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Versions: 00 01 02 03 04 RFC 2538

INTERNET-DRAFT                                          CERTs in the DNS
                                                              March 1998
                                                  Expires September 1998



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

                         Donald E. Eastlake 3rd
                           Olafur Gudmundsson



Status of This Document

   This draft, file name draft-ietf-dnssec-certs-02.txt, is intended to
   become a Proposed Standard RFC.  Distribution of this document is
   unlimited. Comments should be sent to the DNSSEC mailing list <dns-
   security@tis.com> or to the authors.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months.  Internet-Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet-
   Drafts as reference material or to cite them other than as a
   ``working draft'' or ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   1id-abstracts.txt listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (East USA), ftp.isi.edu (West USA),
   ftp.nordu.net (North Europe), ftp.nis.garr.it (South Europe),
   munnari.oz.au (Pacific Rim), or ftp.is.co.za (Africa).



Abstract

   Cryptographic public key are frequently published and their
   authenticity demonstrated by certificate systems.  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).








D. Eastlake, O. Gudmundsson                                     [Page 1]


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Table of Contents

      Status of This Document....................................1
      Abstract...................................................1

      Table of Contents..........................................2

      1. Introduction............................................3

      2. The CERT Resource Record................................4
      2.1 Certificate Type Values................................4
      2.2 Text Representation of CERT RRs........................5
      2.3 X.509 OIDs.............................................6

      3. Appropriate Owner Names for CERT RRs....................7
      3.1 X.509 CERT RR Names....................................7
      3.2 PGP CERT RR Names......................................8

      4. Performance Considerations..............................9
      5. Security Considerations.................................9

      References................................................10
      Authors Addresses.........................................10
      Expiration and File Name..................................10




























D. Eastlake, O. Gudmundsson                                     [Page 2]


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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 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 and 5 below cover performance and security considerations,
   respectively.
































D. Eastlake, O. Gudmundsson                                     [Page 3]


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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 [draft-ietf-dnssec-secext2-*.txt] except that a zero
   algorithm field indicates the algorithm is unknown to a secure DNS,
   which may simply be the result of it 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 [draft-ietf-
   dnssec-secext2-*.txt].  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


D. Eastlake, O. Gudmundsson                                     [Page 4]


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          253   URL      URL 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).

   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 1991 and its extensions and successors.

   The URL private type indicates a certificate format defined by a URL
   prefix.  The certificate portion of the CERT RR MUST begin with a
   null terminated URL [RFC 1738] and the data after the null is the
   private format certificate itself.  The URL 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 URL equality but can use various forms of pattern matching
   so that, for example, subtype or version information can also be
   encoded into the URL.

   The OID private type indicates a private format certificate specified
   by an ISO OID prefix.  The certificate section will start with a one
   byte unsigned OID length and then an 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 [draft-ietf-dnssec-secext2-*.txt].

   The certificate / CRL portion is represented in base 64 and may be


D. Eastlake, O. Gudmundsson                                     [Page 5]


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   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 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



















D. Eastlake, O. Gudmundsson                                     [Page 6]


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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.



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".

   The following is 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
      4.2 below.
   (4) 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.


D. Eastlake, O. Gudmundsson                                     [Page 7]


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3.2 PGP CERT RR Names

   PGP signed keys (certificates) use a general character string name
   [RFC 1991]. 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.










































D. Eastlake, O. Gudmundsson                                     [Page 8]


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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. Security Considerations

   By definition, certificates contains 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.




















D. Eastlake, O. Gudmundsson                                     [Page 9]


INTERNET-DRAFT                                          CERTs in the DNS


References

   RFC 1034 - P. Mockapetris, "Domain Names - Concepts and Facilities",
   STD 13, November 1987.

   RFC 1035 - P. Mockapetris, "Domain Names - Implementation and
   Specifications", STD 13, November 1987.

   RFC 1738 - T. Berners-Lee, L. Masinter & M.  McCahill, "Uniform
   Resource Locators (URL)", December 1994.

   RFC 1991 - D. Atkins, W. Stallings & P.  Zimmermann, "PGP Message
   Exchange Formats", August 1996.

   RFC 2247 - S. Kille, M. Wahl, A. Grimstad, R. Huber, S. Sataluri,
   "Using Domains in LDAP/X.500 Distinguished Names", January 1998.

   draft-ietf-dnssec-secext2-*.txt - D. Eastlake, "Domain Name System
   (DNS) Security Extensions", ?.



Authors Addresses

   Donald E. Eastlake 3rd
   CyberCash, Inc.
   318 Acton Street
   Carlisle, MA 01741 USA

   Telephone:   +1 978 287 4877
                +1 703 620-4200 (main office, Reston, VA)
   FAX:         +1 978 371 7148
   email:       dee@cybercash.com


   Olafur Gudmundsson
   Trusted Information Systems
   3060 Washington Road, Route 97
   Glenwood, MD 21738 USA

   Telephone:   +1 301 854 6889
   email:       ogud@tis.com



Expiration and File Name

   This draft expires September 1998.

   Its file name is draft-ietf-dnssec-certs-02.txt.


D. Eastlake, O. Gudmundsson                                    [Page 10]


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