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PKIX Working Group                                           J. Schlyter
Internet-Draft                                      Carlstedt Research &
Expires: December 9, 2002                                     Technology
                                                            L. Johansson
                                                    Stockholm University
                                                           June 10, 2002


                 DNS as X.509 PKIX Certificate Storage
                       draft-schlyter-pkix-dns-02

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on December 9, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   A major problem facing PKIX deployment and implementation is the
   problem of constructing certificate paths for input to the path
   validation algorithm.  This draft describes the use of the DNS as a
   certificate store and it's implication for path validation in PKIX.








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Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2. Storing PKIX certificates in DNS . . . . . . . . . . . . . . . . 3
   3. Certificate lookup algorithm . . . . . . . . . . . . . . . . . . 4
   4. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5. Security Considerations  . . . . . . . . . . . . . . . . . . . . 4
      References . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
      Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
   A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
      Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 7








































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1. Introduction

   A major problem facing PKIX deployment and implementation is the
   problem of constructing certificate paths for input to the path
   validation algorithm described in RFC 2459 [2].  This problem can be
   solved by successively looking at the issuerAltName extension of each
   certificate and using the information found there together with a
   storage and transport protocol for certificates to find a set of
   candidate certificates associated with the issuerAltName.

   Using the CERT RR [5] a certificate can be published using DNS.  This
   draft describes the use of DNS as a certificate store and it's
   implication for path validation in PKIX.

   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 RFC 2119 [1].

2. Storing PKIX certificates in DNS

   A PKIX certificate is published in DNS using the CERT RR [5] for a
   given domain name which SHOULD be equal to the dnsName component of
   the subjectAltName extension in the certificate.  Multiple
   certificates may be present for each domain name and all SHOULD have
   the same subject DN.  If the domain name does not match the dnsName
   component of the subjectAltName extension the client SHOULD notify
   the user of this and allow the user to decide weather to allow the
   use of the certificate or not.

   When constructing a certificate path for validation the client MAY
   use the AuthorityKeyIdentifier and SubjectKeyIdentifier extensions to
   select the (set of) certificates to use.

   There are a few important cases when multiple CA certificates are
   published in CERT RRs for given domain name:

      Multiple certificates each signed by another member of the same
      set.  This situation occurs when a self-signed certificate issues
      a certificate under the same DN (for the purpose of adding policy
      for instance).

      Multiple certificates, either self-signed or issued by another CA,
      with different validity periods.

      Root key roll-over as described in section 2.4 of RFC 2510 [3]
      where exactly 4 certificates would be published using DNS.





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3. Certificate lookup algorithm

   Given a certificate with a non-empty issuerAltName extension of type
   dnsName, perform a DNS lookup of the corresponding domain name with
   the class IN and type CERT.  For each of the certificates returned
   that are of type PKIX, implementations SHOULD verify that the
   subjectAltName in the certificate contains a component of type
   dnsName with the same domain name as the one where the certificate
   was published using the DNS.

   If a certificate obtained by this algorithm is a self-signed
   certificate and was successfully verified by DNSSEC [4], the user
   SHOULD be given the opportunity to use this certificate as a trust
   anchor.

   The result of this algorithm is a set of of certificates suitable for
   input to the PKIX path validation algorithm.

4. Example

      Client A talks TLS to server B and receives a certificate chain
      ending in a cert (X) with issuerAltName:dnsName set to
      ca.example.com.

      Client A does path validation on the chain and is unable to find X
      in its list of trusted roots.

      Client A queries the DNS for the CERT record for ca.example.com
      and receives a set of certificates.

      Client A looks for X in the set of certificates.  If found, and
      depending on local configuration, A trusts the certificate for use
      as a TLS client trust anchor and adds it to the list of trusted
      roots.

      Path validation now succeeds.


5. Security Considerations

   This document describes a mechanism for automated download of
   certificates from DNS with special provision for bridging trust
   between a PKIX PKI and DNSSEC.  However, if only self-signed end-
   entity PKIX certificates are published using DNS the benefits of PKIX
   policy and key usage management is lost.

   The benefit of this mechanism is a potential for added protection of
   certificate trust anchors in common use on the Internet by leveraging



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   DNSSEC infrastructure.

References

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

   [2]  Housley, R., Ford, W., Polk, T. and D. Solo, "Internet X.509
        Public Key Infrastructure Certificate and CRL Profile", RFC
        2459, January 1999.

   [3]  Adams, C. and S. Farrell, "Internet X.509 Public Key
        Infrastructure Certificate Management Protocols", RFC 2510,
        March 1999.

   [4]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

   [5]  Eastlake, D. and O. Gudmundsson, "Storing Certificates in the
        Domain Name System (DNS)", RFC 2538, March 1999.


Authors' Addresses

   Jakob Schlyter
   Carlstedt Research & Technology
   Stora Badhusgatan 18-20
   Goteborg  SE-411 21
   Sweden

   EMail: jakob@crt.se
   URI:   http://www.crt.se/~jakob/


   Leif Johansson
   Stockholm University
   IT and Media Unit
   Frescati Hagvag 8
   Stockholm  SE-106 91
   Sweden

   Phone: +46 8 16 45 41
   EMail: leifj@it.su.se
   URI:   http://www.it.su.se







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Appendix A. Acknowledgements

   The author gratefully acknowledges, in no particular order, the
   contributions of the following persons:

      Martin Fredriksson

      Niklas Hallqvist

      Edward Lewis









































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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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Acknowledgement

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



















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