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Versions: 00 01 02 03 04 draft-ietf-dane-smime

Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: March 10, 2013                                         Kirei AB
                                                       September 6, 2012


Using Secure DNS to Associate Certificates with Domain Names For S/MIME
                      draft-hoffman-dane-smime-04

Abstract

   This document describes how to use secure DNS to associate an S/MIME
   user's certificate with the intended domain name, similar to the way
   that DANE (RFC 6698) does for TLS.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 March 10, 2013.

Copyright Notice

   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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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  The SMIMEA Resource Record  . . . . . . . . . . . . . . . . . . 3
   3.  Domain Names for S/MIME Certificate Associations  . . . . . . . 4
   4.  SMIMEA RR Examples  . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Mandatory-to-Implement Features . . . . . . . . . . . . . . . . 4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
     6.1.  TLSA RRtype . . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6



































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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 to authenticate the association of the sender's
   certificate with the sender without trusting a configured trust
   anchor.  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.

   This document describes a mechanism for associating a user's
   certificate with the domain that is similar to that described in
   [RFC6698].  Most of the operational and security considerations for
   using the mechanism in this document are described in RFC 6698, and
   are not described here at all.  Only the major differences between
   this mechanism and those used in RFC 6698 are described here.  Thus,
   the reader must be familiar with RFC 6698 before reading this
   document.

1.1.  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

   The SMIMEA DNS resource record (RR) is used to associate an end
   entity certificate or public key with the associated email address,
   thus forming a "SMIMEA certificate association".  The semantics of
   how the SMIMEA RR is interpreted are given later in this document.



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   The type value for the SMIMEA RR type is defined in Section 6.1.  The
   SMIMEA RR is class independent.  The SMIMEA RR has no special TTL
   requirements.  The SMIMEA wire format and presentation format are the
   same as for the TLSA record.


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 [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.


4.  SMIMEA RR Examples

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


5.  Mandatory-to-Implement Features

   S/MIME MUAs conforming to this specification MUST be able to
   correctly interpret SMIMEA records with certificate usages 0, 1, 2,
   and 3.  S/MIME MUAs conforming to this specification MUST be able to
   compare a certificate association with a certificate offered by



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   another S/MIME MUA using selector types 0 and 1, and matching type 0
   (no hash used) and matching type 1 (SHA-256), and SHOULD be able to
   make such comparisons with matching type 2 (SHA-512).


6.  IANA Considerations

6.1.  TLSA RRtype

   This document uses a new DNS RR type, SMIMEA, whose value will be
   allocated by IANA from the Resource Record (RR) TYPEs subregistry of
   the Domain Name System (DNS) Parameters registry.


7.  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.


8.  Acknowledgements

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


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.




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   [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.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

9.2.  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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