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rfc:rfc2538

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

Eastlake & Gudmundsson Standards Track [Page 1] 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.

Eastlake & Gudmundsson Standards Track [Page 2] 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

Eastlake & Gudmundsson Standards Track [Page 3] 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:

Eastlake & Gudmundsson Standards Track [Page 4] 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:

Eastlake & Gudmundsson Standards Track [Page 5] RFC 2538 Storing Certificates in the DNS March 1999

 (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 RFC 2247.
 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
 [RFC 2440]. However, it is recommended by PGP that such names include
 the RFC 822 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 822
 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

Eastlake & Gudmundsson Standards Track [Page 6] RFC 2538 Storing Certificates in the DNS March 1999

 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 [RFC 2434] (and this
 document assigns 0x0001 through 0x0003 and 0x00FD and 0x00FE).
 Certificate types 0x0100 through 0xFEFF are assigned through IETF
 Consensus [RFC 2434] 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.

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.

Eastlake & Gudmundsson Standards Track [Page 7] RFC 2538 Storing Certificates in the DNS March 1999

References

 RFC 1034   Mockapetris, P., "Domain Names - Concepts and Facilities",
            STD 13, RFC 1034, November 1987.
 RFC 1035   Mockapetris, P., "Domain Names - Implementation and
            Specifications", STD 13, RFC 1035, November 1987.
 RFC 2119   Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 RFC 2247   Kille, S., Wahl, M., Grimstad, A., Huber, R. and S.
            Sataluri, "Using Domains in LDAP/X.500 Distinguished
            Names", RFC 2247, January 1998.
 RFC 2396   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
            Resource Identifiers (URI): Generic Syntax", RFC 2396,
            August 1998.
 RFC 2440   Callas, J., Donnerhacke, L., Finney, H. and R.  Thayer,
            "OpenPGP Message Format", RFC 2240, November 1998.
 RFC 2434   Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 2434,
            October 1998.
 RFC 2535   Eastlake, D., "Domain Name System (DNS) Security
            Extensions", RFC 2535, March 1999.
 RFC 2459   Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
            X.509 Public Key Infrastructure Certificate and CRL
            Profile", RFC 2459, January 1999.

Eastlake & Gudmundsson Standards Track [Page 8] RFC 2538 Storing Certificates in the DNS March 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

Eastlake & Gudmundsson Standards Track [Page 9] RFC 2538 Storing Certificates in the DNS March 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Eastlake & Gudmundsson Standards Track [Page 10]

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