Network Working Group                                         P. Hoffman
Internet-Draft                                            VPN Consortium
Intended status: Standards Track                             J. Schlyter
Expires: February 23, 2015                                      Kirei AB
                                                         August 22, 2014

Using Secure DNS to Associate Certificates with Domain Names For S/MIME


   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

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   This Internet-Draft will expire on February 23, 2015.

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Table of Contents

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

1.  Introduction

   S/MIME [RFC5751] messages often contain a certificate (some messages
   contain more than one certificate).  These certificates assist 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 who
   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.  There are other requirements on the MUA, such as
   associating the identity in the certificate with that of the message,
   that are out of scope for this document.

   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.

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   This document describes a mechanism for associating a user's
   certificate with the domain that is similar to that described in DANE
   itself [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.

   serious discussion about what the DANE set of specifications covering
   TLS for HTTP, TLS for SMTP, S/MIME, OpenPGP, and so on are meant for.
   They could be used for acquisition of key assocation material, for
   discovering services that use the keying material, for having
   assurance that a service that uses the keying material should be
   available, or some combination of these.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 PKIX [RFC5280], DNSSEC [RFC4033], [RFC4034],
   [RFC4035], and SMIME [RFC5751] 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.
   Note that the information returned in the SMIMEA record might be for
   the end entity certificate, or it might be for the trust anchor or an
   intermediate certificate.

   The type value for the SMIMEA RRtype is defined in Section 5.1.  The
   SMIMEA resource record is class independent.  The SMIMEA resource
   record has no special TTL requirements.

   The SMIMEA wire format and presentation format are the same as for
   the TLSA record as described in section 2.1 of RFC 6698.  The
   certificate usage field, the selector field, and the matching type
   field have the same format; the semantics are also the same except
   where RFC 6698 talks about TLS at the target protocol for the
   certificate information.

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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 the mail message format definition [RFC2822]
       and the "local part" in the specification for internationalized
       email [RFC6530]), is hashed using the SHA2-224 [RFC5754]
       algorithm (with the hash being represented in its hexadecimal
       representation, 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.  The
       string that is used for the local part is a Unicode string
       encoded in UTF-8.

   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 "", calculate the SHA-224 of "chris",
   which is 0x3f51f4663b2b798560c5b9e16d6069a28727f62518c3a1b33f7f5214.
   The request is thus:

   The corresponding resource record in the zone might look
      IN SMIMEA (
      0 0 1 d2abde240d7cd3ee6b4b28c54df034b9
            7983a1d16e8a410e4561cb106618e971 )

   Design note: Hashing the user name with SHA-224 and using the
   hexidecimal encoding of that hash allows local parts that have
   characters that would prevent their use in domain names in typical
   applications.  Even though the DNS protocol itself can use any octet
   value in a label, most applications that use DNS names are limited to
   a much smaller set of allowed characters.  For example, a period
   (".") is a valid character in a local part, but would wreak havoc in
   a domain name unless the application using the name somehow quoted
   it.  Similarly, RFC 6530 allows non-ASCII characters in local parts,

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   and encoding a local part with non-ASCII characters as the hex of the
   SHA-224 renders the name usable in applications that use the DNS.

   Wildcards can be more useful for SMIMEA than they are for TLSA.  If a
   site publishes a trust anchor certificate for all users on the site
   (certificate usage 0 or 2), it could make sense to use a wildcard
   resource record such as "*".

4.  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
   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).

5.  IANA Considerations

5.1.  SMIMEA RRtype

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

   TODO: there needs to be new registries for certificate usages,
   selectors, and maching types, pre-populated with the values from

6.  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.

   Client treatment of any information included in the trust anchor is a
   matter of local policy.  This specification does not mandate that
   such information be inspected or validated by the domain name

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7.  Acknowledgements

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

8.  References

8.1.  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.

   [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
              Message Syntax", RFC 5754, 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.

8.2.  Informative References

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

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

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Authors' Addresses

   Paul Hoffman
   VPN Consortium


   Jakob Schlyter
   Kirei AB


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