rfc2538.txt

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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






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RFC 2538            Storing Certificates in the DNS           March 1999


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 [RFC 2535] 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.



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RFC 2538            Storing Certificates in the DNS           March 1999


   The key tag field is the 16 bit value computed for the key embedded
   in the certificate as specified in the DNSSEC Standard [RFC 2535].
   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 RFC 2440 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 [RFC 2396] 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



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RFC 2538            Storing Certificates in the DNS           March 1999


   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 [RFC 2535].

   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:









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RFC 2538            Storing Certificates in the DNS           March 1999


    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:




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