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

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 25, 2005.

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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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




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