RFC2538 - Storing Certificates in the Domain Name System (DNS)

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　　Network Working Group D. Eastlake
Request for Comments: 2538 IBM
Category: Standards Track O. Gudmundsson
TIS Labs
March 1999

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 (1999). All Rights Reserved.

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).

Table of Contents

Abstract...................................................1
1. Introduction............................................2
2. The CERT Resource Record................................2
2.1 Certificate Type Values................................3
2.2 Text Representation of CERT RRs........................4
2.3 X.509 OIDs.............................................4
3. Appropriate Owner Names for CERT RRs....................5
3.1 X.509 CERT RR Names....................................5
3.2 PGP CERT RR Names......................................6
4. Performance Considerations..............................6
5. IANA Considerations.....................................7
6. Security Considerations.................................7
References.................................................8
Authors' Addresses.........................................9
Full Copyright Notice.....................................10

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 PGP
certificates/revocations used by PGP 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 [RFC2119].

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
KEY and SIG RRs [RFC2535] 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
secure DNS.

The key tag field is the 16 bit value computed for the key embedded
in the certificate as specified in the DNSSEC Standard [RFC2535].
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 KEY 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 cert
3 PGP PGP cert
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 a Pretty Good Privacy certificate as described
in RFC2440 and its extensions and successors.

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 [RFC2396] 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 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
integer or as a mnemonic symbol as listed in section 2.1 above.

The key tag field is represented as an unsigned integer.

The algorithm field is represented as an unsigned integer or a
mnemonic symbol as listed in [RFC2535].

The certificate / CRL portion is represented in base 64 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:

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.

3.1 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 in
3.2 below.
(5) If none of the above apply, then the distinguished name (DN)
should be mapped into a domain name as specified in RFC2247.

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
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 PGP CERT RR Names

PGP signed keys (certificates) use a general character string User ID
[RFC2440]. However, it is recommended by PGP that such names include
the RFC822 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 RFC822
name can be extracted from the string name no specific domain name is
recommended.

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.

5. IANA Considerations

Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can
only be assigned by an IETF standards action [RFC2434] (and this
document assigns 0x0001 through 0x0003 and 0x00FD and 0x00FE).
Certificate types 0x0100 through 0xFEFF are assigned through IETF
Consensus [RFC2434] based on RFCdocumentation of the certificate
type. The availability of private types under 0x00FD and 0x00FE
should satisfy most requirements for proprietary or private types.

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.

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.

References

RFC1034 Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC1034, November 1987.

RFC1035 Mockapetris, P., "Domain Names - Implementation and
Specifications", STD 13, RFC1035, November 1987.

RFC2119 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

RFC2247 Kille, S., Wahl, M., Grimstad, A., Huber, R. and S.
Sataluri, "Using Domains in LDAP/X.500 Distinguished
Names", RFC2247, January 1998.

RFC2396 Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC2396,
August 1998.

RFC2440 Callas, J., Donnerhacke, L., Finney, H. and R. Thayer,
"OpenPGP Message Format", RFC2240, November 1998.

RFC2434 Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC2434,
October 1998.

RFC2535 Eastlake, D., "Domain Name System (DNS) Security
Extensions", RFC2535, March 1999.

RFC2459 Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and CRL
Profile", RFC2459, January 1999.

Authors' Addresses

Donald E. Eastlake 3rd
IBM
65 Shindegan Hill Road
RR#1
Carmel, NY 10512 USA

Phone: +1-914-784-7913 (w)
+1-914-276-2668 (h)
Fax: +1-914-784-3833 (w-fax)
EMail: dee3@us.ibm.com

Olafur Gudmundsson
TIS Labs at Network Associates
3060 Washington Rd, Route 97
Glenwood MD 21738

Phone: +1 443-259-2389
EMail: ogud@tislabs.com

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