Using Secure DNS to Associate Certificates with Domain Names For S/MIME
draft-hoffman-dane-smime-02

Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: August 25, 2012                                        Kirei AB
                                                       February 22, 2012

Using Secure DNS to Associate Certificates with Domain Names For S/MIME
                      draft-hoffman-dane-smime-02

Abstract

   S/MIME uses certificates for authenticating and encrypting messages.
   Users want their mail user agents to securely associate a certificate
   with the sender of an encrypted and/or signed message.  DNSSEC
   provides a mechanism for a zone operator to sign DNS information
   directly.  This way, bindings of certificates to users within a
   domain are asserted not by external entities, but by the entities
   that operate the DNS.  This document describes how to use secure DNS
   to associate an S/MIME user's certificate with the intended domain
   name.

   IMPORTANT NOTE: This draft is intentionally sketchy.  It is meant as
   a possible starting point for the DANE WG if it wants to consider
   making a protocol similar to TLSA, as described in
   draft-ietf-dane-protocol, but that applies to S/MIME.  The WG may or
   may not want to adopt such work, or if it does, may want to use a
   very different scheme from the one described here.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 25, 2012.

Copyright Notice

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Certificate Associations  . . . . . . . . . . . . . . . . . 3
     1.2.  Securing Certificate Associations . . . . . . . . . . . . . 3
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  The SMIMEA Resource Record  . . . . . . . . . . . . . . . . . . 3
   3.  Domain Names for S/MIME Certificate Associations  . . . . . . . 4
   4.  Use of S/MIME Certificate Associations in S/MIME  . . . . . . . 4
   5.  Mandatory-to-Implement Algorithms . . . . . . . . . . . . . . . 4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 5

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1.  Introduction

   S/MIME [RFC5751] messages often contain a certificate.  This
   certificate assists in authenticating the sender of the message and
   can be used for encrypting messages that will be sent in reply.  In
   order for the S/MIME receiver to authenticate that a message is from
   the sender whom is identified in the message the receiver's mail user
   agent (MUA) must validate that this certificate is associated with
   the purported sender.  Currently, the MUA must trust a trust anchor
   upon which the sender's certificate is rooted, and must successfully
   validate the certificate.

   Some people want a different way to authenticate the association of
   the sender's certificate with the sender without trusting a CA.
   Given that the DNS administrator for a domain name is authorized to
   give identifying information about the zone, it makes sense to allow
   that administrator to also make an authoritative binding between
   email messages purporting to come from the domain name and a
   certificate that might be used by someone authorized to send mail
   from those servers.  The easiest way to do this is to use the DNS.

   [[ More here about additional uses, such as CMS that is not S/MIME
   where the certificates have email addresses for the subject name. ]]

1.1.  Certificate Associations

   [[ Will mostly duplicate the text from Section 1.1 of
   draft-ietf-dane-protocol ]]

1.2.  Securing Certificate Associations

   [[ Will mostly duplicate the text from Section 1.2 of
   draft-ietf-dane-protocol ]]

1.3.  Terminology

   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 [RFC2119].

   This document also makes use of standard PKIX, DNSSEC, and S/MIME
   terminology.  See [RFC5280], [RFC4033], [RFC4034], [RFC4035], and
   [RFC5751] respectively, for these terms.

2.  The SMIMEA Resource Record

   [[ Mostly copied from draft-ietf-dane-protocol, but will define

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   "SMIMEA" instead of "TLSA". ]]

3.  Domain Names for S/MIME Certificate Associations

   Domain names are prepared for requests in the following manner.

   1.  The user name (the "left-hand side" of the email address, called
       the "local-part" in RFC 2822 [RFC2822]) becomes the left-most
       label in the prepared domain name.  This does not include the "@"
       character that separates the left and right sides of the email
       address.

   2.  The string "_smimecert" becomes the second left-most label in the
       prepared domain name.

   3.  The domain name (the "right-hand side" of the email address,
       called the "domain" in RFC 2822) is appended to the result of
       step 2 to complete the prepared domain name.

   For example, to request a SMIMEA resource record for a user whose
   address is "chris@example.com", you would use
   "chris._smimecert.example.com" in the request.

   [[ Need to discuss back-quoting, such as for chris.smith@example.com
   becoming chris\.smith._smimecert.example.com ]]

4.  Use of S/MIME Certificate Associations in S/MIME

   [[ Stuff here that sounds like TLSA but is actually about S/MIME
   senders and receivers.  Lots of text lifted from
   draft-ietf-dane-protocol. ]]

5.  Mandatory-to-Implement Algorithms

   [[ Mostly copied from draft-ietf-dane-protocol. ]]

6.  IANA Considerations

   [[ Mostly copied from draft-ietf-dane-protocol but using "SMIMEA"
   instead. ]]

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7.  Security Considerations

   [[ Mostly copied from draft-ietf-dane-protocol but is actually about
   S/MIME senders and receivers. ]]

8.  Acknowledgements

   [[ Mostly copied from draft-ietf-dane-protocol ]]

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010.

9.2.  Informative References

   [RFC2822]  Resnick, P., "Internet Message Format", RFC 2822,
              April 2001.

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Authors' Addresses

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org

   Jakob Schlyter
   Kirei AB

   Email: jakob@kirei.se

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