| Rfc | 8224 |
|---|
| Title | Authenticated Identity Management in the Session Initiation Protocol
(SIP) |
|---|
| Author | J. Peterson, C. Jennings, E. Rescorla, C. Wendt |
|---|
| Date | February
2018 |
|---|
| Format: | TXT, HTML |
|---|
| Obsoletes | RFC4474 |
|---|
| Updated by | RFC8946 |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Internet Engineering Task Force (IETF) J. Peterson
Request for Comments: 8224 NeuStar
Obsoletes: 4474 C. Jennings
Category: Standards Track Cisco
ISSN: 2070-1721 E. Rescorla
RTFM, Inc.
C. Wendt
Comcast
February 2018
Authenticated Identity Management
in the Session Initiation Protocol (SIP)
Abstract
The baseline security mechanisms in the Session Initiation Protocol
(SIP) are inadequate for cryptographically assuring the identity of
the end users that originate SIP requests, especially in an
interdomain context. This document defines a mechanism for securely
identifying originators of SIP requests. It does so by defining a
SIP header field for conveying a signature used for validating the
identity and for conveying a reference to the credentials of the
signer.
This document obsoletes RFC 4474.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8224.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
3. Architectural Overview ..........................................5
4. Identity Header Field Syntax ....................................7
4.1. PASSporT Construction ......................................8
4.1.1. Example Full and Compact Forms of PASSporT
in Identity ........................................10
5. Example of Operations ..........................................11
5.1. Example Identity Header Construction ......................13
6. Signature Generation and Validation ............................14
6.1. Authentication Service Behavior ...........................14
6.1.1. Handling Repairable Errors .........................16
6.2. Verifier Behavior .........................................17
6.2.1. Authorization of Requests ..........................19
6.2.2. Failure Response Codes Sent by a
Verification Service ...............................19
6.2.3. Handling Retried Requests ..........................21
6.2.4. Handling the Full Form of PASSporT .................21
7. Credentials ....................................................22
7.1. Credential Use by the Authentication Service ..............22
7.2. Credential Use by the Verification Service ................23
7.3. "info" Parameter URIs .....................................24
7.4. Credential System Requirements ............................25
8. Identity Types .................................................26
8.1. Differentiating Telephone Numbers from URIs ...............26
8.2. Authority for Telephone Numbers ...........................27
8.3. Telephone Number Canonicalization Procedures ..............28
8.4. Authority for Domain Names ................................29
8.5. URI Normalization .........................................30
9. Extensibility ..................................................31
10. Backwards Compatibility with RFC 4474 .........................32
11. Privacy Considerations ........................................32
12. Security Considerations .......................................34
12.1. Protected Request Fields .................................34
12.1.1. Protection of the To Header and Retargeting .......36
12.2. Unprotected Request Fields ...............................37
12.3. Malicious Removal of Identity Headers ....................37
12.4. Securing the Connection to the Authentication Service ....38
12.5. Authorization and Transitional Strategies ................39
12.6. Display-Names and Identity ...............................40
13. IANA Considerations ...........................................40
13.1. SIP Header Fields ........................................40
13.2. SIP Response Codes .......................................41
13.3. Identity-Info Parameters .................................41
13.4. Identity-Info Algorithm Parameter Values .................41
14. Changes from RFC 4474 .........................................41
15. References ....................................................42
15.1. Normative References .....................................42
15.2. Informative References ...................................43
Acknowledgments ...................................................46
Authors' Addresses ................................................46
1. Introduction
This document provides enhancements to the existing mechanisms for
authenticated identity management in the Session Initiation Protocol
(SIP) [RFC3261]. An identity, for the purposes of this document, is
defined as either
o a canonical address-of-record (AoR) SIP URI employed to reach a
user (such as "sip:alice@atlanta.example.com") or
o a telephone number, which commonly appears either in a tel URI
[RFC3966] or as the user portion of a SIP URI.
[RFC3261] specifies several places within a SIP request where users
can express an identity for themselves, most prominently the
user-populated From header field. However, in the absence of some
sort of cryptographic authentication mechanism, the recipient of a
SIP request has no way to verify that the From header field has been
populated appropriately. This leaves SIP vulnerable to a category of
abuses such as impersonation attacks that facilitate or enable
robocalling, voicemail hacking, swatting, and related problems as
described in [RFC7340]. Ideally, a cryptographic approach to
identity can provide a much stronger assurance of identity than the
Caller ID services that the telephone network provides today, and one
less vulnerable to spoofing.
[RFC3261] encourages user agents (UAs) to implement a number of
potential authentication mechanisms, including Digest authentication,
Transport Layer Security (TLS), and S/MIME (implementations may
support other security schemes as well). However, few SIP UAs today
support the end-user certificates necessary to authenticate
themselves (via S/MIME, for example), and for its part Digest
authentication is limited by the fact that the originator and
destination must share a prearranged secret. Practically speaking,
originating UAs need to be able to securely communicate their users'
identities to destinations with which they have no previous
association.
As an initial attempt to address this gap, [RFC4474] specified a
means of signing portions of SIP requests in order to provide an
identity assurance. However, [RFC4474] was in several ways
misaligned with deployment realities (see [SIP-RFC4474-CONCERNS]).
Most significantly, [RFC4474] did not deal well with telephone
numbers as identifiers, despite their enduring use in SIP
deployments. [RFC4474] also provided a signature over material that
intermediaries in existing deployments commonly altered. This
specification therefore deprecates the syntax and behavior specified
by [RFC4474], reconsidering the problem space in light of the threat
model in [RFC7375] and aligning the signature format with PASSporT
(Personal Assertion Token) [RFC8225]. Backwards-compatibility
considerations are given in Section 10.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119].
In addition, this document uses three terms specific to the
mechanism:
o Identity: An identifier for the user of a communications service;
for the purposes of SIP, either a SIP URI or a telephone number.
Identities are derived from an "identity field" in a SIP request
such as the From header field.
o Authentication Service: A logical role played by a SIP entity that
adds Identity headers to SIP requests.
o Verification Service (or "Verifier"): A logical role played by a
SIP entity that validates Identity headers in a SIP request.
3. Architectural Overview
The identity architecture for SIP defined in this specification
depends on a logical "authentication service" that validates outgoing
requests. An authentication service may be implemented either as
part of a UA or as a proxy server; typically, it is a component of a
network intermediary like a proxy to which originating UAs send
unsigned requests. Once the originator of the message has been
authenticated, through prearranged means with the authentication
service, the authentication service then creates and adds an Identity
header field to the request. This requires computing cryptographic
information -- including a digital signature over some components of
messages -- that lets other SIP entities verify that the sending user
has been authenticated and its claim of a particular identity has
been authorized. These "verification services" validate the
signature and enable policy decisions to be made based on the results
of the validation.
Policy decisions made after validation depend heavily on the
verification service's trust for the credentials that the
authentication service uses to sign requests. As robocalling,
voicemail hacking, and swatting usually involve impersonation of
telephone numbers, credentials that will be trusted by relying
parties to sign for telephone numbers are a key component of the
architecture. Authority over telephone numbers is, however, not as
easy to establish on the Internet as authority over traditional
domain names. This document assumes the existence of credentials for
establishing authority over telephone numbers for cases where the
telephone number is the identity of the user, but does not mandate or
specify a credential system; [RFC8226] describes a credential system
compatible with this architecture.
Although addressing the vulnerabilities in the Secure Telephone
Identity Revisited (STIR) problem statement [RFC7340] and threat
model mostly requires dealing with telephone number as identities,
SIP must also handle signing for SIP URIs as identities. This is
typically easier to deal with, as these identities are issued by
organizations that have authority over Internet domains. When a new
user becomes associated with example.com, for example, the
administrator of the SIP service for that domain can issue them an
identity in that namespace, such as sip:alice@example.com. Alice may
then send REGISTER requests to example.com that make her UAs eligible
to receive requests for sip:alice@example.com. In other cases, Alice
may herself be the owner of her own domain and may issue herself
identities as she chooses. But ultimately, it is the controller of
the SIP service at example.com that must be responsible for
authorizing the use of names in the example.com domain. Therefore,
for the purposes of SIP as defined in [RFC3261], the necessary
credentials needed to prove that a user is authorized to use a
particular From header field must ultimately derive from the domain
owner: either (1) a UA gives requests to the domain name owner in
order for them to be signed by the domain owner's credentials or
(2) the UA must possess credentials that prove that the domain owner
has given the UA the right to a name.
In order to share a cryptographic assurance of end-user SIP identity
in an interdomain or intradomain context, an authentication service
constructs tokens based on the PASSporT format [RFC8225], which is
special encoding of a JSON [RFC8259] object comprising values derived
from certain header field values in the SIP request. The
authentication service computes a signature over those JSON elements
as PASSporT specifies. An encoding of the resulting PASSporT is then
placed in the SIP Identity header field. In order to assist in the
validation of the Identity header field, this specification also
describes a parameter of the Identity header field that can be used
by the recipient of a request to recover the credentials of the
signer.
Note that the scope of this document is limited to providing an
identity assurance for SIP requests; solving this problem for SIP
responses is outside the scope of this work (see [RFC4916]). Future
work might specify ways that a SIP implementation could gateway
PASSporTs to other protocols.
4. Identity Header Field Syntax
The Identity and Identity-Info header fields that were previously
defined in [RFC4474] are deprecated by this document. This revised
specification collapses the grammar of Identity-Info into the
Identity header field via the "info" parameter. Note that unlike the
prior specification in [RFC4474], the Identity header field is now
allowed to appear more than one time in a SIP request. The revised
grammar for the Identity header field builds on the ABNF [RFC5234] in
[RFC3261], Section 25. It is as follows:
Identity = "Identity" HCOLON signed-identity-digest SEMI
ident-info *( SEMI ident-info-params )
signed-identity-digest = 1*(base64-char / ".")
ident-info = "info" EQUAL ident-info-uri
ident-info-uri = LAQUOT absoluteURI RAQUOT
ident-info-params = ident-info-alg / ident-type /
ident-info-extension
ident-info-alg = "alg" EQUAL token
ident-type = "ppt" EQUAL token
ident-info-extension = generic-param
base64-char = ALPHA / DIGIT / "/" / "+"
In addition to the "info" parameter, and the "alg" parameter
previously defined in [RFC4474], this specification defines the
optional "ppt" parameter (PASSporT Type). The "absoluteURI" portion
of ident-info-uri MUST contain a URI; see Section 7.3 for more on
choosing how to advertise credentials through this parameter.
The signed-identity-digest contains a base64 encoding of a PASSporT
[RFC8225], which secures the request with a signature that PASSporT
generates over the JSON header and payload objects; some of those
header and claim element values will mirror values of the SIP
request.
4.1. PASSporT Construction
For SIP implementations to populate the PASSporT header JSON object
with fields from a SIP request, the following elements MUST be placed
as the values corresponding to the designated JSON keys:
o First, per the baseline PASSporT specification [RFC8225], the JSON
"typ" key MUST have the value "passport".
o Second, the JSON key "alg" MUST mirror the value of the optional
"alg" parameter in the SIP Identity header field. Note that if
the "alg" parameter is absent from the Identity header, the
default value is "ES256".
o Third, the JSON key "x5u" MUST have a value equivalent to the
quoted URI in the "info" parameter, per the simple string
comparison rules of [RFC3986], Section 6.2.1.
o Fourth, if a PASSporT extension is in use, then the optional JSON
key "ppt" MUST be present and have a value equivalent to the
quoted value of the "ppt" parameter of the Identity header field.
An example of the PASSporT header JSON object without any
extension is:
{ "typ":"passport",
"alg":"ES256",
"x5u":"https://www.example.com/cert.cer" }
To populate the PASSporT payload JSON object from a SIP request, the
following elements MUST be placed as values corresponding to the
designated JSON keys:
o First, the JSON "orig" object MUST be populated. If the
originating identity is a telephone number, then the array MUST be
populated with a JSON object containing a "tn" element with a
value set to the value of the quoted originating identity, a
canonicalized telephone number (see Section 8.3). Otherwise, the
object MUST be populated with a JSON object containing a "uri"
element, set to the value of the AoR of the UA sending the message
as taken from the addr-spec of the From header field, per the
procedures in Section 8.5.
o Second, the JSON "dest" array MUST be populated. If the
destination identity is a telephone number, then the array MUST be
populated with a JSON object containing a "tn" element with a
value set to the value of the quoted destination identity, a
canonicalized telephone number (see Section 8.3). Otherwise, the
array MUST be populated with a JSON object containing a "uri"
element, set to the value of the addr-spec component of the
To header field, which is the AoR to which the request is being
sent, per the procedures in Section 8.5. Multiple JSON objects
are permitted in "dest" for future compatibility reasons.
o Third, the JSON key "iat" MUST appear. The authentication service
SHOULD set the value of "iat" to an encoding of the value of the
SIP Date header field as a JSON NumericDate (as UNIX time, per
[RFC7519], Section 2), though an authentication service MAY set
the value of "iat" to its own current clock time. If the
authentication service uses its own clock time, then the use of
the full form of PASSporT is REQUIRED. In either case, the
authentication service MUST NOT generate a PASSporT for a SIP
request if the Date header is outside of its local policy for
freshness (sixty seconds is RECOMMENDED).
o Fourth, if the request contains a Session Description Protocol
(SDP) message body and if that SDP contains one or more
"a=fingerprint" attributes, then the JSON key "mky" MUST appear
with the algorithm(s) and value(s) of the fingerprint attributes
(if they differ), following the format given in [RFC8225],
Section 5.2.2.
For example:
{ "orig":{"tn":"12155551212"},
"dest":{"tn":["12155551213"]},
"iat":1443208345 }
For information on the security properties of these SIP message
elements and why their inclusion mitigates replay attacks, see
Section 12. Note that future extensions to PASSporT could introduce
new claims and that further SIP procedures could be required to
extract information from the SIP request to populate the values of
those claims; see Section 9 of this document.
The "orig" and "dest" arrays may contain identifiers of heterogeneous
type; for example, the "orig" array might contain a "tn" claim, while
the "dest" contains a "uri" claim. Also note that in some cases, the
"dest" array may be populated with more than one value. This could,
for example, occur when multiple "dest" identities are specified in a
meshed conference. Defining how a SIP implementation would align
multiple destination identities in PASSporT with such systems is left
as a subject for future specifications.
After these two JSON objects, the header and the payload, have been
constructed and base64-encoded, they must each be hashed and signed
per [RFC8225], Section 6. The header, payload, and signature
components comprise a full PASSporT object. The resulting PASSporT
may be carried in SIP in either (1) a full form, which includes the
header and payload as well as the signature or (2) a compact form,
which only carries the signature per [RFC8225], Section 7. The
hashing and signing algorithm is specified by the "alg" parameter of
the Identity header field and the mirrored "alg" parameter of
PASSporT. All implementations of this specification MUST support the
required signing algorithms of PASSporT. At present, there is one
mandatory-to-support value for the "alg" parameter: "ES256", as
defined in [RFC7519], which connotes an Elliptic Curve Digital
Signature Algorithm (ECDSA) P-256 digital signature.
4.1.1. Example Full and Compact Forms of PASSporT in Identity
As Appendix F of the JSON Web Signature (JWS) specification [RFC7515]
notes, there are cases where "it is useful to integrity-protect
content that is not itself contained in a JWS." Since the fields
that make up the majority of the PASSporT header and payload have
values replicated in the SIP request, the SIP usage of PASSporT may
exclude the base64-encoded version of the header and payload JSON
objects from the Identity header field and instead present a detached
signature: what PASSporT calls its compact form; see [RFC8225],
Section 7.
When an authentication service constructs an Identity header, the
contents of the signed-identity-digest field MUST contain either a
full or compact PASSporT. Use of the compact form is RECOMMENDED in
order to reduce message size, but note that extensions often require
the full form (see Section 9).
For example, a full form of PASSporT in an Identity header might look
as follows (backslashes shown for line folding only):
Identity: eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1I \
joiaHR0cHM6Ly9jZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJ \
kZXN0Ijp7InVyaSI6WyJzaXA6YWxpY2VAZXhhbXBsZS5jb20iXX0sImlhdC \
I6IjE0NDMyMDgzNDUiLCJvcmlnIjp7InRuIjoiMTIxNTU1NTEyMTIifX0.r \
q3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \
ojNCpTzO3QfPOlckGaS6hEck7w;info=<https://biloxi.example.org \
/biloxi.cert>
The compact form of the same PASSporT object would appear in the
Identity header as:
Identity: ..rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qj \
pjlk-cpFYpFYsojNCpTzO3QfPOlckGaS6hEck7w; \
info=<https://biloxi.example.org/biloxi.cert>
5. Example of Operations
This section provides an informative (non-normative) high-level
example of the operation of the mechanisms described in this
document.
Imagine a case where Bob, who has the home proxy of example.com and
the AoR sip:12155551212@example.com;user=phone, wants to communicate
with Alice at sip:alice@example.com. They have no prior
relationship, and Alice implements best practices to prevent
impersonation attacks.
Bob's UA generates an INVITE and places his AoR in the From header
field of the request. He then sends an INVITE to an authentication
service proxy for his domain.
............................ ..............................
. . . .
. +-------+ . . +-------+ .
. Signs for | | . Signed . | | .
. 12125551xxx| Auth |------------> | Verif | .
. | Svc | . INVITE . | Svc | .
. | Proxy | . . | Proxy | .
. > +-------+ . . +-------+ \ .
. / | . -> \ .
. / | . --. \ .
. / | . -- . \ .
. / | . -- . \ .
. / +-------+. -- . \ .
. / | |.<- . \ .
. / | Cert |. . > .
. +-------+ | Store |. . +-------+ .
. | | | |. . | | .
. | Bob | +-------+. . | Alice | .
. | UA | . . | UA | .
. | | . . | | .
. +-------+ . . +-------+ .
. Domain A . . Domain B .
............................ ..............................
The proxy authenticates Bob and validates that he is authorized to
assert the identity that he populated in the From header field. The
proxy authentication service then constructs a PASSporT that contains
a JSON representation of values that mirror certain parts of the SIP
request, including the identity in the From header field value. As a
part of generating the PASSporT, the authentication service signs a
hash of that JSON header and payload with the private key associated
with the appropriate credential for the identity (in this example, a
certificate with authority to sign for numbers in a range from
12155551000 to 12155551999), and the signature is inserted by the
proxy server into the Identity header field value of the request as a
compact form of PASSporT. Alternatively, the JSON header and payload
themselves might also have been included in the object when using the
full form of PASSporT.
The proxy authentication service, as the holder of a private key with
authority over Bob's telephone number, is asserting that the
originator of this request has been authenticated and that he is
authorized to claim the identity that appears in the From header
field. The proxy inserts an "info" parameter into the Identity
header field that tells Alice how to acquire keying material
necessary to validate its credentials (a public key), in case she
doesn't already have it.
When Alice's domain receives the request, a proxy verification
service validates the signature provided in the Identity header field
and then determines that the authentication service credentials
demonstrate authority over the identity in the From header field.
This same validation operation might be performed by a verification
service in Alice's UA server (UAS). Ultimately, this valid request
is rendered to Alice. If the validation were unsuccessful, some
other treatment could be applied by the receiving domain or
Alice's UA.
5.1. Example Identity Header Construction
For the following SIP request:
INVITE sip:alice@example.com SIP/2.0
Via: SIP/2.0/TLS pc33.atlanta.example.com;branch=z9hG4bKnashds8
To: Alice <sip:alice@example.com>
From: Bob <sip:12155551212@example.com;user=phone>;tag=1928301774>
Call-ID: a84b4c76e66710
CSeq: 314159 INVITE
Max-Forwards: 70
Date: Fri, 25 Sep 2015 19:12:25 GMT
Contact: <sip:12155551212@gateway.example.com>
Content-Type: application/sdp
Content-Length: ...
v=0
o=UserA 2890844526 2890844526 IN IP4 pc33.atlanta.example.com
s=Session SDP
c=IN IP4 pc33.atlanta.example.com
t=0 0
m=audio 49172 RTP/AVP 0
a=rtpmap:0 PCMU/8000
An authentication service will create a corresponding PASSporT
object. The properly serialized PASSporT header and payload JSON
objects would look as follows. For the header, the values chosen by
the authentication service at "example.com" might read:
{"alg":"ES256","typ":"passport","x5u":"https://cert.example.org/
passport.cer"}
The serialized payload will derive values from the SIP request (the
From, To, and Date header field values) as follows:
{"dest":{"uri":["sip:alice@example.com"]},"iat":1443208345,
"orig":{"tn":"12155551212"}}
The authentication service would then generate the signature over the
object, following the procedures in [RFC8225], Section 6. That
signature would look as follows:
rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \
ojNCpTzO3QfPOlckGaS6hEck7w
An authentication service signing this request and using the compact
form of PASSporT would thus generate and add to the request an
Identity header field of the following form:
Identity: ..rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpj \
lk-cpFYpFYsojNCpTzO3QfPOlckGaS6hEck7w; \
info=<https://cert.example.org/passport.cer>
6. Signature Generation and Validation
SIP entities that instantiate the authentication service and
verification service roles will, respectively, generate and validate
the Identity header and the signature it contains.
6.1. Authentication Service Behavior
Any entity that instantiates the authentication service role MUST
possess the private key of one or more credentials that can be used
to sign for a domain or a telephone number (see Section 7.1). The
authentication service role can be instantiated, for example, by an
intermediary such as a proxy server or by a UA. Intermediaries that
instantiate this role MUST be capable of authenticating one or more
SIP users who can register for that identity. Commonly, this role
will be instantiated by a proxy server, since proxy servers are more
likely to have a static hostname, hold corresponding credentials, and
have access to SIP registrar capabilities that allow them to
authenticate users. It is also possible that the authentication
service role might be instantiated by an entity that acts as a
redirect server, but that is left as a topic for future work.
An authentication service adds the Identity header field to SIP
requests. The procedures below define the steps that must be taken
when each Identity header field is added. More than one Identity
header field may appear in a single request, and an authentication
service may add an Identity header field to a request that already
contains one or more Identity header fields.
Entities instantiating the authentication service role perform the
following steps, in order, to generate an Identity header field for a
SIP request:
Step 1: Check Authority for the Identity
First, the authentication service must determine whether it is
authoritative for the identity of the originator of the request. The
authentication service extracts the identity from the URI value from
the "identity field"; in ordinary operations, that is the addr-spec
component of the From header field. In order to determine whether
the signature for the identity field should be over the entire
identity field URI or just a telephone number, the authentication
service MUST follow the process described in Section 8.1. The
information in that section will lead to either the telephone number
canonicalization procedures in Section 8.3 for telephone numbers or
the URI normalization procedures described in Section 8.5 for domain
names. Whichever the result, if the authentication service is not
authoritative for the identity in question, it SHOULD process and
forward the request normally unless the local policy is to block such
requests. The authentication service MUST NOT add an Identity header
field if the authentication service does not have the authority to
make the claim it asserts.
Step 2: Authenticate the Originator
The authentication service MUST then determine whether or not the
originator of the request is authorized to claim the identity given
in the identity field. In order to do so, the authentication service
MUST authenticate the originator of the message. Some possible ways
in which this authentication might be performed include the
following:
o If the authentication service is instantiated by a SIP
intermediary (proxy server), it may authenticate the request with
the authentication scheme used for registration in its domain
(e.g., Digest authentication).
o If the authentication service is instantiated by a SIP UA, a UA
may authenticate its own user through any system-specific means,
perhaps simply by virtue of having physical access to the UA.
Authorization of the use of a particular username or telephone number
in the user part of the From header field is a matter of local policy
for the authentication service; see Section 7.1 for more information.
Note that this check is performed only on the addr-spec in the
identity field (e.g., the URI of the originator, like
"sip:alice@atlanta.example.com"); it does not cover the display-name
portion of the From header field (e.g., "Alice Atlanta"). For more
information, see Section 12.6.
Step 3: Verify Date is Present and Valid
An authentication service MUST add a Date header field to SIP
requests that do not have one. The authentication service MUST
ensure that any preexisting Date header field in the request is
accurate. Local policy can dictate precisely how accurate the Date
must be; a RECOMMENDED maximum discrepancy of sixty seconds will
ensure that the request is unlikely to upset any verifiers. If the
Date header field value contains a time different by more than
one minute from the current time noted by the authentication service,
the authentication service SHOULD reject the request. Finally, the
authentication service MUST verify that both the Date header field
and the current time fall within the validity period of its
credential.
See Section 12.1 for information on how the Date header field assists
verifiers.
Step 4: Populate and Add the Identity Header
Subsequently, the authentication service MUST form a PASSporT object
and add a corresponding Identity header field to the request
containing either the full or compact form of PASSporT. For the
baseline PASSporT header (headers containing no "ppt" parameter),
this follows the procedures in Section 4; if the authentication
service is using an alternative "ppt" format, it MUST add an
appropriate "ppt" parameter and follow the procedures associated with
that extension (see Section 9). After the Identity header field has
been added to the request, the authentication service MUST also add
an "info" parameter to the Identity header field. The "info"
parameter contains a URI from which the authentication service's
credential can be acquired; see Section 7.3 for more on credential
acquisition.
An authentication service MAY use the full form of the PASSporT in
the Identity header field. The presence of the full form is OPTIONAL
because the information carried in the baseline PASSporT headers and
claims is usually redundant with information already carried
elsewhere in the SIP request. Using the compact form can
significantly reduce SIP message size, especially when the PASSporT
payload contains media keys. The syntax of the compact form is given
in [RFC8225], Section 7; essentially, it contains only the signature
component of the PASSporT.
Note that per the behavior specified in [RFC8225], use of the full
form is mandatory when optional extensions are included. See
Section 9.
6.1.1. Handling Repairable Errors
Also, in some cases, a request signed by an authentication service
will be rejected by the verification service on the receiving side,
and the authentication service will receive a SIP 4xx status code in
the backwards direction, such as a 438 ("Invalid Identity Header")
response indicating a verification failure. If the authentication
service did not originally send the full form of the PASSporT object
in the Identity header field, it SHOULD retry the request with the
full form after receiving a 438 response; however, implementations
SHOULD NOT retry the request more than once. Authentication services
implemented at proxy servers would retry such a request as a
sequential fork, by reprocessing the destination as a new target and
handling it serially as described in Section 16.6 of [RFC3261].
The information in the full form is useful on the verification side
for debugging errors, and there are some known causes of verification
failures (such as the Date header field value changing in transit;
see Section 12.1 for more information) that can be resolved by the
inclusion of the full form of PASSporT.
Finally, the authentication service forwards the message normally.
6.2. Verifier Behavior
This document specifies a logical role for SIP entities; this role is
called a verification service, or verifier. When a verifier receives
a SIP message containing one or more Identity header fields, it
inspects the signature(s) to verify the identity of the originator of
the message. The results of a verification are provided as input to
an authorization process that is outside the scope of this document.
A SIP request may contain zero, one, or more Identity header fields.
A verification service performs the steps below on each Identity
header field that appears in a request. If a verification service
cannot use any Identity header in a request, due to the absence of
Identity headers or unsupported "ppt" parameters, and the presence of
an Identity header field is required by local policy (for example,
based on a per-sending-domain policy or a per-sending-user policy),
then a 428 "Use Identity Header" response MUST be sent in the
backwards direction. For more on this and other verifier responses,
see Section 6.2.2.
In order to verify an Identity header field in a message, an entity
acting as a verifier MUST perform the following steps, in the order
specified below. Note that when an Identity header field contains a
full-form PASSporT object, the verifier MUST follow the additional
procedures in Section 6.2.4.
Step 1: Check for an Unsupported "ppt"
The verifier MUST inspect any optional "ppt" parameter appearing in
the Identity header. If no "ppt" parameter is present, then the
verifier proceeds normally with Steps 2 through 5. If a "ppt"
parameter value is present and the verifier does not support it,
it MUST ignore the Identity header field. If a supported "ppt"
parameter value is present, the verifier proceeds with Step 2 and
will ultimately follow the "ppt" variations described in Step 5.
Step 2: Determine the Originator's Identity
In order to determine whether the signature for the identity field
should be over the entire identity field URI or just a telephone
number, the verification service MUST follow the process described in
Section 8.1. The information in that section will lead to either the
telephone number canonicalization procedures in Section 8.3 for
telephone numbers or the URI normalization procedures described in
Section 8.5 for domain names.
Step 3: Identify Credential for Validation
The verifier must ensure that it has access to the proper keying
material to validate the signature in the Identity header field; this
usually involves dereferencing a URI in the "info" parameter of the
Identity header field. See Section 7.2 for more information on these
procedures. If the verifier does not support the credential
described in the "info" parameter, then it treats the credential for
this header field as unsupported.
Step 4: Check the Freshness of Date
The verifier furthermore ensures that the value of the Date header
field of the request meets local policy for freshness (sixty seconds
is RECOMMENDED) and that it falls within the validity period of the
credential used to sign the Identity header field. For more on the
attacks this prevents, see Section 12.1. If the full form of the
PASSporT is present, the verifier SHOULD compare the "iat" value in
the PASSporT to the Date header field value in the request. If the
two are different, and the "iat" value differs from the Date header
field value but remains within verification service policy for
freshness, the verification service SHOULD perform the computation
required by Step 5, using the "iat" value instead of the Date header
field value.
Step 5: Validate the Signature
The verifier MUST validate the signature in the Identity header field
over the PASSporT object. For baseline PASSporT objects (with no
Identity header field "ppt" parameter), the verifier MUST follow the
procedures for generating the signature over a PASSporT object as
described in Section 4. If a "ppt" parameter is present (and, per
Step 1, is supported), the verifier follows the procedures for that
"ppt" (see Section 9). If a verifier determines that the signature
in the Identity header field does not correspond to the reconstructed
signed-identity-digest, then the Identity header field should be
considered invalid.
6.2.1. Authorization of Requests
The verification of an Identity header field does not entail any
particular treatment of the request. The handling of the message
after the verification process depends on how the verification
service is implemented and on local policy. This specification
does not propose any authorization policy for UAs or proxy servers to
follow based on the presence of a valid Identity header field, the
presence of an invalid Identity header field, the absence of an
Identity header field, or the presence of a stale Date header field
value. However, it is anticipated that local policies could involve
making different forwarding decisions in intermediary
implementations, or changing how the user is alerted or how identity
is rendered in UA implementations.
The presence of multiple Identity header fields within a message
raises the prospect that a verification service could receive a
message containing both valid and invalid Identity header fields. As
a guideline, this specification recommends that only if a verifier
determines that all Identity header fields within a message are
invalid should the request be considered to have an invalid identity.
If at least one Identity header field value is valid and from a
trusted source, then relying parties can use that header for
authorization decisions regardless of whether other untrusted or
invalid Identity headers appear in a request.
6.2.2. Failure Response Codes Sent by a Verification Service
[RFC4474] originally defined four response codes for failure
conditions specific to the Identity header field and its original
mechanism. These status codes are retained in this specification,
with some slight modifications. Also, this specification details
responding with a 403 "Forbidden" response when a stale Date header
field value is received; see below.
A 428 response will be sent (per Section 6.2) when an Identity header
field is required but no Identity header field without a "ppt"
parameter or with a supported "ppt" value has been received. In the
case where one or more Identity header fields with unsupported "ppt"
values have been received, then a verification service may send a 428
with a human-readable reason phrase like "Use Supported PASSporT
Format". Note, however, that this specification gives no guidance on
how a verification service might decide to require an Identity header
field for a particular SIP request. Such authorization policies are
outside the scope of this specification.
The 436 "Bad Identity Info" response code indicates an inability to
acquire the credentials needed by the verification service for
validating the signature in an Identity header field. Again, given
the potential presence of multiple Identity header fields, this
response code should only be sent when the verification service is
unable to dereference the URIs and/or acquire the credentials
associated with all Identity header fields in the request. This
failure code could be repairable if the authentication service
resends the request with an "info" parameter pointing to a credential
that the verification service can access.
The 437 "Unsupported Credential" response (previously
"Unsupported Certificate"; see Section 13.2) is sent when a
verification service can acquire, or already holds, the credential
represented by the "info" parameter of at least one Identity header
field in the request but does not support said credential(s), for
reasons such as failing to trust the issuing certification authority
(CA) or failing to support the algorithm with which the credential
was signed.
The 438 "Invalid Identity Header" response indicates that of the set
of Identity header fields in a request, no header field with a valid
and supported PASSporT object has been received. Like the 428
response, this is sent by a verification service when its local
policy dictates that a broken signature in an Identity header field
is grounds for rejecting a request. Note that in some cases, an
Identity header field may be broken for other reasons than that an
originator is attempting to spoof an identity: for example, when a
transit network alters the Date header field of the request. Sending
a full-form PASSporT can repair some of these conditions (see
Section 6.2.4), so the recommended way to attempt to repair this
failure is to retry the request with the full form of PASSporT if it
had originally been sent with the compact form. The alternative
reason phrase "Invalid PASSporT" can be used when an extended
full-form PASSporT lacks required headers or claims, or when an
extended full-form PASSporT signaled with the "ppt" parameter lacks
required claims for that extension. Sending a string along these
lines will help humans debugging the sending system.
Finally, a 403 response may be sent when the verification service
receives a request with a Date header field value that is older than
the local policy for freshness permits. The same response may be
used when the "iat" in the full form of a PASSporT has a value older
than the local policy for freshness permits. The reason phrase
"Stale Date" can be sent to help humans debug the failure.
Future specifications may explore ways, including Reason codes or
Warning headers, to communicate further information that could be
used to disambiguate the source of errors in cases with multiple
Identity headers in a single request or to provide similar detailed
feedback for debugging purposes.
6.2.3. Handling Retried Requests
If a verification service sends a failure response in the backwards
direction, the authentication service may retry the request as
described in Section 6.1.1. If the authentication service is
instantiated at a proxy server, then it will retry the request as a
sequential fork. Verification services implemented at a proxy server
will recognize this request as a spiral rather than a loop due to the
proxy behavior fix documented in [RFC5393], Section 4.2. However, if
the verification service is implemented in an endpoint, the endpoint
will need to override the default UAS behavior (in particular, the
SHOULD in [RFC3261], Section 8.2.2.2) to accept this request as a
spiral rather than a loop.
6.2.4. Handling the Full Form of PASSporT
If the full form of PASSporT is present in an Identity header, this
permits the use of optional extensions as described in [RFC8225],
Section 8.3. Furthermore, the verification service can extract from
the "orig" and "dest" elements of the PASSporT full form the
canonical telephone numbers created by the authentication service, as
well as an "iat" claim corresponding to the Date header field that
the authentication service used. These values may be used to debug
canonicalization problems or to avoid unnecessary signature breakage
caused by intermediaries that alter certain SIP header field values
in transit.
However, the verification service MUST NOT treat the value in the
"orig" of a full-form PASSporT as the originating identity of the
call: the originating identity of the call is always derived from the
SIP signaling, and it is that value, per the procedures above in
Section 6.2 Step 2, that is used to recompute the signature at the
verification service. That value, rather than the value inside the
PASSporT object, is rendered to an end user in ordinary SIP
operations, and if a verification service were to simply trust that
the value in the "orig" corresponded to the call that it received
without comparing it to the call signaling, this would enable various
cut-and-paste attacks. As an optimization, when the full form is
present, the verification service MAY delay performing that
cryptographic operation and first compute its own canonicalization of
an originating telephone number to compare it to the values in the
"orig" element of PASSporT. This would allow the verification
service to ascertain whether or not the two ends agree on the
canonical number form; if they do not, then surely the signature
validation would fail.
7. Credentials
This section gives general guidance on the use of credential systems
by authentication and verification services, as well as requirements
that must be met by credential systems that conform with this
architecture. It does not mandate any specific credential system.
Furthermore, this specification allows either a UA or a proxy server
to provide the authentication service function and/or the
verification service function. For the purposes of end-to-end
security, it is obviously preferable for end systems to acquire their
own credentials; in this case, UAs can act as authentication
services. However, for some deployments, end-user credentials may be
neither practical nor affordable, given the potentially large number
of SIP UAs (phones, PCs, laptops, PDAs, gaming devices) that may be
employed by a single user. Synchronizing keying material across
multiple devices may be prohibitively complex and require quite a
good deal of additional endpoint behavior. Managing several
credentials for the various devices could also be burdensome. Thus,
for reasons of credential management alone, implementing the
authentication service at an intermediary may be more practical.
This trade-off needs to be understood by implementers of this
specification.
7.1. Credential Use by the Authentication Service
In order to act as an authentication service, a SIP entity must
possess the private keying material of one or more credentials that
cover domain names or telephone numbers. These credentials may
represent authority over one domain (such as example.com) or a set of
domains enumerated by the credential. Similarly, a credential may
represent authority over a single telephone number or a range of
telephone numbers. The way that the scope of a credential's
authority is expressed is specific to the credential mechanism.
Authorization of the use of a particular username or telephone number
in the From header field value is a matter of local policy for the
authentication service, one that depends greatly on the manner in
which authentication is performed. For non-telephone number user
parts, one policy might be as follows: the username given in the
"username" parameter of the Proxy-Authorization header field must
correspond exactly to the username in the From header field of the
SIP message. However, there are many cases in which this is too
limiting or inappropriate; a realm might use "username" parameters in
the Proxy-Authorization header field that do not correspond to the
user portion of From header fields, or a user might manage multiple
accounts in the same administrative domain. In this latter case, a
domain might maintain a mapping between the values in the "username"
parameter of the Proxy-Authorization header field and a set of one or
more SIP URIs that might legitimately be asserted for that
"username". For example, the username can correspond to the "private
identity" as defined by the Third Generation Partnership Project
(3GPP) [TS-3GPP.23.228], in which case the From header field can
contain any one of the public identities associated with this private
identity. In this instance, another policy might be as follows: the
URI in the From header field must correspond exactly to one of the
mapped URIs associated with the "username" given in the
Proxy-Authorization header field. This is a suitable approach for
telephone numbers in particular.
This specification could also be used with credentials that cover a
single name or URI, such as alice@example.com or
sip:alice@example.com. This would require a modification to
authentication service behavior to operate on a whole URI rather than
a domain name. Because this is not believed to be a pressing use
case, this is deferred to future work, but implementers should note
this as a possible future direction.
Exceptions to such authentication service policies arise for cases
like anonymity; if the AoR asserted in the From header field uses a
form like "sip:anonymous@example.com" (see [RFC3323]), then the
"example.com" proxy might authenticate only that the user is a valid
user in the domain and insert the signature over the From header
field as usual.
7.2. Credential Use by the Verification Service
In order to act as a verification service, a SIP entity must have a
way to acquire credentials for authorities over particular domain
names, telephone numbers, and/or number ranges. Dereferencing the
URI found in the "info" parameter of the Identity header field (as
described in Section 7.3) MUST be supported by all verification
service implementations to create a baseline means of credential
acquisition. Provided that the credential used to sign a message is
not previously known to the verifier, SIP entities SHOULD discover
this credential by dereferencing the "info" parameter, unless they
have some implementation-specific way of acquiring the needed keying
material, such as an offline store of periodically updated
credentials. The 436 "Bad Identity Info" response exists for cases
where the verification service cannot dereference the URI in the
"info" parameter.
This specification does not propose any particular policy for a
verification service to determine whether or not the holder of a
credential is the appropriate party to sign for a given SIP identity.
Guidance on this is deferred to credential mechanism specifications.
Verification service implementations supporting this specification
may wish to have some means of retaining credentials (in accordance
with normal practices for credential lifetimes and revocation) in
order to prevent themselves from needlessly downloading the same
credential every time a request from the same identity is received.
Credentials cached in this manner may be indexed in accordance with
local policy: for example, by their scope of authority or by the URI
given in the "info" parameter value. Further consideration of how to
cache credentials is deferred to the credential mechanism
specifications.
7.3. "info" Parameter URIs
An "info" parameter MUST contain a URI that dereferences to a
resource that contains the public key components of the credential
used by the authentication service to sign a request. It is
essential that a URI in the "info" parameter be dereferencable by any
entity that could plausibly receive the request. For common cases,
this means that the URI SHOULD be dereferencable by any entity on the
public Internet. In constrained deployment environments, a service
private to the environment MAY be used instead.
Beyond providing a means of accessing credentials for an identity,
the "info" parameter further serves as a means of differentiating
which particular credential was used to sign a request, when there
are potentially multiple authorities eligible to sign. For example,
imagine a case where a domain implements the authentication service
role for a range of telephone numbers and a UA belonging to Alice has
acquired a credential for a single telephone number within that
range. Either would be eligible to sign a SIP request for the number
in question. Verification services, however, need a means to
differentiate which one performed the signature. The "info"
parameter performs that function.
7.4. Credential System Requirements
This document makes no recommendation for the use of any specific
credential system. Today, there are two primary credential systems
in place for proving ownership of domain names: certificates (e.g.,
X.509 v3; see [RFC5280]) and the domain name system itself (e.g.,
DNS-Based Authentication of Named Entities (DANE); see [RFC6698]).
It is envisioned that either could be used in the SIP identity
context: an "info" parameter could, for example, give an HTTP URL of
the Content-Type "application/pkix-cert" pointing to a certificate
(following the conventions of [RFC2585]). The "info" parameter might
use the DNS URL scheme (see [RFC4501]) to designate keys in the DNS.
While no comparable public credentials exist for telephone numbers,
either approach could be applied to telephone numbers. A credential
system based on certificates is given in [RFC8226], but this
specification can work with other credential systems; for example,
using the DNS was proposed in [CIDER].
In order for a credential system to work with this mechanism, its
specification must detail:
o which URI schemes the credential will use in the "info" parameter,
and any special procedures required to dereference the URIs,
o how the verifier can learn the scope of the credential,
o any special procedures required to extract keying material from
the resources designated by the URI,
o any algorithms required to validate the credentials (e.g., for
certificates, any algorithms used by certificate authorities to
sign certificates themselves), and
o how the associated credentials will support the mandatory signing
algorithm(s) required by PASSporT [RFC8225].
SIP entities cannot reliably predict where SIP requests will
terminate. When choosing a credential scheme for deployments of this
specification, it is therefore essential that the trust anchor(s) for
credentials be widely trusted or that deployments restrict the use of
this mechanism to environments where the reliance on particular trust
anchors is assured by business arrangements or similar constraints.
Note that credential systems must address key lifecycle management
concerns: were a domain to change the credential available at the
Identity header field "info" parameter URI before a verifier
evaluates a request signed by an authentication service, this would
cause obvious verifier failures. When a rollover occurs,
authentication services SHOULD thus provide new "info" URIs for each
new credential and SHOULD continue to make older key acquisition URIs
available for a duration longer than the plausible lifetime of a SIP
transaction (a minute would most likely suffice).
8. Identity Types
The STIR problem statement [RFC7340] focuses primarily on cases where
the called and calling parties identified in the To and From header
field values use telephone numbers, as this remains the dominant use
case in the deployment of SIP. However, the Identity header
mechanism also works with SIP URIs without telephone numbers (of the
form "sip:user@host") and, potentially, other identifiers when SIP
interworks with other protocols.
Authentication services confirm the identity of the originator of a
call, which is typically found in the From header field value. The
guidance in this specification also applies to extracting the URI
containing the originator's identity from the P-Asserted-Identity
header field value instead of the From header field value. In some
trusted environments, the P-Asserted-Identity header field is used
in lieu of the From header field to convey the AoR or telephone
number of the originator of a request; where it does, local policy
might therefore dictate that the canonical identity derives from the
P-Asserted-Identity header field rather than the From header field.
Ultimately, in any case where local policy canonicalizes the identity
into a form different from how it appears in the From header field,
the use of the full form of PASSporT by authentication services is
RECOMMENDED, but because the "orig" claim of PASSporT itself could
then divulge information about users or networks, implementers should
be mindful of the guidelines in Section 11.
8.1. Differentiating Telephone Numbers from URIs
In order to determine whether or not the user portion of a SIP URI is
a telephone number, authentication services and verification services
MUST perform the following procedure on any SIP URI they inspect that
contains a numeric user part. Note that the same procedures are
followed for creating the canonical form of a URI found in the From
header field as the procedures used for a URI found in the To header
field or the P-Asserted-Identity header field.
First, implementations will ascertain if the user portion of the URI
constitutes a telephone number. Telephone numbers most commonly
appear in SIP header field values in the username portion of a SIP
URI (e.g., "sip:+17005551008@chicago.example.com;user=phone"). The
user part of SIP URIs with the "user=phone" parameter conforms to the
syntax of the tel URI scheme [RFC3966]. It is also possible for a
tel URI to appear in SIP header fields outside the context of a SIP
or Session Initiation Protocol Secure (SIPS) URI (e.g.,
"tel:+17005551008"). Thus, in standards-compliant environments,
numbers will be explicitly labeled by the use of tel URIs or the
"user=phone" parameter.
Alternatively, implementations in environments that do not conform to
those standards MAY follow local policies for identifying telephone
numbers. For example, implementations could infer that the user part
is a telephone number due to the presence of the "+" indicator at the
start of the user portion. Absent even that indication, if there are
numbers present in the user portion, implementations might
conceivably also detect that the user portion of the URI contains a
telephone number by determining whether or not those numbers would be
dialable or routable in the local environment -- bearing in mind that
the telephone number may be a valid E.164 number [E.164], a
nationally specific number, or even a private branch exchange number.
Implementations could also rely on external hints: for example, a
verification service implementation could infer from the type of
credential that signed a request that the signature must be over a
telephone number.
Regardless of how the implementation detects telephone numbers, once
a telephone number has been detected, implementations SHOULD follow
the procedures in Section 8.3. If the URI field does not contain a
telephone number or if the result of the canonicalization of the From
header field value does not form a valid E.164 telephone number, the
authentication service and/or verification service SHOULD treat the
entire URI as a SIP URI and apply the procedures in Section 8.5.
These URI normalization procedures are invoked to canonicalize the
URI before it is included in a PASSporT object in, for example, a
"uri" claim. See Section 8.5 for that behavior.
8.2. Authority for Telephone Numbers
In order for telephone numbers to be used with the mechanism
described in this document, authentication services must receive
credentials from an authority for telephone numbers or telephone
number ranges, and verification services must trust the authority
employed by the authentication service that signs a request. Per
Section 7.4, enrollment procedures and credential management are
outside the scope of this document; approaches to credential
management for telephone numbers are discussed in [RFC8226].
8.3. Telephone Number Canonicalization Procedures
Once an implementation has identified a telephone number, it must
construct a number string. That requires performing the following
steps:
o Implementations MUST drop any "+"s, internal dashes, parentheses,
or other non-numeric characters, except for the "#" or "*" keys
used in some special service numbers (typically, these will appear
only in the To header field value). This MUST result in an ASCII
string limited to "#", "*", and digits without whitespace or
visual separators.
o Next, an implementation must assess if the number string is a
valid, globally routable number with a leading country code.
If not, implementations SHOULD convert the number into E.164
format, adding a country code if necessary; this may involve
transforming the number from a dial string (see [RFC3966]),
removing any national or international dialing prefixes or
performing similar procedures. It is only in the case that an
implementation cannot determine how to convert the number to a
globally routable format that this step may be skipped. This will
be the case, for example, for nationally specific service numbers
(e.g., 911, 112); however, calls to those numbers are routed in a
very strict fashion, which ordinarily prevents them from reaching
entities that don't understand the numbers.
o Some domains may need to take unique steps to convert their
numbers into a global format, and such transformations during
canonicalization can also be made in accordance with specific
policies used within a local domain. For example, one domain may
only use local number formatting and need to convert all To/From
header field user portions to E.164 by prepending country-code and
region-code digits; another domain might have prefixed usernames
with trunk-routing codes, in which case the canonicalization will
need to remove the prefix. This specification cannot anticipate
all of the potential transformations that might be useful.
o The resulting canonical number string will be used as input to the
hash calculation during signing and verifying processes.
The ABNF of this number string is:
tn-spec = 1*tn-char
tn-char = "#" / "*" / DIGIT
The resulting number string is used in the construction of the
telephone number field(s) in a PASSporT object.
8.4. Authority for Domain Names
To use a SIP URI as an identity in this mechanism requires
authentication and verification systems to support standard
mechanisms for proving authority over a domain name: that is, the
domain name in the host portion of the SIP URI.
A verifier MUST evaluate the correspondence between the user's
identity and the signing credential by following the procedures
defined in [RFC5922], Section 7.2. While [RFC5922] deals with the
use of TLS and is specific to certificates, the procedures described
are applicable to verifying identity if one substitutes the "hostname
of the server" for the domain portion of the user's identity in the
From header field of a SIP request with an Identity header field.
This process is complicated by two deployment realities. In the
first place, credentials have varying ways of describing their
subjects and may indeed have multiple subjects, especially in
"virtual hosting" cases where multiple domains are managed by a
single application (see [RFC5922], Section 7.8). Secondly, some SIP
services may delegate SIP functions to a subordinate domain and
utilize the procedures in [RFC3263] that allow requests for, say,
"example.com" to be routed to "sip.example.com". As a result, a user
with the AoR "sip:alice@example.com" may process requests through a
host like "sip.example.com", and it may be that latter host that acts
as an authentication service.
To address the second of these problems, a domain that deploys an
authentication service on a subordinate host might supply that host
with the private keying material associated with a credential whose
subject is a domain name that corresponds to the domain portion of
the AoRs that the domain distributes to users. Note that this
corresponds to the comparable case of routing inbound SIP requests to
a domain. When the NAPTR and SRV procedures of [RFC3263] are used to
direct requests to a domain name other than the domain in the
original Request-URI (e.g., for "sip:alice@example.com", the
corresponding SRV records point to the service "sip1.example.org"),
the client expects that the certificate passed back in any TLS
exchange with that host will correspond exactly with the domain of
the original Request-URI, not the domain name of the host.
Consequently, in order to make inbound routing to such SIP services
work, a domain administrator must similarly be willing to share the
domain's private key with the service. This design decision was made
to compensate for the insecurity of the DNS, and it makes certain
potential approaches to DNS-based "virtual hosting" unsecurable for
SIP in environments where domain administrators are unwilling to
share keys with hosting services.
8.5. URI Normalization
Just as telephone numbers may undergo a number of syntactic
transformations during transit, the same can happen to SIP and SIPS
URIs without telephone numbers as they traverse certain
intermediaries. Therefore, when generating a PASSporT object based
on a SIP request, any SIP and SIPS URIs must be transformed into a
canonical form that captures the AoR represented by the URI before
they are provisioned in PASSporT claims such as "uri". The URI
normalization procedures required are as follows.
Following the ABNF of [RFC3261], the SIP or SIPS URI in question MUST
discard all elements after the "hostport" of the URI, including all
uri-parameters and escaped headers, from its syntax. Of the userinfo
component of the SIP URI, only the user element will be retained: any
password (and any leading ":" before the password) MUST be removed,
and since this userinfo necessarily does not contain a
telephone-subscriber component, no further parameters can appear in
the user portion.
The hostport portion of the SIP or SIPS URI MUST similarly be
stripped of any trailing port along with the ":" that proceeds the
port, leaving only the host.
The ABNF of this canonical URI form (following the syntax defined in
[RFC3261]) is:
canon-uri = ( "sip" / "sips" ) ":" user "@" host
Finally, the URI will be subject to the syntax-based URI
normalization procedures of [RFC3986], Section 6.2.2.
Implementations MUST perform case normalization (rendering the
scheme, user, and host all lowercase) and percent-encoding
normalization (decoding any percent-encoded octet that corresponds to
an unreserved character, per [RFC3986], Section 2.3). However, note
that normalization procedures face known challenges in some
internationalized environments (see [IRI-COMPARISON]) and that
perfect normalization of URIs may not be possible in those
environments.
For future PASSporT applications, it may be desirable to provide an
identifier without an attached protocol scheme. Future
specifications that define PASSporT claims for SIP as a using
protocol could use these basic procedures but could eliminate the
scheme component. A more exact definition is left to future
specifications.
9. Extensibility
As future requirements may warrant increasing the scope of the
Identity mechanism, this specification specifies an optional "ppt"
parameter of the Identity header field, which mirrors the "ppt"
header in PASSporT. The "ppt" parameter value MUST consist of a
token containing an extension specification, which denotes an
extended set of one or more signed claims per the type extensibility
mechanism specified in [RFC8225], Section 8. Note that per the
guidance in that section, "ppt" is used only to enforce a mandatory
extension: optional claims may be added to any PASSporT object
without requiring the use of "ppt", but the compact form of PASSporT
MUST NOT be used when optional claims are present in the PASSporT
payload.
The potential for extensions is one of the primary motivations for
allowing the presence of multiple Identity header fields in the same
SIP request. It is envisioned that future extensions might allow for
alternate information to be signed or explicitly allow different
parties to provide the signatures than the authorities envisioned by
baseline STIR. A request might, for example, have one Identity added
by an authentication service at the originating administrative domain
and then another Identity header field added by some further
intermediary using a PASSporT extension. While this specification
does not define any such specific purpose for multiple Identity
header fields, implementations MUST support receiving multiple header
fields for reasons of future compatibility.
An authentication service cannot assume that verifiers will
understand any given extension. Verifiers that do support an
extension may then trigger appropriate application-level behavior in
the presence of an extension; authors of extensions should provide
appropriate extension-specific guidance to application developers on
this point.
10. Backwards Compatibility with RFC 4474
This specification introduces several significant changes from the
version of the Identity header field defined by [RFC4474]. However,
due to the problems enumerated in [SIP-RFC4474-CONCERNS], it is not
believed that the original Identity header field has seen any
deployment, or even implementation in deployed products.
As such, this mechanism contains no provisions for signatures
generated with this specification to work with implementations
compliant with [RFC4474], nor does it contain any related backwards-
compatibility provisions. Hypothetically, were an implementation
compliant with [RFC4474] to receive messages containing this revised
version of the Identity header field, it would likely fail the
request with a 436 response code due to the absence of an
Identity-Info header field (Section 4). Implementations of this
specification, for debugging purposes, might interpret a 436 with a
reason phrase of "Bad Identity Info" (previously "Bad Identity-Info";
see Section 13.2) as an indication that the request has failed
because it reached a (hypothetical) verification service that is
compliant with [RFC4474].
11. Privacy Considerations
The purpose of this mechanism is to provide a reliable identification
of the originator of a SIP request, specifically a cryptographic
assurance that an authority asserts the originator can claim the URI
the identity stipulated in the request. This URI may contain or
imply a variety of personally identifying information, including the
name of a human being, their place of work or service provider, and,
possibly, further details. The intrinsic privacy risks associated
with that URI are, however, no different from those of baseline SIP.
Per the guidance in [RFC6973], implementers should make users aware
of the privacy trade-off of providing secure identity.
The identity mechanism presented in this document is compatible with
the standard SIP practices for privacy described in [RFC3323]. A SIP
proxy server can act as both a privacy service as described in
[RFC3323] and an authentication service. Since a UA can provide any
From header field value that the authentication service is willing to
authorize, there is no reason why private SIP URIs that contain
legitimate domains (e.g., sip:anonymous@example.com) cannot be signed
by an authentication service. The construction of the Identity
header field is the same for private URIs as it is for any other sort
of URIs. Similar practices could be used to support opportunistic
signing of SIP requests for UA-integrated authentication services
with self-signed certificates, though that is outside the scope of
this specification and is left as a matter for future investigation.
Note, however, that even when using anonymous SIP URIs, an
authentication service must possess a certificate corresponding to
the host portion of the addr-spec of the From header field value of
the request; accordingly, using domains like "anonymous.invalid"
will not be usable by privacy services that simultaneously act as
authentication services. The assurance offered by the usage of
anonymous URIs with a valid domain portion is "this is a known user
in my domain that I have authenticated, but I am keeping its identity
private."
It is worth noting two features of this more anonymous form of
identity. One can eliminate any identifying information in a domain
through the use of the domain "anonymous.invalid", but we must then
acknowledge that it is difficult for a domain to be both anonymous
and authenticated. The use of the domain "anonymous.invalid" entails
that no corresponding authority for the domain can exist, and as a
consequence, authentication service functions for that domain are
meaningless. The second feature is more germane to the threats this
document mitigates [RFC7375]. None of the relevant attacks, all of
which rely on the attacker taking on the identity of a victim or
hiding their identity using someone else's identity, are enabled by
an anonymous identity. As such, the inability to assert an authority
over an anonymous domain is irrelevant to our threat model.
[RFC3325] defines the "id" priv-value token, which is specific to the
P-Asserted-Identity header field. The sort of assertion provided by
the P-Asserted-Identity header field is very different from the
Identity header field presented in this document. It contains
additional information about the originator of a message that may go
beyond what appears in the From header field; P-Asserted-Identity
holds a definitive identity for the originator that is somehow known
to a closed network of intermediaries. Presumably, that network will
use this identity for billing or security purposes. The danger of
this network-specific information leaking outside of the closed
network motivated the "id" priv-value token. The "id" priv-value
token has no implications for the Identity header field, and privacy
services MUST NOT remove the Identity header field when a priv-value
of "id" appears in a Privacy header field.
The full form of the PASSporT object provides the complete JSON
objects used to generate the signed-identity-digest of the Identity
header field value, including the canonicalized form of the telephone
number of the originator of a call if the signature is over a
telephone number. In some contexts, local policy may require a
canonicalization that differs substantially from the original From
header field. Depending on those policies, potentially the full form
of PASSporT might divulge information about the originating network
or user that might not appear elsewhere in the SIP request. Were it
to be used to reflect the contents of the P-Asserted-Identity header
field, for example, then the object would need to be converted to the
compact form when the P-Asserted-Identity header is removed to avoid
any such leakage outside of a trust domain. Since, in those
contexts, the canonical form of the originator's identity could not
be reassembled by a verifier and thus the Identity signature
validation process would fail, using P-Asserted-Identity with the
full form of PASSporT in this fashion is NOT RECOMMENDED outside of
environments where SIP requests will never leave the trust domain.
As a side note, history shows that closed networks never stay closed
and one should design their implementation assuming connectivity to
the broader Internet.
Finally, note that unlike [RFC3325], the mechanism described in this
specification adds no information to SIP requests that has privacy
implications -- apart from disclosing that an authentication service
is willing to sign for an originator.
12. Security Considerations
This document describes a mechanism that provides a signature over
the Date header field of SIP requests, parts of the To and From
header fields, and (when present) any media keying material in the
message body. In general, the considerations related to the security
of these header fields are the same as those given in [RFC3261] for
including header fields in tunneled "message/sip" MIME bodies (see
Section 23 of [RFC3261] in particular). This section details the
individual security properties obtained by including each of these
header fields within the signature; collectively, this set of header
fields provides the necessary properties to prevent impersonation.
It addresses the solution-specific attacks against in-band solutions
enumerated in [RFC7375], Section 4.1.
12.1. Protected Request Fields
The From header field value (in ordinary operations) indicates the
identity of the originator of the message; for the purposes of this
document, either the SIP AoR URI or an embedded telephone number
provides the identity of a SIP user. Note that in some deployments
the identity of the originator may reside in P-Asserted-Identity
instead. The originator's identity is the key piece of information
that this mechanism secures; the remainder of the signed parts of a
SIP request are present to provide reference integrity and to prevent
certain types of cut-and-paste attacks.
The Date header field value protects against cut-and-paste attacks,
as described in [RFC3261], Section 23.4.2. That specification
recommends that implementations notify the user of a potential
security issue if the signed Date header field value is stale by an
hour or more. To prevent cut-and-paste of recently observed
messages, this specification instead RECOMMENDS a shorter interval of
sixty seconds. Implementations of this specification MUST NOT deem
valid a request with an outdated Date header field. Note that per
the behavior described in [RFC3893], Section 10, servers can keep
state of recently received requests, and thus if an Identity header
field is replayed by an attacker within the Date interval, verifiers
can detect that it is spoofed because a message with an identical
Date from the same source had recently been received.
It has been observed in the wild that some networks change the Date
header field value of SIP requests in transit; to accommodate that
type of scenario, alternative behavior might be necessary.
Verification services that observe a signature validation failure MAY
therefore reconstruct the Date header field component of the
signature from the "iat" carried in the full form of PASSporT:
provided that time recorded by "iat" falls within the local policy
for freshness that would ordinarily apply to the Date header, the
verification service MAY treat the signature as valid, provided it
keeps adequate state to detect recent replays. Note that this will
require the inclusion of the full form of the PASSporT object by
authentication services in networks where such failures are observed.
The To header field value provides the identity of the SIP user that
this request originally targeted. Covering the identity in the To
header field with the Identity signature serves two purposes. First,
it prevents cut-and-paste attacks in which an Identity header field
from a legitimate request for one user is cut-and-pasted into a
request for a different user. Second, it preserves the starting URI
scheme of the request; this helps prevent downgrade attacks against
the use of SIPS. The To identity offers additional protection
against cut-and-paste attacks beyond the Date header field. For
example, without a signature over the To identity, an attacker who
receives a call from a target could immediately cut-and-paste the
Identity and From header field value from that INVITE into a new
request to the target's voicemail service within the Date interval,
and the voicemail service would have no way of knowing that the
Identity header field it received had been originally signed for a
call intended for a different number. However, note the caveats
below in Section 12.1.1.
When signing a request that contains a fingerprint of keying material
in SDP for DTLS-SRTP [RFC5763], this mechanism always provides a
signature over that fingerprint. This signature prevents certain
classes of impersonation attacks in which an attacker forwards or
cut-and-pastes a legitimate request. Although the target of the
attack may accept the request, the attacker will be unable to
exchange media with the target, as they will not possess a key
corresponding to the fingerprint. For example, there are some
baiting attacks, launched with the REFER method or through social
engineering, where the attacker receives a request from the target
and reoriginates it to a third party. These might not be prevented
by only a signature over the From, To, and Date, but they could be
prevented by securing a fingerprint for DTLS-SRTP. While this is a
different form of impersonation than is commonly used for
robocalling, ultimately there is little purpose in establishing the
identity of the user that originated a SIP request if this assurance
is not coupled with a comparable assurance over the contents of the
subsequent media communication. This signature also reduces the
potential for active eavesdropping attacks against the SIP media. In
environments where DTLS-SRTP is unsupported, however, no field is
signed and no protections are provided.
12.1.1. Protection of the To Header and Retargeting
Armed with the original value of the To header field, the recipient
of a request may be tempted to compare it to their own identity in
order to determine whether or not the identity information in this
call might have been replayed. However, any request may be
legitimately retargeted as well, and as a result legitimate requests
may reach a SIP endpoint whose user is not identified by the URI
designated in the To header field value. It is therefore difficult
for any verifier to decide whether or not some prior retargeting was
"legitimate". Retargeting can also cause confusion when identity
information is provided for requests sent in the backwards direction
in a dialog, as the dialog identifiers may not match credentials held
by the ultimate target of the dialog. For further information on the
problems of response identity, see [SIP-RETARGET].
Any means for authentication services or verifiers to anticipate
retargeting is outside the scope of this document and is likely to
have the same applicability to response identity as it does to
requests in the backwards direction within a dialog. Consequently,
no special guidance is given for implementers here regarding the
"connected party" problem (see [RFC4916]); authentication service
behavior is unchanged if retargeting has occurred for a dialog-
forming request. Ultimately, the authentication service provides an
Identity header field for requests in the dialog only when the user
is authorized to assert the identity given in the From header field,
and if they are not, an Identity header field is not provided. And
per the threat model of [RFC7375], resolving problems with
"connected" identity has little bearing on detecting robocalling or
related impersonation attacks.
12.2. Unprotected Request Fields
[RFC4474] originally provided protections for Contact, Call-ID, and
CSeq. This document removes protection for these fields. The
absence of these header field values creates some opportunities for
determined attackers to impersonate based on cut-and-paste attacks;
however, the absence of these header field values does not seem
impactful to the primary focus of this document, which is the
prevention of the simple unauthorized claiming of an identity for the
purposes of robocalling, voicemail hacking, or swatting.
It might seem attractive to provide a signature over some of the
information present in the Via header field value(s). For example,
without a signature over the sent-by field of the topmost Via header
field, an attacker could remove that Via header field and insert its
own in a cut-and-paste attack, which would cause all responses to the
request to be routed to a host of the attacker's choosing. However,
a signature over the topmost Via header field does not prevent
attacks of this nature, since the attacker could leave the topmost
Via intact and merely insert a new Via header field directly after
it, which would cause responses to be routed to the attacker's host
"on their way" to the valid host; the end result would be exactly the
same. Although it is possible that an intermediary-based
authentication service could guarantee that no Via hops are inserted
between the sending UA and the authentication service, it could not
prevent an attacker from adding a Via hop after the authentication
service and thereby preempting responses. It is necessary for the
proper operation of SIP for subsequent intermediaries to be capable
of inserting such Via header fields, and thus it cannot be prevented.
As such, though it is desirable, securing Via is not possible through
the sort of identity mechanism described in this document; the best
known practice for securing Via is the use of SIPS.
12.3. Malicious Removal of Identity Headers
In the end analysis, the Identity header field cannot protect itself.
Any attacker could remove the header field from a SIP request and
modify the request arbitrarily afterwards. However, this mechanism
is not intended to protect requests from men-in-the-middle who
interfere with SIP messages; it is intended only to provide a way
that the originators of SIP requests can prove that they are who they
claim to be. At best, by stripping identity information from a
request, a man-in-the-middle could make it impossible to distinguish
any illegitimate messages he would like to send from those messages
sent by an authorized user. However, it requires a considerably
greater amount of energy to mount such an attack than it does to
mount trivial impersonations by just copying someone else's
From header field. This mechanism provides a way that an authorized
user can provide a definitive assurance of his identity that an
unauthorized user, an impersonator, cannot.
12.4. Securing the Connection to the Authentication Service
In the absence of UA-based authentication services, the assurance
provided by this mechanism is strongest when a UA forms a direct
connection, preferably one secured by TLS, to an intermediary-based
authentication service. The reasons for this are twofold:
o If a user does not receive a certificate from the authentication
service over the TLS connection that corresponds to the expected
domain (especially when the user receives a challenge via a
mechanism such as Digest), then it is possible that a rogue server
is attempting to pose as an authentication service for a domain
that it does not control, possibly in an attempt to collect shared
secrets for that domain. A similar practice could be used for
telephone numbers, though the application of certificates for
telephone numbers to TLS is left as a matter for future study.
o Without TLS, the various header field values and the body of the
request will not have integrity protection when the request
arrives at an authentication service. Accordingly, a prior
legitimate or illegitimate intermediary could modify the message
arbitrarily.
Of these two concerns, the first is most material to the intended
scope of this mechanism. This mechanism is intended to prevent
impersonation attacks, not man-in-the-middle attacks; integrity over
parts of the header and body is provided by this mechanism only to
prevent replay attacks. However, it is possible that applications
relying on the presence of the Identity header field could leverage
this integrity protection for services other than replay protection.
Accordingly, direct TLS connections SHOULD be used between the
UA client (UAC) and the authentication service whenever possible.
The opportunistic nature of this mechanism, however, makes it very
difficult to constrain UAC behavior, and moreover there will be some
deployment architectures where a direct connection is simply
infeasible and the UAC cannot act as an authentication service
itself. Accordingly, when a direct connection and TLS are not
possible, a UAC should use the SIPS mechanism, Digest "auth-int" for
body integrity, or both when it can. The ultimate decision to add an
Identity header field to a request lies with the authentication
service, of course; domain policy must identify those cases where the
UAC's security association with the authentication service is
too weak.
12.5. Authorization and Transitional Strategies
Ultimately, the worth of an assurance provided by an Identity header
field is limited by the security practices of the authentication
service that issues the assurance. Relying on an Identity header
field generated by a remote administrative domain assumes that the
issuing domain uses recommended administrative practices to
authenticate its users. However, it is possible that some
authentication services will implement policies that effectively make
users unaccountable (e.g., ones that accept unauthenticated
registrations from arbitrary users). The value of an Identity header
field from such authentication services is questionable. While there
is no magic way for a verifier to distinguish "good" from "bad"
signers by inspecting a SIP request, it is expected that further work
in authorization practices could be built on top of this identity
solution; without such an identity solution, many promising
approaches to authorization policy are impossible. That much said,
it is RECOMMENDED that authentication services based on proxy servers
employ strong authentication practices.
One cannot expect the Identity header field to be supported by every
SIP entity overnight. This leaves the verifier in a difficult
position; when it receives a request from a given SIP user, how can
it know whether or not the originator's domain supports Identity? In
the absence of ubiquitous support for Identity, some transitional
strategies are necessary.
o A verifier could remember when it receives a request from a domain
or telephone number that uses Identity and, in the future, view
messages received from that source without an Identity header
field with skepticism.
o A verifier could consult some sort of directory that indicates
whether a given caller should have a signed identity. There are a
number of potential ways in which this could be implemented. This
is left as a subject for future work.
In the long term, some sort of identity mechanism, either the one
documented in this specification or a successor, must become
mandatory-to-use for SIP; that is the only way to guarantee that this
protection can always be expected by verifiers.
Finally, it is worth noting that the presence or absence of the
Identity header fields cannot be the sole factor in making an
authorization decision. Permissions might be granted to a message on
the basis of the specific verified Identity or really on any other
aspect of a SIP request. Authorization policies are outside the
scope of this specification, but this specification advises any
future authorization work not to assume that messages with valid
Identity header fields are always good.
12.6. Display-Names and Identity
As a matter of interface design, SIP UAs might render the
display-name portion of the From header field of a caller as the
identity of the caller; there is a significant precedent in email
user interfaces for this practice. Securing the display-name
component of the From header field value is outside the scope of this
document but may be the subject of future work, such as through the
"ppt" name mechanism.
In the absence of signing the display-name, authentication services
might check and validate it, and compare it to a list of acceptable
display-names that may be used by the originator; if the display-name
does not meet policy constraints, the authentication service could
return a 403 response code. In this case, the reason phrase should
indicate the nature of the problem: for example, "Inappropriate
Display Name". However, the display-name is not always present, and
in many environments the requisite operational procedures for
display-name validation may not exist, so no normative guidance is
given here.
13. IANA Considerations
IANA has completed a number of actions described in this document.
Primarily, the previous references to [RFC4474] in the "Session
Initiation Protocol (SIP) Parameters" registry have been updated to
point to this document, unless specified otherwise below.
13.1. SIP Header Fields
The Identity-Info header in the SIP "Header Fields" registry has been
marked as deprecated by this document.
Also, the Identity-Info header reserved the compact form "n" at its
time of registration. That compact form has been removed from the
registry. The Identity header, however, retains the compact form "y"
reserved by [RFC4474].
13.2. SIP Response Codes
The 436 "Bad Identity-Info" default reason phrase has been changed to
"Bad Identity Info" in the SIP "Response Codes" registry.
The 437 "Unsupported Certificate" default reason phrase has been
changed to "Unsupported Credential".
13.3. Identity-Info Parameters
IANA manages a registry for Identity-Info parameters. Per this
specification, IANA has changed the name of this registry to
"Identity Parameters".
This specification defines one new value for the registry: "info" as
defined in Section 7.3.
13.4. Identity-Info Algorithm Parameter Values
IANA managed an "Identity-Info Algorithm Parameter Values" registry;
per this specification, IANA has deprecated and closed this registry.
Since the algorithms for signing PASSporTs are defined in [RFC8225]
rather than in this specification, there is no longer a need for an
algorithm parameter registry for the Identity header field.
14. Changes from RFC 4474
The following are salient changes from the original RFC 4474:
o The credential mechanism has been generalized; credential
enrollment, acquisition, and trust are now outside the scope of
this document.
o This document reduces the scope of the Identity signature to
remove CSeq, Call-ID, Contact, and the message body; signing of
key fingerprints in SDP is now included.
o The Identity-Info header field has been deprecated, and its
components have been relocated into parameters of the Identity
header field (which obsoletes the previous version of the header
field).
o The Identity header field can now appear multiple times in one
request.
o The previous signed-identity-digest format has been replaced with
PASSporT (signing algorithms are now defined in a separate
specification).
o Status code descriptions have been revised.
15. References
15.1. Normative References
[E.164] International Telecommunication Union, "The international
public telecommunication numbering plan",
ITU-T Recommendation E.164, November 2010,
<https://www.itu.int/rec/T-REC-E.164/en>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
DOI 10.17487/RFC3263, June 2002,
<https://www.rfc-editor.org/info/rfc3263>.
[RFC3966] Schulzrinne, H., "The tel URI for Telephone Numbers",
RFC 3966, DOI 10.17487/RFC3966, December 2004,
<https://www.rfc-editor.org/info/rfc3966>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5922] Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, DOI 10.17487/RFC5922, June 2010,
<https://www.rfc-editor.org/info/rfc5922>.
[RFC8225] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
<https://www.rfc-editor.org/info/rfc8225>.
15.2. Informative References
[CIDER] Kaplan, H., "A proposal for Caller Identity in a DNS-based
Entrusted Registry (CIDER)", Work in Progress,
draft-kaplan-stir-cider-00, July 2013.
[IRI-COMPARISON]
Masinter, L. and M. Duerst, "Comparison, Equivalence and
Canonicalization of Internationalized Resource
Identifiers", Work in Progress, draft-ietf-iri-
comparison-02, October 2012.
[RFC2585] Housley, R. and P. Hoffman, "Internet X.509 Public Key
Infrastructure Operational Protocols: FTP and HTTP",
RFC 2585, DOI 10.17487/RFC2585, May 1999,
<https://www.rfc-editor.org/info/rfc2585>.
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session
Initiation Protocol (SIP)", RFC 3323,
DOI 10.17487/RFC3323, November 2002,
<https://www.rfc-editor.org/info/rfc3323>.
[RFC3325] Jennings, C., Peterson, J., and M. Watson, "Private
Extensions to the Session Initiation Protocol (SIP) for
Asserted Identity within Trusted Networks", RFC 3325,
DOI 10.17487/RFC3325, November 2002,
<https://www.rfc-editor.org/info/rfc3325>.
[RFC3893] Peterson, J., "Session Initiation Protocol (SIP)
Authenticated Identity Body (AIB) Format", RFC 3893,
DOI 10.17487/RFC3893, September 2004,
<https://www.rfc-editor.org/info/rfc3893>.
[RFC4474] Peterson, J. and C. Jennings, "Enhancements for
Authenticated Identity Management in the Session
Initiation Protocol (SIP)", RFC 4474,
DOI 10.17487/RFC4474, August 2006,
<https://www.rfc-editor.org/info/rfc4474>.
[RFC4501] Josefsson, S., "Domain Name System Uniform Resource
Identifiers", RFC 4501, DOI 10.17487/RFC4501, May 2006,
<https://www.rfc-editor.org/info/rfc4501>.
[RFC4916] Elwell, J., "Connected Identity in the Session Initiation
Protocol (SIP)", RFC 4916, DOI 10.17487/RFC4916,
June 2007, <https://www.rfc-editor.org/info/rfc4916>.
[RFC5393] Sparks, R., Ed., Lawrence, S., Hawrylyshen, A., and B.
Campen, "Addressing an Amplification Vulnerability in
Session Initiation Protocol (SIP) Forking Proxies",
RFC 5393, DOI 10.17487/RFC5393, December 2008,
<https://www.rfc-editor.org/info/rfc5393>.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763,
May 2010, <https://www.rfc-editor.org/info/rfc5763>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698,
August 2012, <https://www.rfc-editor.org/info/rfc6698>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/info/rfc8259>.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
RFC 7340, DOI 10.17487/RFC7340, September 2014,
<https://www.rfc-editor.org/info/rfc7340>.
[RFC7375] Peterson, J., "Secure Telephone Identity Threat Model",
RFC 7375, DOI 10.17487/RFC7375, October 2014,
<https://www.rfc-editor.org/info/rfc7375>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515,
May 2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[RFC8226] Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", RFC 8226,
DOI 10.17487/RFC8226, February 2018,
<https://www.rfc-editor.org/info/rfc8226>.
[SIP-RETARGET]
Peterson, J., "Retargeting and Security in SIP: A
Framework and Requirements", Work in Progress,
draft-peterson-sipping-retarget-00, February 2005.
[SIP-RFC4474-CONCERNS]
Rosenberg, J., "Concerns around the Applicability of
RFC 4474", Work in Progress, draft-rosenberg-sip-rfc4474-
concerns-00, February 2008.
[TS-3GPP.23.228]
3GPP, "IP Multimedia Subsystem (IMS); Stage 2", 3GPP
TS 23.228 7.7.0, March 2007,
<http://www.3gpp.org/ftp/Specs/html-info/23228.htm>.
Acknowledgments
The authors would like to thank Adam Roach, Jim Schaad, Ning Zhang,
Syed Ali, Olle Jacobson, Dave Frankel, Robert Sparks, Dave Crocker,
Stephen Kent, Brian Rosen, Alex Bobotek, Paul Kyzivat, Jonathan
Lennox, Richard Shockey, Martin Dolly, Andrew Allen, Hadriel Kaplan,
Sanjay Mishra, Anton Baskov, Pierce Gorman, David Schwartz, Eric
Burger, Alan Ford, Christer Holmberg, Philippe Fouquart, Michael
Hamer, Henning Schulzrinne, and Richard Barnes for their comments.
Authors' Addresses
Jon Peterson
Neustar, Inc.
1800 Sutter St. Suite 570
Concord, CA 94520
United States of America
Email: jon.peterson@neustar.biz
Cullen Jennings
Cisco
400 3rd Avenue SW, Suite 350
Calgary, AB T2P 4H2
Canada
Email: fluffy@cisco.com
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
United States of America
Email: ekr@rtfm.com
Chris Wendt
Comcast
One Comcast Center
Philadelphia, PA 19103
United States of America
Email: chris-ietf@chriswendt.net