Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: September 4, 2011                                      Kirei AB
                                                           March 3, 2011


Using Secure DNS to Associate Certificates with Domain Names For S/MIME
                      draft-hoffman-dane-smime-00

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 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
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   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 4, 2011.

Copyright Notice




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   Copyright (c) 2011 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
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   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 . . . . . . . . . . . . . . . . . . . . . . . . 4
   2.  Getting S/MIME Certificate Associations from the DNS  . . . . . 4
     2.1.  Requested Domain Name . . . . . . . . . . . . . . . . . . . 5
     2.2.  Format of the Resource Record . . . . . . . . . . . . . . . 5
     2.3.  Making Certificate Associations . . . . . . . . . . . . . . 5
     2.4.  Presentation Format . . . . . . . . . . . . . . . . . . . . 5
     2.5.  Wire Format . . . . . . . . . . . . . . . . . . . . . . . . 6
   3.  Use of S/MIME Certificate Associations in S/MIME  . . . . . . . 6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

















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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 the 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

   In this document, a certificate association is based on a
   cryptographic hash of a certificate (sometimes called a
   "fingerprint") or on the certificate itself.  For a fingerprint, a
   hash is taken of the binary, DER-encoded certificate, and that hash
   is the certificate association; the type of hash function used can be
   chosen by the DNS administrator.  When using the certificate itself
   in the certificate association, the entire certificate in the normal
   format is used.  This document also only applies to PKIX [RFC5280]
   certificates.

   Certificate associations are made between a certificate or the hash
   of a certificate and an email address (sometimes called an "RFC 822
   address" or a variation of that term).  A DNS query can return
   multiple certificate associations, such as in the case of a mail user
   who is changing from one certificate to another.

1.2.  Securing Certificate Associations

   This document defines a secure method to associate the certificate
   that is in an S/MIME email message (or was received in some similar
   fashion) with a domain name using DNS protected by DNSSEC.  Because
   the certificate association was retrieved based on a DNS query, the



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   domain name in the query is by definition associated with the
   certificate.

   DNSSEC, which is defined in RFCs 4033, 4034, and 4035 ([RFC4033],
   [RFC4034], and [RFC4035]), uses cryptographic keys and digital
   signatures to provide authentication of DNS data.  Information
   retrieved from the DNS and that is validated using DNSSEC is thereby
   proved to be the authoritative data.  The DNSSEC signature MUST be
   validated on all responses in order to assure the proof of origin of
   the data.

   This document only relates to securely getting the DNS information
   for the certificate association using DNSSEC; other secure DNS
   mechanisms are out of scope.

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].

   A note on terminology: Some people have said that this protocol is a
   form of "certificate exclusion".  This is true, but in a very unusual
   sense.  That is, a DNS reply that contains two of the certificate
   types defined here inherently excludes every other possible
   certificate in the universe other than those found with a pre-image
   attack against one of those two.  The certificate type defined here
   is better thought of as "enumeration" of a small number of
   certificate associations, not "exclusion" of a near-infinite number
   of other certificates.

   Some of the terminology in this draft may not match with the
   terminology used in RFC 5280.  This will be fixed in future versions
   of this draft, with help from the PKIX community.  In specific, we
   need to say (in a PKIX-appropriate way) that when we say "valid up
   to" and "chains to", full RFC 5280 path processing including
   revocation status checking is intended.


.  Getting S/MIME Certificate Associations from the DNS

   This document defines a new DNS resource record type, "SMIMEA".  A
   query on a prepared domain name for the SMIMEA RR can return one or
   more records of the type SMIMEA.  The SMIMEA RRType is TBD.







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2.1.  Requested Domain Name

   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 ]]

2.2.  Format of the Resource Record

   [[ This will be the same as for TLSA because there is no reason for
   the two to diverge.  Lots of text lifted from the TLSA document. ]]

2.3.  Making Certificate Associations

   [[ Stuff here that sounds like TLSA but is actually about S/MIME
   senders and receivers.  Lots of text lifted from the TLSA document.
   ]]

2.4.  Presentation Format

   The RDATA of the presentation format of the SMIMEA resource record
   consists of two numbers (certificate and hash type) followed by the
   bytes containing the certificate or the hash of the associated
   certificate itself, presented in hex.  An example of a SHA-256 hash
   (type 2) of an end-entity certificate (type 1) would be:

   chris._smimecert.example.com. IN SMIMEA (
      1 2 5c1502a6549c423be0a0aa9d9a16904de5ef0f5c98
          c735fcca79f09230aa7141 )

   An example of an unhashed CA certificate (type 2) would be:



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   chris._smimecert.example.com. IN SMIMEA (
      2 0 308202c5308201ada00302010202090... )

   Because the length of hashes and certificates can be quite long,
   presentation format explicitly allows line breaks and white space in
   the hex values; those characters are removed when converting to the
   wire format.

2.5.  Wire Format

   The wire format is:

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Cert type   |   Hash type   |                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
   /                                                               /
   /                    Certificate for association                /
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The wire format for the RDATA in the first example given above would
   be:

   chris._smimecert.example.com. IN TYPE65534 \# 34 ( 01025c1502a6549c42
                 3be0a0aa9d9a16904de5ef0f5c98c735fcca79f09230aa7141 )

   The wire format for the RDATA in the second example given above would
   be:

   chris._smimecert.example.com. IN TYPE65534 \# 715 0200308202c5308...

   Note that in the preceding examples, "TYPE65534" is given as an
   example.  That RR Type is in the IANA "private use" range; the real
   RR Type for SMIMEA will be issued by IANA, as described in the IANA
   Considerations section below.


.  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 the TLSA document.
   ]]







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.  IANA Considerations

   [[ Mostly copied from TLSA but using "SMIMEA" instead. ]]


.  Security Considerations

   [[ Stuff here that sounds like TLSA but is actually about S/MIME
   senders and receivers.  Lots of text lifted from the TLSA document,
   but with some significant differences. ]]


.  Acknowledgements

   Many of the ideas in this document have been discussed over many
   years.  More recently, the ideas have been discussed by the authors
   and others in a more focused fashion.  In particular, some of the
   ideas here originated with Paul Vixie, Dan Kaminsky, Jeff Hodges,
   Phill Hallam-Baker, Simon Josefsson, Warren Kumari, Adam Langley,
   Ilari Liusvaara, Scott Schmit, and Ondrej Sury.


.  References

7.1.  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



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              Specification", RFC 5751, January 2010.

7.2.  Informative References

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


Authors' Addresses

   Paul Hoffman
   VPN Consortium

   Email: paul.hoffman@vpnc.org


   Jakob Schlyter
   Kirei AB

   Email: jakob@kirei.se































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