DNS CERT Record: Storing Certificates in the Domain Name System (DNS)

Network Working Group                                       S. Josefsson
Request for Comments: 4398                                    March 2006
Obsoletes: 2538
Category: Standards Track


          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 (2006).

Abstract

   Cryptographic public keys are frequently published, and their
   authenticity is 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).

   This document obsoletes RFC 2538.

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 ............................6
      2.3. X.509 OIDs .................................................6
   3. Appropriate Owner Names for CERT RRs ............................7
      3.1. Content-Based X.509 CERT RR Names ..........................8
      3.2. Purpose-Based X.509 CERT RR Names ..........................9
      3.3. Content-Based OpenPGP CERT RR Names ........................9
      3.4. Purpose-Based OpenPGP CERT RR Names .......................10
      3.5. Owner Names for IPKIX, ISPKI, IPGP, and IACPKIX ...........10
   4. Performance Considerations .....................................11
   5. Contributors ...................................................11
   6. Acknowledgements ...............................................11
   7. Security Considerations ........................................12
   8. IANA Considerations ............................................12
   9. Changes since RFC 2538 .........................................13
   10. References ....................................................14
      10.1. Normative References .....................................14
      10.2. Informative References ...................................15
   Appendix A.  Copying Conditions ...................................16

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 of 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 specifies a CERT resource record (RR) for the storage of
   certificates in the Domain Name System [1] [2].

   Section 3 discusses appropriate owner names for CERT RRs.

   Sections 4, 7, and 8 cover performance, security, and IANA
   considerations, respectively.

   Section 9 explains the changes in this document compared to RFC 2538.

   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 [3].

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 defined in Section 2.1
   below.

   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 [12].  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 to be examined.  Note that two different
   keys can have the same key tag.  However, the key MUST 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 complies to limits (such as key size)
   defined for DNS security.  If it is not, the algorithm field MUST be
   zero and the tag field is meaningless and SHOULD be zero.

   The algorithm field has the same meaning as the algorithm field in
   DNSKEY and RRSIG RRs [12], except that a zero algorithm field
   indicates that the algorithm is unknown to a secure DNS, which may
   simply be the result of the algorithm not having been standardized
   for DNSSEC [11].

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  IPKIX     The URL of an X.509 data object
             5  ISPKI     The URL of an SPKI certificate
             6  IPGP      The fingerprint and URL of an OpenPGP packet
             7  ACPKIX    Attribute Certificate
             8  IACPKIX   The URL of an Attribute Certificate
         9-252            Available for IANA assignment
           253  URI       URI private
           254  OID       OID private
           255            Reserved
     256-65279            Available for IANA assignment
   65280-65534            Experimental
         65535            Reserved

   These values represent the initial content of the IANA registry; see
   Section 8.

   The PKIX type is reserved to indicate an X.509 certificate conforming
   to the profile defined by the IETF PKIX working group [8].  The
   certificate section will start with a one-octet unsigned OID length
   and then an X.500 OID indicating the nature of the remainder of the
   certificate section (see Section 2.3, below).  (NOTE: X.509
   certificates do not include their X.500 directory-type-designating
   OID as a prefix.)

   The SPKI and ISPKI types are reserved to indicate the SPKI
   certificate format [15], for use when the SPKI documents are moved
   from experimental status.  The format for these two CERT RR types
   will need to be specified later.

   The PGP type indicates an OpenPGP packet as described in [5] and its
   extensions and successors.  This is used 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 ACPKIX type indicates an Attribute Certificate format [9].

   The IPKIX and IACPKIX types indicate a URL that will serve the
   content that would have been in the "certificate, CRL, or URL" field
   of the corresponding type (PKIX or ACPKIX, respectively).

   The IPGP type contains both an OpenPGP fingerprint for the key in
   question, as well as a URL.  The certificate portion of the IPGP CERT
   RR is defined as a one-octet fingerprint length, followed by the
   OpenPGP fingerprint, followed by the URL.  The OpenPGP fingerprint is
   calculated as defined in RFC 2440 [5].  A zero-length fingerprint or
   a zero-length URL are legal, and indicate URL-only IPGP data or
   fingerprint-only IPGP data, respectively.  A zero-length fingerprint
   and a zero-length URL are meaningless and invalid.

   The IPKIX, ISPKI, IPGP, and IACPKIX types are known as "indirect".
   These types MUST be used when the content is too large to fit in the
   CERT RR and MAY be used at the implementer's discretion.  They SHOULD
   NOT be used where the DNS message is 512 octets or smaller and could
   thus be expected to fit a UDP packet.

   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 [10], 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 an ISO OID prefix.  The certificate section will start with a
   one-octet 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 [12].

   The certificate/CRL portion is represented in base 64 [16] and may be
   divided 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.  However, 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 DNS names with
   characters that require DNS quoting as per Section 5.1 of RFC 1035
   [2].

   The choice of name under which CERT RRs are stored is important to
   clients that perform CERT queries.  In some situations, the clients
   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 email 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.

   Therefore, two owner name guidelines are defined: 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 a name that a client retrieving
   CERT RRs ought to know already; for example, the host name of an
   X.509 protected service or the Key ID of an OpenPGP key.  The
   content-based and purpose-based owner name may be the same; for
   example, when a client looks up a key based on the From: address of
   an incoming email.

   Implementations SHOULD use the purpose-based owner name guidelines
   described in this document and MAY use CNAME RRs at content-based
   owner names (or other names), pointing to the purpose-based owner
   name.

   Note that this section describes an application-based mapping from
   the name space used in a certificate to the name space used by DNS.
   The DNS does not infer any relationship amongst CERT resource records
   based on similarities or differences of the DNS owner name(s) of CERT
   resource records.  For example, if multiple labels are used when
   mapping from a CERT identifier to a domain name, then care must be
   taken in understanding wildcard record synthesis.

3.1.  Content-Based X.509 CERT RR Names

   Some X.509 versions, such as the PKIX profile of X.509 [8], permit
   multiple names to be associated with subjects and issuers under
   "Subject Alternative Name" and "Issuer Alternative Name".  For
   example, the PKIX profile has such Alternate Names with an ASN.1
   specification as follows:

      GeneralName ::= CHOICE {
           otherName                       [0]     OtherName,
           rfc822Name                      [1]     IA5String,
           dNSName                         [2]     IA5String,
           x400Address                     [3]     ORAddress,
           directoryName                   [4]     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 ought to 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 ought to be used.
   3.  If neither of the above is used, but a URI containing a domain
       name is present, that domain name ought to be used.
   4.  If none of the above is included but a character string name is
       included, then it ought to be treated as described below for
       OpenPGP names.
   5.  If none of the above apply, then the distinguished name (DN)
       ought to be mapped into a domain name as specified in [4].

   Example 1: 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/>.  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: 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>".  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

   Due to the difficulty for clients that do not already possess a
   certificate to reconstruct the content-based owner name,
   purpose-based owner names are recommended in this section.
   Recommendations for purpose-based owner names vary per scenario.  The
   following table summarizes the purpose-based X.509 CERT RR owner name
   guidelines for use with S/MIME [17], SSL/TLS [13], and IPsec [14]:

    Scenario             Owner name
    ------------------   ----------------------------------------------
    S/MIME Certificate   Standard translation of an RFC 2822 email
                         address.  Example: An S/MIME certificate for
                         "postmaster@example.org" will use a standard
                         hostname translation of the owner name,
                         "postmaster.example.org".

    TLS Certificate      Hostname of the TLS server.

    IPsec Certificate    Hostname of the IPsec machine and/or, for IPv4
                         or IPv6 addresses, the fully qualified domain
                         name in the appropriate reverse domain.

   An alternate approach for IPsec is to store raw public keys [18].

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
   ought to 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:

      $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 containing encrypted or
   signed data 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
   auxiliary data that may be available.  In this case, use of an owner
   name identical to the key fingerprint and the key ID expressed in
   hexadecimal [16] is recommended.  For example:

      $ORIGIN example.org.
      0424D4EE81A0E3D119C6F835EDA21E94B565716F IN CERT PGP ...
      F835EDA21E94B565716F                     IN CERT PGP ...
      B565716F                                 IN CERT PGP ...

   If the same key material is stored for several owner names, the use
   of CNAME may help avoid data duplication.  Note that CNAME is not
   always applicable, because it maps one owner name to the other for
   all purposes, which may be sub-optimal when two keys with the same
   Key ID are stored.

3.5.  Owner Names for IPKIX, ISPKI, IPGP, and IACPKIX

   These types are stored under the same owner names, both purpose- and
   content-based, as the PKIX, SPKI, PGP, and ACPKIX types.

4.  Performance Considerations

   The Domain Name System (DNS) protocol was designed for small
   transfers, typically below 512 octets.  While larger transfers will
   perform correctly and work is underway to make larger transfers more
   efficient, it is still advisable at this time that every reasonable
   effort be made to minimize the size of certificates stored within the
   DNS.  Steps that can be taken may include using the fewest possible
   optional or extension fields and using short field values for
   necessary variable-length fields.

   The RDATA field in the DNS protocol may only hold data of size 65535
   octets (64kb) or less.  This means that each CERT RR MUST NOT contain
   more than 64kb of payload, even if the corresponding certificate or
   certificate revocation list is larger.  This document addresses this
   by defining "indirect" data types for each normal type.

   Deploying CERT RRs to support digitally signed email changes the
   access patterns of DNS lookups from per-domain to per-user.  If
   digitally signed email and a key/certificate lookup based on CERT RRs
   are deployed on a wide scale, this may lead to an increased DNS load,
   with potential performance and cache effectiveness consequences.
   Whether or not this load increase will be noticeable is not known.

5.  Contributors

   The majority of this document is copied verbatim from RFC 2538, by
   Donald Eastlake 3rd and Olafur Gudmundsson.

6.  Acknowledgements

   Thanks to David Shaw and Michael Graff for their contributions to
   earlier works that motivated, and served as inspiration for, this
   document.

   This document was improved by suggestions and comments from Olivier
   Dubuisson, Scott Hollenbeck, Russ Housley, Peter Koch, Olaf M.
   Kolkman, Ben Laurie, Edward Lewis, John Loughney, Allison Mankin,
   Douglas Otis, Marcos Sanz, Pekka Savola, Jason Sloderbeck, Samuel
   Weiler, and Florian Weimer.  No doubt the list is incomplete.  We
   apologize to anyone we left out.

7.  Security Considerations

   By definition, certificates contain their own authenticating
   signatures.  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.

   If an organization chooses to issue certificates for its employees,
   placing CERT RRs in the DNS by owner name, and if DNSSEC (with NSEC)
   is in use, it is possible for someone to enumerate all employees of
   the organization.  This is usually not considered desirable, for the
   same reason that enterprise phone listings are not often publicly
   published and are even marked confidential.

   Using the URI type introduces another level of indirection that may
   open a new vulnerability.  One method of securing that indirection is
   to include a hash of the certificate in the URI itself.

   If DNSSEC is used, then the non-existence of a CERT RR and,
   consequently, certificates or revocation lists can be securely
   asserted.  Without DNSSEC, this is not possible.

8.  IANA Considerations

   The IANA has created a new registry for CERT RR: certificate types.
   The initial contents of this registry is:

       Decimal   Type     Meaning                           Reference
       -------   ----     -------                           ---------
             0            Reserved                          RFC 4398
             1   PKIX     X.509 as per PKIX                 RFC 4398
             2   SPKI     SPKI certificate                  RFC 4398
             3   PGP      OpenPGP packet                    RFC 4398
             4   IPKIX    The URL of an X.509 data object   RFC 4398
             5   ISPKI    The URL of an SPKI certificate    RFC 4398
             6   IPGP     The fingerprint and URL           RFC 4398
                          of an OpenPGP packet
             7   ACPKIX   Attribute Certificate             RFC 4398
             8   IACPKIX  The URL of an Attribute           RFC 4398
                             Certificate

         9-252            Available for IANA assignment
                             by IETF Standards action
           253   URI      URI private                       RFC 4398
           254   OID      OID private                       RFC 4398
           255            Reserved                          RFC 4398
     256-65279            Available for IANA assignment
                          by IETF Consensus
   65280-65534            Experimental                      RFC 4398
         65535            Reserved                          RFC 4398

   Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can
   only be assigned by an IETF standards action [6].  This document
   assigns 0x0001 through 0x0008 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 ought to satisfy most
   requirements for proprietary or private types.

   The CERT RR reuses the DNS Security Algorithm Numbers registry.  In
   particular, the CERT RR requires that algorithm number 0 remain
   reserved, as described in Section 2.  The IANA will reference the
   CERT RR as a user of this registry and value 0, in particular.

9.  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.   The SPKI types has been reserved, until RFC 2692/2693 is moved
        from the experimental status.
   10.  Added indirect types IPKIX, ISPKI, IPGP, and IACPKIX.

   11.  An IANA registry of CERT type values was created.

10.  References

10.1.  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]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

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

   [8]   Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
         Public Key Infrastructure Certificate and Certificate
         Revocation List (CRL) Profile", RFC 3280, April 2002.

   [9]   Farrell, S. and R. Housley, "An Internet Attribute Certificate
         Profile for Authorization", RFC 3281, April 2002.

   [10]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
         Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
         January 2005.

   [11]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "DNS Security Introduction and Requirements", RFC 4033,
         March 2005.

   [12]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
         "Resource Records for the DNS Security Extensions", RFC 4034,
         March 2005.

10.2.  Informative References

   [13]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
         RFC 2246, January 1999.

   [14]  Kent, S. and K. Seo, "Security Architecture for the Internet
         Protocol", RFC 4301, December 2005.

   [15]  Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas, B.,
         and T. Ylonen, "SPKI Certificate Theory", RFC 2693,
         September 1999.

   [16]  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
         RFC 3548, July 2003.

   [17]  Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
         (S/MIME) Version 3.1 Message Specification", RFC 3851,
         July 2004.

   [18]  Richardson, M., "A Method for Storing IPsec Keying Material in
         DNS", RFC 4025, March 2005.

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.

Author's Address

   Simon Josefsson

   EMail: simon@josefsson.org

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