Network Working Group S. Josefsson
Internet-Draft January 24, 2005
Expires: July 25, 2005
Storing Certificates in the Domain Name System (DNS)
draft-ietf-dnsext-rfc2538bis-00
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Cryptographic public key are frequently published and their
authenticity 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).
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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 . . . . . . . . . . . . . 5
2.3 X.509 OIDs . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Appropriate Owner Names for CERT RRs . . . . . . . . . . . . . 6
3.1 Content-based X.509 CERT RR Names . . . . . . . . . . . . 7
3.2 Purpose-based X.509 CERT RR Names . . . . . . . . . . . . 8
3.3 Content-based OpenPGP CERT RR Names . . . . . . . . . . . 8
3.4 Purpose-based OpenPGP CERT RR Names . . . . . . . . . . . 9
4. Performance Considerations . . . . . . . . . . . . . . . . . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Changes since RFC 2538 . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 11
9.2 Informative References . . . . . . . . . . . . . . . . . . . 12
A. Copying conditions . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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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 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 below specifies a CERT resource record (RR) for the storage
of certificates in the Domain Name System.
Section 3 discusses appropriate owner names for CERT RRs.
Sections 4, 5, and 6 below cover performance, IANA, and security
considerations, respectively.
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 [10].
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 define in section 2.1
below.
The algorithm field has the same meaning as the algorithm field in
DNSKEY and RRSIG RRs [9] except that a zero algorithm field indicates
the algorithm is unknown to a secure DNS, which may simply be the
result of the algorithm not having been standardized for DNSSEC.
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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 [9]. 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 be examined. However, the key must always
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 limits such as key size
limits) defined for DNS security. If it is not, the algorithm field
MUST BE zero and the tag field is meaningless and SHOULD BE zero.
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
4-252 available for IANA assignment
253 URI URI private
254 OID OID private
255-65534 available for IANA assignment
65535 reserved
The PKIX type is reserved to indicate an X.509 certificate conforming
to the profile being defined by the IETF PKIX working group. The
certificate section will start with a one byte unsigned OID length
and then an X.500 OID indicating the nature of the remainder of the
certificate section (see 2.3 below). (NOTE: X.509 certificates do
not include their X.500 directory type designating OID as a prefix.)
The SPKI type is reserved to indicate a certificate formated as to be
specified by the IETF SPKI working group.
The PGP type indicates an OpenPGP packet as described in [5] and its
extensions and successors. Two uses are 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 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 [4] and the data after the null is the private
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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 a an ISO OID prefix. The certificate section will start with a
one byte 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 [9].
The certificate / CRL portion is represented in base 64 [11] and may
be divided up 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. But 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:
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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 the use in DNS
names of characters that require DNS quoting which is to use a
backslash followed by the octal representation of the ASCII code for
the character such as \000 for NULL.
The choice of name under which CERT RRs are stored is important to
clients that perform CERT queries. In some situations, the client
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 e-mail 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.
This motivate describing two different owner name guidelines. We
call the two rules 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
selected to be a name that clients that wishes to retrieve CERT RRs
are expected to know; for example the host name of a X.509 protected
service or a Key ID of an OpenPGP key. Note that in some situations,
the content-based and purpose-based owner name can be the same; for
example when a client look up keys based on e-mail addresses for
incoming e-mail.
Implementations SHOULD use the purpose-based owner name guidelines
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described in this document, and MAY use CNAMEs at content-based owner
names (or other names), pointing to the purpose-based owner name.
3.1 Content-based X.509 CERT RR Names
Some X.509 versions permit multiple names to be associated with
subjects and issuers under "Subject Alternate Name" and "Issuer
Alternate Name". For example, x.509v3 has such Alternate Names with
an ASN.1 specification as follows:
GeneralName ::= CHOICE {
otherName [0] INSTANCE OF OTHER-NAME,
rfc822Name [1] IA5String,
dNSName [2] IA5String,
x400Address [3] EXPLICIT OR-ADDRESS.&Type,
directoryName [4] EXPLICIT 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 should 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 should be used.
3. If neither of the above it used but a URI containing a domain
name is present, that domain name should be used.
4. If none of the above is included but a character string name is
included, then it should be treated as described for PGP names
below.
5. If none of the above apply, then the distinguished name (DN)
should be mapped into a domain name as specified in [3].
Example 1: Assume that 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/>. Then
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: Assume that an X.509v3 certificate is issued to /CN=James
Hacker/L=Basingstoke/O=Widget Inc/C=GB/ with Subject Alternate names
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of (a) domain name widget.foo.example, (b) IPv4 address
10.251.13.201, and (c) string "James Hacker
<hacker@mail.widget.foo.example>". Then 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
It is difficult for clients that do not already posses a certificate
to reconstruct the content-based owner name that should be used to
retrieve the certificate. For this reason, purpose-based owner names
are recommended in this section. Because purpose-based owner names
by nature depend on the specific scenario, or purpose, for which the
certificate will be used, there are more than one recommendation.
The following table summarize the purpose-based X.509 CERT RR owner
name guidelines.
Scenario Owner name
-------------------------------------------------------------------
S/MIME Certificate Standard translation of RFC 822 email address.
Example: A S/MIME certificate for
"postmaster@example.org" will use a standard
hostname translation of the owner name,
i.e. "postmaster.example.org".
SSL Certificate Hostname of the SSL server.
IPSEC Certificate Hostname of the IPSEC machine, and/or
for the in-addr.arpa reverse lookup IP address.
CRLs Hostname of the issuing CA.
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
should 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:
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$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 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 auxilliary data that may be available. For
these situations, it is recommended to use an owner name identical to
the key fingerprint and key ID expressed in hexadecimal [11]. For
example:
$ORIGIN example.org.
0424D4EE81A0E3D119C6F835EDA21E94B565716F IN CERT PGP ...
F835EDA21E94B565716F IN CERT PGP ...
B565716F IN CERT PGP ...
If the same key material is stored at several owner names, the use of
CNAME may be used to avoid data duplication. Note that CNAME is not
always applicable, because it map an owner names to the other for all
purposes, and this may be sub-optimal when two keys with the same Key
ID are stored.
4. Performance Considerations
Current Domain Name System (DNS) implementations are optimized for
small transfers, typically not more than 512 bytes including
overhead. While larger transfers will perform correctly and work is
underway to make larger transfers more efficient, it is still
advisable at this time to make every reasonable effort to minimize
the size of certificates stored within the DNS. Steps that can be
taken may include using the fewest possible optional or extensions
fields and using short field values for variable length fields that
must be included.
The RDATA field in the DNS protocol may only hold data of size 65535
octets (64kb) or less. This means that each CERT RR cannot contain
more than 64kb worth of payload, even if the corresponding
certificate or certificate revocation list is larger. This document
do not address this limitation.
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5. Acknowledgements
The majority of this document is copied verbatim from RFC 2538, by
Donald Eastlake 3rd and Olafur Gudmundsson.
The author wishes to thank David Shaw and Michael Graff for their
contributions to the earlier work that motivated this revised
document.
Florian Weimer suggested to clarify wording regarding what data can
be stored in RRDATA portion of OpenPGP CERT RRs, and that the URI
type may include hashes to secure the indirection. Olivier Dubuisson
confirmed that the X.509 OID were indeed correct.
6. Security Considerations
By definition, certificates contain their own authenticating
signature. 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.
When the URI type is used, it should be understood that is introduce
an additional indirection that may allow for a new attack vector.
One method to secure that indirection is to include a hash of the
certificate in the URI itself.
CERT RRs are not used in connection with securing the DNS security
additions so there are no security considerations related to CERT RRs
and securing the DNS itself.
7. IANA Considerations
Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can
only be assigned by an IETF standards action [6]. This document
assigns 0x0001 through 0x0003 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 should satisfy most
requirements for proprietary or private types.
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8. 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. References
9.1 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] Kille, S., Wahl, M., Grimstad, A., Huber, R. and S. Sataluri,
"Using Domains in LDAP/X.500 Distinguished Names", RFC 2247,
January 1998.
[4] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 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.
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[8] Arends, R., Austein, R., Massey, D., Larson, M. and S. Rose,
"DNS Security Introduction and Requirements",
draft-ietf-dnsext-dnssec-intro-13 (work in progress), October
2004.
[9] Arends, R., "Resource Records for the DNS Security Extensions",
draft-ietf-dnsext-dnssec-records-11 (work in progress), October
2004.
9.2 Informative References
[10] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[11] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
Author's Address
Simon Josefsson
EMail: simon@josefsson.org
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.
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Intellectual Property Statement
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Copyright Statement
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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