additional
protocol constants and other defined values required for extensions
to the Kerberos protocol will be administered by the Kerberos working
group. Following the recommendations outlined in [RFC2434], guidance
is provided to the IANA as follows:
"reserved" realm name types in Section 6.1 and "other" realm types
except those beginning with "X-" or "x-" will not be registered
without IETF standards action, at which point guidelines for further
assignment will be specified. Realm name types beginning with "X-"
or "x-" are for private use.
For host address types described in Section 7.1, negative values are
for private use. Assignment of additional positive numbers is
subject to review by the Kerberos working group or other expert
review.
Additional key usage numbers, as defined in Section 7.5.1, will be
assigned subject to review by the Kerberos working group or other
expert review.
Additional preauthentication data type values, as defined in section
7.5.2, will be assigned subject to review by the Kerberos working
group or other expert review.
Additional authorization data types as defined in Section 7.5.4, will
be assigned subject to review by the Kerberos working group or other
expert review. Although it is anticipated that there may be
significant demand for private use types, provision is intentionally
not made for a private use portion of the namespace because conflicts
between privately assigned values could have detrimental security
implications.
Additional transited encoding types, as defined in Section 7.5.5,
present special concerns for interoperability with existing
implementations. As such, such assignments will only be made by
standards action, except that the Kerberos working group or another
other working group with competent jurisdiction may make preliminary
assignments for documents that are moving through the standards
process.
Additional Kerberos message types, as described in Section 7.5.7,
will be assigned subject to review by the Kerberos working group or
other expert review.
Additional name types, as described in Section 7.5.8, will be
assigned subject to review by the Kerberos working group or other
expert review.
Additional error codes described in Section 7.5.9 will be assigned
subject to review by the Kerberos working group or other expert
review.
10. Security Considerations
As an authentication service, Kerberos provides a means of verifying
the identity of principals on a network. By itself, Kerberos does
not provide authorization. Applications should not accept the
issuance of a service ticket by the Kerberos server as granting
authority to use the service, since such applications may become
vulnerable to the bypass of this authorization check in an
environment where they inter-operate with other KDCs or where other
options for application authentication are provided.
Denial of service attacks are not solved with Kerberos. There are
places in the protocols where an intruder can prevent an application
from participating in the proper authentication steps. Because
authentication is a required step for the use of many services,
successful denial of service attacks on a Kerberos server might
result in the denial of other network services that rely on Kerberos
for authentication. Kerberos is vulnerable to many kinds of denial
of service attacks: those on the network, which would prevent clients
from contacting the KDC; those on the domain name system, which could
prevent a client from finding the IP address of the Kerberos server;
and those by overloading the Kerberos KDC itself with repeated
requests.
Interoperability conflicts caused by incompatible character-set usage
(see 5.2.1) can result in denial of service for clients that utilize
character-sets in Kerberos strings other than those stored in the KDC
database.
Authentication servers maintain a database of principals (i.e., users
and servers) and their secret keys. The security of the
authentication server machines is critical. The breach of security
of an authentication server will compromise the security of all
servers that rely upon the compromised KDC, and will compromise the
authentication of any principals registered in the realm of the
compromised KDC.
Principals must keep their secret keys secret. If an intruder
somehow steals a principal's key, it will be able to masquerade as
that principal or impersonate any server to the legitimate principal.
Password-guessing attacks are not solved by Kerberos. If a user
chooses a poor password, it is possible for an attacker to
successfully mount an off-line dictionary attack by repeatedly
attempting to decrypt, with successive entries from a dictionary,
messages obtained that are encrypted under a key derived from the
user's password.
Unless pre-authentication options are required by the policy of a
realm, the KDC will not know whether a request for authentication
succeeds. An attacker can request a reply with credentials for any
principal. These credentials will likely not be of much use to the
attacker unless it knows the client's secret key, but the
availability of the response encrypted in the client's secret key
provides the attacker with ciphertext that may be used to mount brute
force or dictionary attacks to decrypt the credentials, by guessing
the user's password. For this reason it is strongly encouraged that
Kerberos realms require the use of pre-authentication. Even with
pre-authentication, attackers may try brute force or dictionary
attacks against credentials that are observed by eavesdropping on the
network.
Because a client can request a ticket for any server principal and
can attempt a brute force or dictionary attack against the server
principal's key using that ticket, it is strongly encouraged that
keys be randomly generated (rather than generated from passwords) for
any principals that are usable as the target principal for a
KRB_TGS_REQ or KRB_AS_REQ messages. [RFC4086]
Although the DES-CBC-MD5 encryption method and DES-MD5 checksum
methods are listed as SHOULD be implemented for backward
compatibility, the single DES encryption algorithm on which these are
based is weak, and stronger algorithms should be used whenever
possible.
Each host on the network must have a clock that is loosely
synchronized to the time of the other hosts; this synchronization is
used to reduce the bookkeeping needs of application servers when they
do replay detection. The degree of "looseness" can be configured on
a per-server basis, but it is typically on the order of 5 minutes.
If the clocks are synchronized over the network, the clock
synchronization protocol MUST itself be secured from network
attackers.
Principal identifiers must not recycled on a short-term basis. A
typical mode of access control will use access control lists (ACLs)
to grant permissions to particular principals. If a stale ACL entry
remains for a deleted principal and the principal identifier is
reused, the new principal will inherit rights specified in the stale
ACL entry. By not reusing principal identifiers, the danger of
inadvertent access is removed.
Proper decryption of an KRB_AS_REP message from the KDC is not
sufficient for the host to verify the identity of the user; the user
and an attacker could cooperate to generate a KRB_AS_REP format
message that decrypts properly but is not from the proper KDC. To
authenticate a user logging on to a local system, the credentials
obtained in the AS exchange may first be used in a TGS exchange to
obtain credentials for a local server. Those credentials must then
be verified by a local server through successful completion of the
Client/Server exchange.
Many RFC 1510-compliant implementations ignore unknown authorization
data elements. Depending on these implementations to honor
authorization data restrictions may create a security weakness.
Kerberos credentials contain clear-text information identifying the
principals to which they apply. If privacy of this information is
needed, this exchange should itself be encapsulated in a protocol
providing for confidentiality on the exchange of these credentials.
Applications must take care to protect communications subsequent to
authentication, either by using the KRB_PRIV or KRB_SAFE messages as
appropriate, or by applying their own confidentiality or integrity
mechanisms on such communications. Completion of the KRB_AP_REQ and
KRB_AP_REP exchange without subsequent use of confidentiality and
integrity mechanisms provides only for authentication of the parties
to the communication and not confidentiality and integrity of the
subsequent communication. Applications applying confidentiality and
integrity protection mechanisms other than KRB_PRIV and KRB_SAFE must
make sure that the authentication step is appropriately linked with
the protected communication channel that is established by the
application.
Unless the application server provides its own suitable means to
protect against replay (for example, a challenge-response sequence
initiated by the server after authentication, or use of a server-
generated encryption subkey), the server must utilize a replay cache
to remember any authenticator presented within the allowable clock
skew. All services sharing a key need to use the same replay cache.
If separate replay caches are used, then an authenticator used with
one such service could later be replayed to a different service with
the same service principal.
If a server loses track of authenticators presented within the
allowable clock skew, it must reject all requests until the clock
skew interval has passed, providing assurance that any lost or
replayed authenticators will fall outside the allowable clock skew
and can no longer be successfully replayed.
Implementations of Kerberos should not use untrusted directory
servers to determine the realm of a host. To allow this would allow
the compromise of the directory server to enable an attacker to
direct the client to accept authentication with the wrong principal
(i.e., one with a similar name, but in a realm with which the
legitimate host was not registered).
Implementations of Kerberos must not use DNS to map one name to
another (canonicalize) in order to determine the host part of the
principal name with which one is to communicate. To allow this
canonicalization would allow a compromise of the DNS to result in a
client obtaining credentials and correctly authenticating to the
wrong principal. Though the client will know who it is communicating
with, it will not be the principal with which it intended to
communicate.
If the Kerberos server returns a TGT for a realm 'closer' than the
desired realm, the client may use local policy configuration to
verify that the authentication path used is an acceptable one.
Alternatively, a client may choose its own authentication path rather
than rely on the Kerberos server to select one. In either case, any
policy or configuration information used to choose or validate
authentication paths, whether by the Kerberos server or client, must
be obtained from a trusted source.
The Kerberos protocol in its basic form does not provide perfect
forward secrecy for communications. If traffic has been recorded by
an eavesdropper, then messages encrypted using the KRB_PRIV message,
or messages encrypted using application-specific encryption under
keys exchanged using Kerberos can be decrypted if the user's,
application server's, or KDC's key is subsequently discovered. This
is because the session key used to encrypt such messages, when
transmitted over the network, is encrypted in the key of the
application server. It is also encrypted under the session key from
the user's TGT when it is returned to the user in the KRB_TGS_REP
message. The session key from the TGT is sent to the user in the
KRB_AS_REP message encrypted in the user's secret key and embedded in
the TGT, which was encrypted in the key of the KDC. Applications
requiring perfect forward secrecy must exchange keys through
mechanisms that provide such assurance, but may use Kerberos for
authentication of the encrypted channel established through such
other means.
11. Acknowledgements
This document is a revision to RFC 1510 which was co-authored with
John Kohl. The specification of the Kerberos protocol described in
this document is the result of many years of effort. Over this
period, many individuals have contributed to the definition of the
protocol and to the writing of the specification. Unfortunately, it
is not possible to list all contributors as authors of this document,
though there are many not listed who are authors in spirit, including
those who contributed text for parts of some sections, who
contributed to the design of parts of the protocol, and who
contributed significantly to the discussion of the protocol in the
IETF common authentication technology (CAT) and Kerberos working
groups.
Among those contributing to the development and specification of
Kerberos were Jeffrey Altman, John Brezak, Marc Colan, Johan
Danielsson, Don Davis, Doug Engert, Dan Geer, Paul Hill, John Kohl,
Marc Horowitz, Matt Hur, Jeffrey Hutzelman, Paul Leach, John Linn,
Ari Medvinsky, Sasha Medvinsky, Steve Miller, Jon Rochlis, Jerome
Saltzer, Jeffrey Schiller, Jennifer Steiner, Ralph Swick, Mike Swift,
Jonathan Trostle, Theodore Ts'o, Brian Tung, Jacques Vidrine, Assar
Westerlund, and Nicolas Williams. Many other members of MIT Project
Athena, the MIT networking group, and the Kerberos and CAT working
groups of the IETF contributed but are not listed.
A. ASN.1 module
KerberosV5Spec2 {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2) modules(4) krb5spec2(2)
} DEFINITIONS EXPLICIT TAGS ::= BEGIN
-- OID arc for KerberosV5
--
-- This OID may be used to identify Kerberos protocol messages
-- encapsulated in other protocols.
--
-- This OID also designates the OID arc for KerberosV5-related OIDs.
--
-- NOTE: RFC 1510 had an incorrect value (5) for "dod" in its OID.
id-krb5 OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) kerberosV5(2)
}
Int32 ::= INTEGER (-2147483648..2147483647)
-- signed values representable in 32 bits
UInt32 ::= INTEGER (0..4294967295)
-- unsigned 32 bit values
Microseconds ::= INTEGER (0..999999)
-- microseconds
KerberosString ::= GeneralString (IA5String)
Realm ::= KerberosString
PrincipalName ::= SEQUENCE {
name-type [0] Int32,
name-string [1] SEQUENCE OF KerberosString
}
KerberosTime ::= GeneralizedTime -- with no fractional seconds
HostAddress ::= SEQUENCE {
addr-type [0] Int32,
address [1] OCTET STRING
}
-- NOTE: HostAddresses is always used as an OPTIONAL field and
-- should not be empty.
HostAddresses -- NOTE: subtly different from rfc1510,
-- but has a value mapping and encodes the same
::= SEQUENCE OF HostAddress
-- NOTE: AuthorizationData is always used as an OPTIONAL field and
-- should not be empty.
AuthorizationData ::= SEQUENCE OF SEQUENCE {
ad-type [0] Int32,
ad-data [1] OCTET STRING
}
PA-DATA ::= SEQUENCE {
-- NOTE: first tag is [1], not [0]
padata-type [1] Int32,
padata-value [2] OCTET STRING -- might be encoded AP-REQ
}
KerberosFlags ::= BIT STRING (SIZE (32..MAX))
-- minimum number of bits shall be sent,
-- but no fewer than 32
EncryptedData ::= SEQUENCE {
etype [0] Int32 -- EncryptionType --,
kvno [1] UInt32 OPTIONAL,
cipher [2] OCTET STRING -- ciphertext
}
EncryptionKey ::= SEQUENCE {
keytype [0] Int32 -- actually encryption type --,
keyvalue [1] OCTET STRING
}
Checksum ::= SEQUENCE {
cksumtype [0] Int32,
checksum [1] OCTET STRING
}
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno [0] INTEGER (5),
realm [1] Realm,
sname [2] PrincipalName,
enc-part [3] EncryptedData -- EncTicketPart
}
-- Encrypted part of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags [0] TicketFlags,
key [1] EncryptionKey,
crealm [2] Realm,
cname [3] PrincipalName,
transited [4] TransitedEncoding,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
caddr [9] HostAddresses OPTIONAL,
authorization-data [10] AuthorizationData OPTIONAL
}
-- encoded Transited field
TransitedEncoding ::= SEQUENCE {
tr-type [0] Int32 -- must be registered --,
contents [1] OCTET STRING
}
TicketFlags ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- may-postdate(5),
-- postdated(6),
-- invalid(7),
-- renewable(8),
-- initial(9),
-- pre-authent(10),
-- hw-authent(11),
-- the following are new since 1510
-- transited-policy-checked(12),
-- ok-as-delegate(13)
AS-REQ ::= [APPLICATION 10] KDC-REQ
TGS-REQ ::= [APPLICATION 12] KDC-REQ
KDC-REQ ::= SEQUENCE {
-- NOTE: first tag is [1], not [0]
pvno [1] INTEGER (5) ,
msg-type [2] INTEGER (10 -- AS -- | 12 -- TGS --),
padata [3] SEQUENCE OF PA-DATA OPTIONAL
-- NOTE: not empty --,
req-body [4] KDC-REQ-BODY
}
KDC-REQ-BODY ::= SEQUENCE {
kdc-options [0] KDCOptions,
cname [1] PrincipalName OPTIONAL
-- Used only in AS-REQ --,
realm [2] Realm
-- Server's realm
-- Also client's in AS-REQ --,
sname [3] PrincipalName OPTIONAL,
from [4] KerberosTime OPTIONAL,
till [5] KerberosTime,
rtime [6] KerberosTime OPTIONAL,
nonce [7] UInt32,
etype [8] SEQUENCE OF Int32 -- EncryptionType
-- in preference order --,
addresses [9] HostAddresses OPTIONAL,
enc-authorization-data [10] EncryptedData OPTIONAL
-- AuthorizationData --,
additional-tickets [11] SEQUENCE OF Ticket OPTIONAL
-- NOTE: not empty
}
KDCOptions ::= KerberosFlags
-- reserved(0),
-- forwardable(1),
-- forwarded(2),
-- proxiable(3),
-- proxy(4),
-- allow-postdate(5),
-- postdated(6),
-- unused7(7),
-- renewable(8),
-- unused9(9),
-- unused10(10),
-- opt-hardware-auth(11),
-- unused12(12),
-- unused13(13),
-- 15 is reserved for canonicalize
-- unused15(15),
-- 26 was unused in 1510
-- disable-transited-check(26),
--
-- renewable-ok(27),
-- enc-tkt-in-skey(28),
-- renew(30),
-- validate(31)
AS-REP ::= [APPLICATION 11] KDC-REP
TGS-REP ::= [APPLICATION 13] KDC-REP
KDC-REP ::= SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (11 -- AS -- | 13 -- TGS --),
padata [2] SEQUENCE OF PA-DATA OPTIONAL
-- NOTE: not empty --,
crealm [3] Realm,
cname [4] PrincipalName,
ticket [5] Ticket,
enc-part [6] EncryptedData
-- EncASRepPart or EncTGSRepPart,
-- as appropriate
}
EncASRepPart ::= [APPLICATION 25] EncKDCRepPart
EncTGSRepPart ::= [APPLICATION 26] EncKDCRepPart
EncKDCRepPart ::= SEQUENCE {
key [0] EncryptionKey,
last-req [1] LastReq,
nonce [2] UInt32,
key-expiration [3] KerberosTime OPTIONAL,
flags [4] TicketFlags,
authtime [5] KerberosTime,
starttime [6] KerberosTime OPTIONAL,
endtime [7] KerberosTime,
renew-till [8] KerberosTime OPTIONAL,
srealm [9] Realm,
sname [10] PrincipalName,
caddr [11] HostAddresses OPTIONAL
}
LastReq ::= SEQUENCE OF SEQUENCE {
lr-type [0] Int32,
lr-value [1] KerberosTime
}
AP-REQ ::= [APPLICATION 14] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (14),
ap-options [2] APOptions,
ticket [3] Ticket,
authenticator [4] EncryptedData -- Authenticator
}
APOptions ::= KerberosFlags
-- reserved(0),
-- use-session-key(1),
-- mutual-required(2)
-- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE {
authenticator-vno [0] INTEGER (5),
crealm [1] Realm,
cname [2] PrincipalName,
cksum [3] Checksum OPTIONAL,
cusec [4] Microseconds,
ctime [5] KerberosTime,
subkey [6] EncryptionKey OPTIONAL,
seq-number [7] UInt32 OPTIONAL,
authorization-data [8] AuthorizationData OPTIONAL
}
AP-REP ::= [APPLICATION 15] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (15),
enc-part [2] EncryptedData -- EncAPRepPart
}
EncAPRepPart ::= [APPLICATION 27] SEQUENCE {
ctime [0] KerberosTime,
cusec [1] Microseconds,
subkey [2] EncryptionKey OPTIONAL,
seq-number [3] UInt32 OPTIONAL
}
KRB-SAFE ::= [APPLICATION 20] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (20),
safe-body [2] KRB-SAFE-BODY,
cksum [3] Checksum
}
KRB-SAFE-BODY ::= SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress,
r-address [5] HostAddress OPTIONAL
}
KRB-PRIV ::= [APPLICATION 21] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (21),
-- NOTE: there is no [2] tag
enc-part [3] EncryptedData -- EncKrbPrivPart
}
EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE {
user-data [0] OCTET STRING,
timestamp [1] KerberosTime OPTIONAL,
usec [2] Microseconds OPTIONAL,
seq-number [3] UInt32 OPTIONAL,
s-address [4] HostAddress -- sender's addr --,
r-address [5] HostAddress OPTIONAL -- recip's addr
}
KRB-CRED ::= [APPLICATION 22] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (22),
tickets [2] SEQUENCE OF Ticket,
enc-part [3] EncryptedData -- EncKrbCredPart
}
EncKrbCredPart ::= [APPLICATION 29] SEQUENCE {
ticket-info [0] SEQUENCE OF KrbCredInfo,
nonce [1] UInt32 OPTIONAL,
timestamp [2] KerberosTime OPTIONAL,
usec [3] Microseconds OPTIONAL,
s-address [4] HostAddress OPTIONAL,
r-address [5] HostAddress OPTIONAL
}
KrbCredInfo ::= SEQUENCE {
key [0] EncryptionKey,
prealm [1] Realm OPTIONAL,
pname [2] PrincipalName OPTIONAL,
flags [3] TicketFlags OPTIONAL,
authtime [4] KerberosTime OPTIONAL,
starttime [5] KerberosTime OPTIONAL,
endtime [6] KerberosTime OPTIONAL,
renew-till [7] KerberosTime OPTIONAL,
srealm [8] Realm OPTIONAL,
sname [9] PrincipalName OPTIONAL,
caddr [10] HostAddresses OPTIONAL
}
KRB-ERROR ::= [APPLICATION 30] SEQUENCE {
pvno [0] INTEGER (5),
msg-type [1] INTEGER (30),
ctime [2] KerberosTime OPTIONAL,
cusec [3] Microseconds OPTIONAL,
stime [4] KerberosTime,
susec [5] Microseconds,
error-code [6] Int32,
crealm [7] Realm OPTIONAL,
cname [8] PrincipalName OPTIONAL,
realm [9] Realm -- service realm --,
sname [10] PrincipalName -- service name --,
e-text [11] KerberosString OPTIONAL,
e-data [12] OCTET STRING OPTIONAL
}
METHOD-DATA ::= SEQUENCE OF PA-DATA
TYPED-DATA ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
data-type [0] Int32,
data-value [1] OCTET STRING OPTIONAL
}
-- preauth stuff follows
PA-ENC-TIMESTAMP ::= EncryptedData -- PA-ENC-TS-ENC
PA-ENC-TS-ENC ::= SEQUENCE {
patimestamp [0] KerberosTime -- client's time --,
pausec [1] Microseconds OPTIONAL
}
ETYPE-INFO-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] OCTET STRING OPTIONAL
}
ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY
ETYPE-INFO2-ENTRY ::= SEQUENCE {
etype [0] Int32,
salt [1] KerberosString OPTIONAL,
s2kparams [2] OCTET STRING OPTIONAL
}
ETYPE-INFO2 ::= SEQUENCE SIZE (1..MAX) OF ETYPE-INFO2-ENTRY
AD-IF-RELEVANT ::= AuthorizationData
AD-KDCIssued ::= SEQUENCE {
ad-checksum [0] Checksum,
i-realm [1] Realm OPTIONAL,
i-sname [2] PrincipalName OPTIONAL,
elements [3] AuthorizationData
}
AD-AND-OR ::= SEQUENCE {
condition-count [0] Int32,
elements [1] AuthorizationData
}
AD-MANDATORY-FOR-KDC ::= AuthorizationData
END
B. Changes since RFC 1510
This document replaces RFC 1510 and clarifies specification of items
that were not completely specified. Where changes to recommended
implementation choices were made, or where new options were added,
those changes are described within the document and listed in this
section. More significantly, "Specification 2" in Section 8 changes
the required encryption and checksum methods to bring them in line
with the best current practices and to deprecate methods that are no
longer considered sufficiently strong.
Discussion was added to Section 1 regarding the ability to rely on
the KDC to check the transited field, and on the inclusion of a flag
in a ticket indicating that this check has occurred. This is a new
capability not present in RFC 1510. Pre-existing implementations may
ignore or not set this flag without negative security implications.
The definition of the secret key says that in the case of a user the
key may be derived from a password. In RFC 1510, it said that the
key was derived from the password. This change was made to
accommodate situations where the user key might be stored on a
smart-card, or otherwise obtained independently of a password.
The introduction mentions the use of public key cryptography for
initial authentication in Kerberos by reference. RFC 1510 did not
include such a reference.
Section 1.3 was added to explain that while Kerberos provides
authentication of a named principal, it is still the responsibility
of the application to ensure that the authenticated name is the
entity with which the application wishes to communicate.
Discussion of extensibility has been added to the introduction.
Discussion of how extensibility affects ticket flags and KDC options
was added to the introduction of Section 2. No changes were made to
existing options and flags specified in RFC 1510, though some of the
sections in the specification were renumbered, and text was revised
to make the description and intent of existing options clearer,
especially with respect to the ENC-TKT-IN-SKEY option (now section
2.9.2) which is used for user-to-user authentication. The new option
and ticket flag transited policy checking (Section 2.7) was added.
A warning regarding generation of session keys for application use
was added to Section 3, urging the inclusion of key entropy from the
KDC generated session key in the ticket. An example regarding use of
the sub-session key was added to Section 3.2.6. Descriptions of the
pa-etype-info, pa-etype-info2, and pa-pw-salt pre-authentication data
items were added. The recommendation for use of pre-authentication
was changed from "MAY" to "SHOULD" and a note was added regarding
known plaintext attacks.
In RFC 1510, Section 4 described the database in the KDC. This
discussion was not necessary for interoperability and unnecessarily
constrained implementation. The old Section 4 was removed.
The current Section 4 was formerly Section 6 on encryption and
checksum specifications. The major part of this section was brought
up to date to support new encryption methods, and moved to a separate
document. Those few remaining aspects of the encryption and checksum
specification specific to Kerberos are now specified in Section 4.
Significant changes were made to the layout of Section 5 to clarify
the correct behavior for optional fields. Many of these changes were
made necessary because of improper ASN.1 description in the original
Kerberos specification which left the correct behavior
underspecified. Additionally, the wording in this section was
tightened wherever possible to ensure that implementations conforming
to this specification will be extensible with the addition of new
fields in future specifications.
Text was added describing time_t=0 issues in the ASN.1. Text was
also added, clarifying issues with implementations treating omitted
optional integers as zero. Text was added clarifying behavior for
optional SEQUENCE or SEQUENCE OF that may be empty. Discussion was
added regarding sequence numbers and behavior of some
implementations, including "zero" behavior and negative numbers. A
compatibility note was added regarding the unconditional sending of
EncTGSRepPart regardless of the enclosing reply type. Minor changes
were made to the description of the HostAddresses type. Integer
types were constrained. KerberosString was defined as a
(significantly) constrained GeneralString. KerberosFlags was defined
to reflect existing implementation behavior that departs from the
definition in RFC 1510. The transited-policy-checked(12) and the
ok-as-delegate(13) ticket flags were added. The disable-transited-
check(26) KDC option was added.
Descriptions of commonly implemented PA-DATA were added to Section 5.
The description of KRB-SAFE has been updated to note the existing
implementation behavior of double-encoding.
There were two definitions of METHOD-DATA in RFC 1510. The second
one, intended for use with KRB_AP_ERR_METHOD was removed leaving the
SEQUENCE OF PA-DATA definition.
Section 7, naming constraints, from RFC 1510 was moved to Section 6.
Words were added describing the convention that domain-based realm
names for newly-created realms should be specified as uppercase.
This recommendation does not make lowercase realm names illegal.
Words were added highlighting that the slash-separated components in
the X.500 style of realm names is consistent with existing RFC 1510
based implementations, but that it conflicts with the general
recommendation of X.500 name representation specified in RFC 2253.
Section 8, network transport, constants and defined values, from RFC
1510 was moved to Section 7. Since RFC 1510, the definition of the
TCP transport for Kerberos messages was added, and the encryption and
checksum number assignments have been moved into a separate document.
"Specification 2" in Section 8 of the current document changes the
required encryption and checksum methods to bring them in line with
the best current practices and to deprecate methods that are no
longer considered sufficiently strong.
Two new sections, on IANA considerations and security considerations
were added.
The pseudo-code has been removed from the appendix. The pseudo-code
was sometimes misinterpreted to limit implementation choices and in
RFC 1510, it was not always consistent with the words in the
specification. Effort was made to clear up any ambiguities in the
specification, rather than to rely on the pseudo-code.
An appendix was added containing the complete ASN.1 module drawn from
the discussion in Section 5 of the current document.
END NOTES
(*TM) Project Athena, Athena, and Kerberos are trademarks of the
Massachusetts Institute of Technology (MIT).
Normative References
[RFC3961] Raeburn, K., "Encryption and Checksum
Specifications for Kerberos 5", RFC 3961, February
2005.
[RFC3962] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February
2005.
[ISO-646/ECMA-6] International Organization for Standardization,
"7-bit Coded Character Set for Information
Interchange", ISO/IEC 646:1991.
[ISO-2022/ECMA-35] International Organization for Standardization,
"Character code structure and extension
techniques", ISO/IEC 2022:1994.
[RFC1035] Mockapetris, P., "Domain names - implementation
and specification", STD 13, RFC 1035, November
1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 2434, October 1998.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS
RR for specifying the location of services (DNS
SRV)", RFC 2782, February 2000.
[RFC2253] Wahl, M., Kille, S., and T. Howes, "Lightweight
Directory Access Protocol (v3): UTF-8 String
Representation of Distinguished Names", RFC 2253,
December 1997.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol
Version 6 (IPv6) Addressing Architecture", RFC
3513, April 2003.
[X680] Abstract Syntax Notation One (ASN.1):
Specification of Basic Notation, ITU-T
Recommendation X.680 (1997) | ISO/IEC
International Standard 8824-1:1998.
[X690] ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules
(CER) and Distinguished Encoding Rules (DER),
ITU-T Recommendation X.690 (1997)| ISO/IEC
International Standard 8825-1:1998.
Informative References
[ISO-8859] International Organization for Standardization,
"8-bit Single-byte Coded Graphic Character Sets --
Latin Alphabet", ISO/IEC 8859.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API
Mechanism", RFC 1964, June 1996.
[DGT96] Don Davis, Daniel Geer, and Theodore Ts'o,
"Kerberos With Clocks Adrift: History, Protocols,
and Implementation", USENIX Computing Systems 9:1,
January 1996.
[DS81] Dorothy E. Denning and Giovanni Maria Sacco,
"Time-stamps in Key Distribution Protocols,"
Communications of the ACM, Vol. 24 (8), p. 533-
536, August 1981.
[KNT94] John T. Kohl, B. Clifford Neuman, and Theodore Y.
Ts'o, "The Evolution of the Kerberos
Authentication System". In Distributed Open
Systems, pages 78-94. IEEE Computer Society Press,
1994.
[MNSS87] S. P. Miller, B. C. Neuman, J. I. Schiller, and J.
H. Saltzer, Section E.2.1: Kerberos Authentication
and Authorization System, M.I.T. Project Athena,
Cambridge, Massachusetts, December 21, 1987.
[NS78] Roger M. Needham and Michael D. Schroeder, "Using
Encryption for Authentication in Large Networks of
Computers," Communications of the ACM, Vol. 21
(12), pp. 993-999, December 1978.
[Neu93] B. Clifford Neuman, "Proxy-Based Authorization and
Accounting for Distributed Systems," in
Proceedings of the 13th International Conference
on Distributed Computing Systems, Pittsburgh, PA,
May 1993.
[NT94] B. Clifford Neuman and Theodore Y. Ts'o, "An
Authentication Service for Computer Networks,"
IEEE Communications Magazine, Vol. 32 (9), p. 33-
38, September 1994.
[Pat92] J. Pato, Using Pre-Authentication to Avoid
Password Guessing Attacks, Open Software
Foundation DCE Request for Comments 26 (December
1992.
[RFC1510] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September
1993.
[RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106,
RFC 4086, June 2005.
[SNS88] J. G. Steiner, B. C. Neuman, and J. I. Schiller,
"Kerberos: An Authentication Service for Open
Network Systems," p. 191-202, Usenix Conference
Proceedings, Dallas, Texas, February 1988.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The
Kerberos Version 5 Generic Security Service
Application Program Interface (GSS-API) Mechanism:
Version 2", RFC 4121, July 2005.
Authors' Addresses
Clifford Neuman
Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, CA 90292, USA
EMail: bcn@isi.edu
Tom Yu
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139, USA
EMail: tlyu@mit.edu
Sam Hartman
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139, USA
EMail: hartmans-ietf@mit.edu
Kenneth Raeburn
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139, USA
EMail: raeburn@mit.edu
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