| Rfc | 4398 |
|---|
| Title | Storing Certificates in the Domain Name System (DNS) |
|---|
| Author | S. Josefsson |
|---|
| Date | March 2006 |
|---|
| Format: | TXT, HTML |
|---|
| Obsoletes | RFC2538 |
|---|
| Updated by | RFC6944 |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Network Working Group S. Josefsson
Request for Comments: 4398 March 2006
Obsoletes: 2538
Category: Standards Track
Storing Certificates in the Domain Name System (DNS)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Cryptographic public keys are frequently published, and their
authenticity is demonstrated by certificates. A CERT resource record
(RR) is defined so that such certificates and related certificate
revocation lists can be stored in the Domain Name System (DNS).
This document obsoletes RFC 2538.
Table of Contents
1. Introduction ....................................................3
2. The CERT Resource Record ........................................3
2.1. Certificate Type Values ....................................4
2.2. Text Representation of CERT RRs ............................6
2.3. X.509 OIDs .................................................6
3. Appropriate Owner Names for CERT RRs ............................7
3.1. Content-Based X.509 CERT RR Names ..........................8
3.2. Purpose-Based X.509 CERT RR Names ..........................9
3.3. Content-Based OpenPGP CERT RR Names ........................9
3.4. Purpose-Based OpenPGP CERT RR Names .......................10
3.5. Owner Names for IPKIX, ISPKI, IPGP, and IACPKIX ...........10
4. Performance Considerations .....................................11
5. Contributors ...................................................11
6. Acknowledgements ...............................................11
7. Security Considerations ........................................12
8. IANA Considerations ............................................12
9. Changes since RFC 2538 .........................................13
10. References ....................................................14
10.1. Normative References .....................................14
10.2. Informative References ...................................15
Appendix A. Copying Conditions ...................................16
1. Introduction
Public keys are frequently published in the form of a certificate,
and their authenticity is commonly demonstrated by certificates and
related certificate revocation lists (CRLs). A certificate is a
binding, through a cryptographic digital signature, of a public key,
a validity interval and/or conditions, and identity, authorization,
or other information. A certificate revocation list is a list of
certificates that are revoked, and of incidental information, all
signed by the signer (issuer) of the revoked certificates. Examples
are X.509 certificates/CRLs in the X.500 directory system or OpenPGP
certificates/revocations used by OpenPGP software.
Section 2 specifies a CERT resource record (RR) for the storage of
certificates in the Domain Name System [1] [2].
Section 3 discusses appropriate owner names for CERT RRs.
Sections 4, 7, and 8 cover performance, security, and IANA
considerations, respectively.
Section 9 explains the changes in this document compared to RFC 2538.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [3].
2. The CERT Resource Record
The CERT resource record (RR) has the structure given below. Its RR
type code is 37.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type | key tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| algorithm | /
+---------------+ certificate or CRL /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
The type field is the certificate type as defined in Section 2.1
below.
The key tag field is the 16-bit value computed for the key embedded
in the certificate, using the RRSIG Key Tag algorithm described in
Appendix B of [12]. This field is used as an efficiency measure to
pick which CERT RRs may be applicable to a particular key. The key
tag can be calculated for the key in question, and then only CERT RRs
with the same key tag need to be examined. Note that two different
keys can have the same key tag. However, the key MUST be transformed
to the format it would have as the public key portion of a DNSKEY RR
before the key tag is computed. This is only possible if the key is
applicable to an algorithm and complies to limits (such as key size)
defined for DNS security. If it is not, the algorithm field MUST be
zero and the tag field is meaningless and SHOULD be zero.
The algorithm field has the same meaning as the algorithm field in
DNSKEY and RRSIG RRs [12], except that a zero algorithm field
indicates that the algorithm is unknown to a secure DNS, which may
simply be the result of the algorithm not having been standardized
for DNSSEC [11].
2.1. Certificate Type Values
The following values are defined or reserved:
Value Mnemonic Certificate Type
----- -------- ----------------
0 Reserved
1 PKIX X.509 as per PKIX
2 SPKI SPKI certificate
3 PGP OpenPGP packet
4 IPKIX The URL of an X.509 data object
5 ISPKI The URL of an SPKI certificate
6 IPGP The fingerprint and URL of an OpenPGP packet
7 ACPKIX Attribute Certificate
8 IACPKIX The URL of an Attribute Certificate
9-252 Available for IANA assignment
253 URI URI private
254 OID OID private
255 Reserved
256-65279 Available for IANA assignment
65280-65534 Experimental
65535 Reserved
These values represent the initial content of the IANA registry; see
Section 8.
The PKIX type is reserved to indicate an X.509 certificate conforming
to the profile defined by the IETF PKIX working group [8]. The
certificate section will start with a one-octet unsigned OID length
and then an X.500 OID indicating the nature of the remainder of the
certificate section (see Section 2.3, below). (NOTE: X.509
certificates do not include their X.500 directory-type-designating
OID as a prefix.)
The SPKI and ISPKI types are reserved to indicate the SPKI
certificate format [15], for use when the SPKI documents are moved
from experimental status. The format for these two CERT RR types
will need to be specified later.
The PGP type indicates an OpenPGP packet as described in [5] and its
extensions and successors. This is used to transfer public key
material and revocation signatures. The data is binary and MUST NOT
be encoded into an ASCII armor. An implementation SHOULD process
transferable public keys as described in Section 10.1 of [5], but it
MAY handle additional OpenPGP packets.
The ACPKIX type indicates an Attribute Certificate format [9].
The IPKIX and IACPKIX types indicate a URL that will serve the
content that would have been in the "certificate, CRL, or URL" field
of the corresponding type (PKIX or ACPKIX, respectively).
The IPGP type contains both an OpenPGP fingerprint for the key in
question, as well as a URL. The certificate portion of the IPGP CERT
RR is defined as a one-octet fingerprint length, followed by the
OpenPGP fingerprint, followed by the URL. The OpenPGP fingerprint is
calculated as defined in RFC 2440 [5]. A zero-length fingerprint or
a zero-length URL are legal, and indicate URL-only IPGP data or
fingerprint-only IPGP data, respectively. A zero-length fingerprint
and a zero-length URL are meaningless and invalid.
The IPKIX, ISPKI, IPGP, and IACPKIX types are known as "indirect".
These types MUST be used when the content is too large to fit in the
CERT RR and MAY be used at the implementer's discretion. They SHOULD
NOT be used where the DNS message is 512 octets or smaller and could
thus be expected to fit a UDP packet.
The URI private type indicates a certificate format defined by an
absolute URI. The certificate portion of the CERT RR MUST begin with
a null-terminated URI [10], and the data after the null is the
private format certificate itself. The URI SHOULD be such that a
retrieval from it will lead to documentation on the format of the
certificate. Recognition of private certificate types need not be
based on URI equality but can use various forms of pattern matching
so that, for example, subtype or version information can also be
encoded into the URI.
The OID private type indicates a private format certificate specified
by an ISO OID prefix. The certificate section will start with a
one-octet unsigned OID length and then a BER-encoded OID indicating
the nature of the remainder of the certificate section. This can be
an X.509 certificate format or some other format. X.509 certificates
that conform to the IETF PKIX profile SHOULD be indicated by the PKIX
type, not the OID private type. Recognition of private certificate
types need not be based on OID equality but can use various forms of
pattern matching such as OID prefix.
2.2. Text Representation of CERT RRs
The RDATA portion of a CERT RR has the type field as an unsigned
decimal integer or as a mnemonic symbol as listed in Section 2.1,
above.
The key tag field is represented as an unsigned decimal integer.
The algorithm field is represented as an unsigned decimal integer or
a mnemonic symbol as listed in [12].
The certificate/CRL portion is represented in base 64 [16] and may be
divided into any number of white-space-separated substrings, down to
single base-64 digits, which are concatenated to obtain the full
signature. These substrings can span lines using the standard
parenthesis.
Note that the certificate/CRL portion may have internal sub-fields,
but these do not appear in the master file representation. For
example, with type 254, there will be an OID size, an OID, and then
the certificate/CRL proper. However, only a single logical base-64
string will appear in the text representation.
2.3. X.509 OIDs
OIDs have been defined in connection with the X.500 directory for
user certificates, certification authority certificates, revocations
of certification authority, and revocations of user certificates.
The following table lists the OIDs, their BER encoding, and their
length-prefixed hex format for use in CERT RRs:
id-at-userCertificate
= { joint-iso-ccitt(2) ds(5) at(4) 36 }
== 0x 03 55 04 24
id-at-cACertificate
= { joint-iso-ccitt(2) ds(5) at(4) 37 }
== 0x 03 55 04 25
id-at-authorityRevocationList
= { joint-iso-ccitt(2) ds(5) at(4) 38 }
== 0x 03 55 04 26
id-at-certificateRevocationList
= { joint-iso-ccitt(2) ds(5) at(4) 39 }
== 0x 03 55 04 27
3. Appropriate Owner Names for CERT RRs
It is recommended that certificate CERT RRs be stored under a domain
name related to their subject, i.e., the name of the entity intended
to control the private key corresponding to the public key being
certified. It is recommended that certificate revocation list CERT
RRs be stored under a domain name related to their issuer.
Following some of the guidelines below may result in DNS names with
characters that require DNS quoting as per Section 5.1 of RFC 1035
[2].
The choice of name under which CERT RRs are stored is important to
clients that perform CERT queries. In some situations, the clients
may not know all information about the CERT RR object it wishes to
retrieve. For example, a client may not know the subject name of an
X.509 certificate, or the email address of the owner of an OpenPGP
key. Further, the client might only know the hostname of a service
that uses X.509 certificates or the Key ID of an OpenPGP key.
Therefore, two owner name guidelines are defined: content-based owner
names and purpose-based owner names. A content-based owner name is
derived from the content of the CERT RR data; for example, the
Subject field in an X.509 certificate or the User ID field in OpenPGP
keys. A purpose-based owner name is a name that a client retrieving
CERT RRs ought to know already; for example, the host name of an
X.509 protected service or the Key ID of an OpenPGP key. The
content-based and purpose-based owner name may be the same; for
example, when a client looks up a key based on the From: address of
an incoming email.
Implementations SHOULD use the purpose-based owner name guidelines
described in this document and MAY use CNAME RRs at content-based
owner names (or other names), pointing to the purpose-based owner
name.
Note that this section describes an application-based mapping from
the name space used in a certificate to the name space used by DNS.
The DNS does not infer any relationship amongst CERT resource records
based on similarities or differences of the DNS owner name(s) of CERT
resource records. For example, if multiple labels are used when
mapping from a CERT identifier to a domain name, then care must be
taken in understanding wildcard record synthesis.
3.1. Content-Based X.509 CERT RR Names
Some X.509 versions, such as the PKIX profile of X.509 [8], permit
multiple names to be associated with subjects and issuers under
"Subject Alternative Name" and "Issuer Alternative Name". For
example, the PKIX profile has such Alternate Names with an ASN.1
specification as follows:
GeneralName ::= CHOICE {
otherName [0] OtherName,
rfc822Name [1] IA5String,
dNSName [2] IA5String,
x400Address [3] ORAddress,
directoryName [4] Name,
ediPartyName [5] EDIPartyName,
uniformResourceIdentifier [6] IA5String,
iPAddress [7] OCTET STRING,
registeredID [8] OBJECT IDENTIFIER }
The recommended locations of CERT storage are as follows, in priority
order:
1. If a domain name is included in the identification in the
certificate or CRL, that ought to be used.
2. If a domain name is not included but an IP address is included,
then the translation of that IP address into the appropriate
inverse domain name ought to be used.
3. If neither of the above is used, but a URI containing a domain
name is present, that domain name ought to be used.
4. If none of the above is included but a character string name is
included, then it ought to be treated as described below for
OpenPGP names.
5. If none of the above apply, then the distinguished name (DN)
ought to be mapped into a domain name as specified in [4].
Example 1: An X.509v3 certificate is issued to /CN=John Doe /DC=Doe/
DC=com/DC=xy/O=Doe Inc/C=XY/ with Subject Alternative Names of (a)
string "John (the Man) Doe", (b) domain name john-doe.com, and (c)
URI <https://www.secure.john-doe.com:8080/>. The storage locations
recommended, in priority order, would be
1. john-doe.com,
2. www.secure.john-doe.com, and
3. Doe.com.xy.
Example 2: An X.509v3 certificate is issued to /CN=James Hacker/
L=Basingstoke/O=Widget Inc/C=GB/ with Subject Alternate names of (a)
domain name widget.foo.example, (b) IPv4 address 10.251.13.201, and
(c) string "James Hacker <hacker@mail.widget.foo.example>". The
storage locations recommended, in priority order, would be
1. widget.foo.example,
2. 201.13.251.10.in-addr.arpa, and
3. hacker.mail.widget.foo.example.
3.2. Purpose-Based X.509 CERT RR Names
Due to the difficulty for clients that do not already possess a
certificate to reconstruct the content-based owner name,
purpose-based owner names are recommended in this section.
Recommendations for purpose-based owner names vary per scenario. The
following table summarizes the purpose-based X.509 CERT RR owner name
guidelines for use with S/MIME [17], SSL/TLS [13], and IPsec [14]:
Scenario Owner name
------------------ ----------------------------------------------
S/MIME Certificate Standard translation of an RFC 2822 email
address. Example: An S/MIME certificate for
"postmaster@example.org" will use a standard
hostname translation of the owner name,
"postmaster.example.org".
TLS Certificate Hostname of the TLS server.
IPsec Certificate Hostname of the IPsec machine and/or, for IPv4
or IPv6 addresses, the fully qualified domain
name in the appropriate reverse domain.
An alternate approach for IPsec is to store raw public keys [18].
3.3. Content-Based OpenPGP CERT RR Names
OpenPGP signed keys (certificates) use a general character string
User ID [5]. However, it is recommended by OpenPGP that such names
include the RFC 2822 [7] email address of the party, as in "Leslie
Example <Leslie@host.example>". If such a format is used, the CERT
ought to be under the standard translation of the email address into
a domain name, which would be leslie.host.example in this case. If
no RFC 2822 name can be extracted from the string name, no specific
domain name is recommended.
If a user has more than one email address, the CNAME type can be used
to reduce the amount of data stored in the DNS. For example:
$ORIGIN example.org.
smith IN CERT PGP 0 0 <OpenPGP binary>
john.smith IN CNAME smith
js IN CNAME smith
3.4. Purpose-Based OpenPGP CERT RR Names
Applications that receive an OpenPGP packet containing encrypted or
signed data but do not know the email address of the sender will have
difficulties constructing the correct owner name and cannot use the
content-based owner name guidelines. However, these clients commonly
know the key fingerprint or the Key ID. The key ID is found in
OpenPGP packets, and the key fingerprint is commonly found in
auxiliary data that may be available. In this case, use of an owner
name identical to the key fingerprint and the key ID expressed in
hexadecimal [16] is recommended. For example:
$ORIGIN example.org.
0424D4EE81A0E3D119C6F835EDA21E94B565716F IN CERT PGP ...
F835EDA21E94B565716F IN CERT PGP ...
B565716F IN CERT PGP ...
If the same key material is stored for several owner names, the use
of CNAME may help avoid data duplication. Note that CNAME is not
always applicable, because it maps one owner name to the other for
all purposes, which may be sub-optimal when two keys with the same
Key ID are stored.
3.5. Owner Names for IPKIX, ISPKI, IPGP, and IACPKIX
These types are stored under the same owner names, both purpose- and
content-based, as the PKIX, SPKI, PGP, and ACPKIX types.
4. Performance Considerations
The Domain Name System (DNS) protocol was designed for small
transfers, typically below 512 octets. While larger transfers will
perform correctly and work is underway to make larger transfers more
efficient, it is still advisable at this time that every reasonable
effort be made to minimize the size of certificates stored within the
DNS. Steps that can be taken may include using the fewest possible
optional or extension fields and using short field values for
necessary variable-length fields.
The RDATA field in the DNS protocol may only hold data of size 65535
octets (64kb) or less. This means that each CERT RR MUST NOT contain
more than 64kb of payload, even if the corresponding certificate or
certificate revocation list is larger. This document addresses this
by defining "indirect" data types for each normal type.
Deploying CERT RRs to support digitally signed email changes the
access patterns of DNS lookups from per-domain to per-user. If
digitally signed email and a key/certificate lookup based on CERT RRs
are deployed on a wide scale, this may lead to an increased DNS load,
with potential performance and cache effectiveness consequences.
Whether or not this load increase will be noticeable is not known.
5. Contributors
The majority of this document is copied verbatim from RFC 2538, by
Donald Eastlake 3rd and Olafur Gudmundsson.
6. Acknowledgements
Thanks to David Shaw and Michael Graff for their contributions to
earlier works that motivated, and served as inspiration for, this
document.
This document was improved by suggestions and comments from Olivier
Dubuisson, Scott Hollenbeck, Russ Housley, Peter Koch, Olaf M.
Kolkman, Ben Laurie, Edward Lewis, John Loughney, Allison Mankin,
Douglas Otis, Marcos Sanz, Pekka Savola, Jason Sloderbeck, Samuel
Weiler, and Florian Weimer. No doubt the list is incomplete. We
apologize to anyone we left out.
7. Security Considerations
By definition, certificates contain their own authenticating
signatures. Thus, it is reasonable to store certificates in
non-secure DNS zones or to retrieve certificates from DNS with DNS
security checking not implemented or deferred for efficiency. The
results may be trusted if the certificate chain is verified back to a
known trusted key and this conforms with the user's security policy.
Alternatively, if certificates are retrieved from a secure DNS zone
with DNS security checking enabled and are verified by DNS security,
the key within the retrieved certificate may be trusted without
verifying the certificate chain if this conforms with the user's
security policy.
If an organization chooses to issue certificates for its employees,
placing CERT RRs in the DNS by owner name, and if DNSSEC (with NSEC)
is in use, it is possible for someone to enumerate all employees of
the organization. This is usually not considered desirable, for the
same reason that enterprise phone listings are not often publicly
published and are even marked confidential.
Using the URI type introduces another level of indirection that may
open a new vulnerability. One method of securing that indirection is
to include a hash of the certificate in the URI itself.
If DNSSEC is used, then the non-existence of a CERT RR and,
consequently, certificates or revocation lists can be securely
asserted. Without DNSSEC, this is not possible.
8. IANA Considerations
The IANA has created a new registry for CERT RR: certificate types.
The initial contents of this registry is:
Decimal Type Meaning Reference
------- ---- ------- ---------
0 Reserved RFC 4398
1 PKIX X.509 as per PKIX RFC 4398
2 SPKI SPKI certificate RFC 4398
3 PGP OpenPGP packet RFC 4398
4 IPKIX The URL of an X.509 data object RFC 4398
5 ISPKI The URL of an SPKI certificate RFC 4398
6 IPGP The fingerprint and URL RFC 4398
of an OpenPGP packet
7 ACPKIX Attribute Certificate RFC 4398
8 IACPKIX The URL of an Attribute RFC 4398
Certificate
9-252 Available for IANA assignment
by IETF Standards action
253 URI URI private RFC 4398
254 OID OID private RFC 4398
255 Reserved RFC 4398
256-65279 Available for IANA assignment
by IETF Consensus
65280-65534 Experimental RFC 4398
65535 Reserved RFC 4398
Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can
only be assigned by an IETF standards action [6]. This document
assigns 0x0001 through 0x0008 and 0x00FD and 0x00FE. Certificate
types 0x0100 through 0xFEFF are assigned through IETF Consensus [6]
based on RFC documentation of the certificate type. The availability
of private types under 0x00FD and 0x00FE ought to satisfy most
requirements for proprietary or private types.
The CERT RR reuses the DNS Security Algorithm Numbers registry. In
particular, the CERT RR requires that algorithm number 0 remain
reserved, as described in Section 2. The IANA will reference the
CERT RR as a user of this registry and value 0, in particular.
9. Changes since RFC 2538
1. Editorial changes to conform with new document requirements,
including splitting reference section into two parts and
updating the references to point at latest versions, and to add
some additional references.
2. Improve terminology. For example replace "PGP" with "OpenPGP",
to align with RFC 2440.
3. In Section 2.1, clarify that OpenPGP public key data are binary,
not the ASCII armored format, and reference 10.1 in RFC 2440 on
how to deal with OpenPGP keys, and acknowledge that
implementations may handle additional packet types.
4. Clarify that integers in the representation format are decimal.
5. Replace KEY/SIG with DNSKEY/RRSIG etc, to align with DNSSECbis
terminology. Improve reference for Key Tag Algorithm
calculations.
6. Add examples that suggest use of CNAME to reduce bandwidth.
7. In Section 3, appended the last paragraphs that discuss
"content-based" vs "purpose-based" owner names. Add Section 3.2
for purpose-based X.509 CERT owner names, and Section 3.4 for
purpose-based OpenPGP CERT owner names.
8. Added size considerations.
9. The SPKI types has been reserved, until RFC 2692/2693 is moved
from the experimental status.
10. Added indirect types IPKIX, ISPKI, IPGP, and IACPKIX.
11. An IANA registry of CERT type values was created.
10. References
10.1. Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Kille, S., Wahl, M., Grimstad, A., Huber, R., and S. Sataluri,
"Using Domains in LDAP/X.500 Distinguished Names", RFC 2247,
January 1998.
[5] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[6] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[7] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[8] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 3280, April 2002.
[9] Farrell, S. and R. Housley, "An Internet Attribute Certificate
Profile for Authorization", RFC 3281, April 2002.
[10] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[11] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[12] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
10.2. Informative References
[13] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[14] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
[15] Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas, B.,
and T. Ylonen, "SPKI Certificate Theory", RFC 2693,
September 1999.
[16] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[17] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851,
July 2004.
[18] Richardson, M., "A Method for Storing IPsec Keying Material in
DNS", RFC 4025, March 2005.
Appendix A. Copying Conditions
Regarding the portion of this document that was written by Simon
Josefsson ("the author", for the remainder of this section), the
author makes no guarantees and is not responsible for any damage
resulting from its use. The author grants irrevocable permission to
anyone to use, modify, and distribute it in any way that does not
diminish the rights of anyone else to use, modify, and distribute it,
provided that redistributed derivative works do not contain
misleading author or version information. Derivative works need not
be licensed under similar terms.
Author's Address
Simon Josefsson
EMail: simon@josefsson.org
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Acknowledgement
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Administrative Support Activity (IASA).