Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: September 10, 2012                                     Kirei AB
                                                           March 9, 2012


Using Secure DNS to Associate Certificates with Domain Names For S/MIME
                      draft-hoffman-dane-smime-03

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 September 10, 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   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.  TLSA RDATA Wire Format  . . . . . . . . . . . . . . . . . . . . 4
   4.  TLSA RR Presentation Format . . . . . . . . . . . . . . . . . . 4
   5.  TLSA RR Examples  . . . . . . . . . . . . . . . . . . . . . . . 4
   6.  Domain Names for S/MIME Certificate Associations  . . . . . . . 4
   7.  Use of S/MIME Certificate Associations in S/MIME  . . . . . . . 5
   8.  Mandatory-to-Implement Features . . . . . . . . . . . . . . . . 5
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   10. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     12.1. Normative References  . . . . . . . . . . . . . . . . . . . 5
     12.2. Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

















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.  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.


.  The SMIMEA Resource Record

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



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   "SMIMEA" instead of "TLSA". ]]


.  TLSA RDATA Wire Format

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


.  TLSA RR Presentation Format

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


.  TLSA RR Examples

   [[ Similar in format to draft-ietf-dane-protocol, but with very
   different examples, of course. ]]


.  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 [RFC2822] and the "local part" in [RFC6530]),
       is encoded with Base32 [RFC4648], to become 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 [RFC2822]) 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
   "MNUHE2LT._smimecert.example.com" in the request.

   Design note: Encoding the user name with Base32 allows local parts
   that have characters that would prevent their use in domain names.
   For example, a period (".") is a valid character in a local part, but
   would wreak havoc in a domain name.  Similarly, [RFC6530] allows non-
   ASCII characters in local parts, and encoding a local part with non-
   ASCII characters with Base32 renders the name usable in the DNS.




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.  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. ]]


.  Mandatory-to-Implement Features

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


.  IANA Considerations

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


.  Security Considerations

   DNS zones that are signed with DNSSEC using NSEC for denial of
   existence are susceptible to zone-walking, a mechanism that allow
   someone to enumerate all the names in the zone.  Someone who wanted
   to collect email addresses from a zone that uses SMIMEA might use
   such a mechanism.  DNSSEC-signed zones using NSEC3 for denial of
   existence are significantly less susceptible to zone-walking.
   Someone could still attempt a dictionary attack on the zone to find
   SMIMEA records, just as they can use dictionary attacks on an SMTP
   server to see which addresses are valid.

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


.  Acknowledgements

   Miek Gieben and Martin Pels contributed technical ideas and support
   to this document.


.  References

12.1.  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.



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              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.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [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.

12.2.  Informative References

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

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, February 2012.


Authors' Addresses

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org


   Jakob Schlyter
   Kirei AB

   Email: jakob@kirei.se







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