Network Working Group                                         P. Hoffman
Internet-Draft                                                     ICANN
Intended status: Experimental                                J. Schlyter
Expires: September 17, 2017                                     Kirei AB
                                                          March 16, 2017

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 DNS-Based Authentication of Named Entities (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.

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   This Internet-Draft will expire on September 17, 2017.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Experiment Goal . . . . . . . . . . . . . . . . . . . . .   3
   2.  The SMIMEA Resource Record  . . . . . . . . . . . . . . . . .   4
   3.  Location of the SMIMEA Record . . . . . . . . . . . . . . . .   4
   4.  Email Address Variants and Internationalization
       Considerations  . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Mandatory-to-Implement Features . . . . . . . . . . . . . . .   6
   6.  Application Use of S/MIME Certificate Associations  . . . . .   6
   7.  Certificate Size and DNS  . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     9.1.  Response Size . . . . . . . . . . . . . . . . . . . . . .   8
     9.2.  Email Address Information Leak  . . . . . . . . . . . . .   8
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     11.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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.  Others to want mitigate the difficulty of finding
   certificates from outside the enterprise.  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], as updated by [RFC7218] and [RFC7671]; it is also
   similar to the mechanism given in [RFC7929] for OpenPGP.  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",
   document are to be interpreted as described in [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.

1.2.  Experiment Goal

   This specification is one experiment in improving access to public
   keys for end-to-end email security.  There are a range of ways in
   which this can reasonably be done for OpenPGP or S/MIME, for example,
   using the DNS, or SMTP, or HTTP.  Proposals for each of these have
   been made with various levels of support in terms of implementation
   and deployment.  For each such experiment, specifications such as
   this will enable experiments to be carried out that may succeed or
   that may uncover technical or other impediments to large- or small-
   scale deployments.  The IETF encourages those implementing and
   deploying such experiments to publicly document their experiences so
   that future specifications in this space can benefit.

   This document defines an RRtype whose use is Experimental.  The goal
   of the experiment is to see whether encrypted email usage will
   increase if an automated discovery method is available to MTAs and
   MUAs to help the end user with email encryption key management.

   It is unclear if this RRtype will scale to some of the larger email
   service deployments.  Concerns have been raised about the size of the
   SMIMEA record and the size of the resulting DNS zone files.  This
   experiment hopefully will give the working group some insight into
   whether or not this is a problem.

   If the experiment is successful, it is expected that the findings of
   the experiment will result in an updated document for standards track

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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 resource record 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.  This mechanism is similar to
   the one given in [RFC7929] for OpenPGP.

   The type value for the SMIMEA RRtype is defined in Section 8.  The
   SMIMEA resource record is class independent.

   The SMIMEA wire format and presentation format are the same as for
   the TLSA record as described in section 2.1 of [RFC6698].  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.

3.  Location of the SMIMEA Record

   The DNS does not allow the use of all characters that are supported
   in the "local-part" of email addresses as defined in [RFC5322] and
   [RFC6530].  Therefore, email addresses are mapped into DNS using the
   following method:

   1.  The "left-hand side" of the email address, called the "local-
       part" in both the mail message format definition [RFC5322] and in
       the specification for internationalized email [RFC6530]) is
       encoded in UTF-8 (or its subset ASCII).  If the local-part is
       written in another charset it MUST be converted to UTF-8.

   2.  The local-part is first canonicalized using the following rules.
       If the local-part is unquoted, any whitespace (CFWS) around dots
       (".") is removed.  Any enclosing double quotes are removed.  Any
       literal quoting is removed.

   3.  If the local-part contains any non-ASCII characters, it SHOULD be
       normalized using the Unicode Normalization Form C from
       [Unicode52].  Recommended normalization rules can be found in
       Section 10.1 of [RFC6530].

   4.  The local-part is hashed using the SHA2-256 [RFC5754] algorithm,
       with the hash truncated to 28 octets and represented in its
       hexadecimal representation, to become the left-most label in the
       prepared domain name.

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

   6.  The domain name (the "right-hand side" of the email address,
       called the "domain" in [RFC5322]) is appended to the result of
       step 5 to complete the prepared domain name.

   For example, to request an SMIMEA resource record for a user whose
   email address is "", an SMIMEA query would be placed
   for the following QNAME: "c93f1e400f26708f98cb19d936620da35eec8f72e57".

4.  Email Address Variants and Internationalization Considerations

   Mail systems usually handle variant forms of local-parts.  The most
   common variants are upper and lower case, often automatically
   corrected when a name is recognized as such.  Other variants include
   systems that ignore "noise" characters such as dots, so that local
   parts johnsmith and John.Smith would be equivalent.  Many systems
   allow "extensions" such as john-ext or mary+ext where john or mary is
   treated as the effective local-part, and the ext is passed to the
   recipient for further handling.  This can complicate finding the
   SMIMEA record associated with the dynamically created email address.

   [RFC5321] and its predecessors have always made it clear that only
   the recipient MTA is allowed to interpret the local-part of an
   address.  Therefor, sending MUAs and MTAs supporting this
   specification MUST NOT perform any kind of mapping rules based on the
   email address.  In order to improve chances of finding SMIMEA
   resource records for a particular local-part, domains that allow
   variant forms (such as treating local-parts as case-insensitive)
   might publish SMIMEA resource records for all variants of local-
   parts, might publish variants on first use (for example a webmail
   provider that also controls DNS for a domain can publish variants as
   used by owner of a particular local-part) or just publish SMIMEA
   resource records for the most common variants.

   Section 3 above defines how the local-part is used to determine the
   location in which one looks for an SMIMEA resource record.  Given the
   variety of local-parts seen in email, designing a good experiment for
   this is difficult as: a) some current implementations are known to
   lowercase at least US-ASCII local-parts, b) we know from (many) other
   situations that any strategy based on guessing and making multiple
   DNS queries is not going to achieve consensus for good reasons, and
   c) the underlying issues are just hard - see Section 10.1 of
   [RFC6530] for discussion of just some of the issues that would need
   to be tackled to fully address this problem.

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   However, while this specification is not the place to try to address
   these issues with local-parts, doing so is also not required to
   determine the outcome of this experiment.  If this experiment
   succeeds then further work on email addresses with non-ASCII local-
   parts will be needed and that would be better based on the findings
   from this experiment, rather than doing nothing or starting this
   experiment based on a speculative approach to what is a very complex

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
   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).

   S/MIME MUAs conforming to this specification MUST be able to
   interpret any S/MIME capabilities (defined in [RFC4262]) in any
   certificates that it receives through SMIMEA records.

6.  Application Use of S/MIME Certificate Associations

   The SMIMEA record allows an application or service to obtain an S/
   MIME certificate or public key and use it for verifying a digital
   signature or encrypting a message to the public key.  The DNS answer
   MUST pass DNSSEC validation; if DNSSEC validation reaches any state
   other than "Secure" (as specified in [RFC4035]), the DNSSEC
   validation MUST be treated as a failure.

   If no S/MIME certificates are known for an email address, an SMIMEA
   DNS lookup MAY be performed to seek the certificate or public key
   that corresponds to that email address.  This can then be used to
   verify a received signed message or can be used to send out an
   encrypted email message.  An application whose attempt fails to
   retrieve a DNSSEC verified SMIMEA resource record from the DNS should
   remember that failure for some time to avoid sending out a DNS
   request for each email message the application is sending out; such
   DNS requests constitute a privacy leak.

7.  Certificate Size and DNS

   Due to the expected size of the SMIMEA record, applications SHOULD
   use TCP - not UDP - to perform queries for the SMIMEA resource

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   Although the reliability of the transport of large DNS resource
   records has improved in the last years, it is still recommended to
   keep the DNS records as small as possible without sacrificing the
   security properties of the public key.  The algorithm type and key
   size of certificates should not be modified to accommodate this

8.  IANA Considerations

   This document uses a new DNS RRtype, SMIMEA, whose value (53) was
   allocated by IANA from the Resource Record (RR) TYPEs subregistry of
   the Domain Name System (DNS) Parameters registry.

9.  Security Considerations

   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

   DNSSEC does not protect the queries from Pervasive Monitoring as
   defined in [RFC7258].  Since DNS queries are currently mostly
   unencrypted, a query to lookup a target SMIMEA record could reveal
   that a user using the (monitored) recursive DNS server is attempting
   to send encrypted email to a target.

   Various components could be responsible for encrypting an email
   message to a target recipient.  It could be done by the sender's MUA
   or a MUA plugin or the sender's MTA.  Each of these have their own
   characteristics.  A MUA can ask the user to make a decision before
   continuing.  The MUA can either accept or refuse a message.  The MTA
   might deliver the message as-is, or encrypt the message before
   delivering.  Each of these components should attempt to encrypt an
   unencrypted outgoing message whenever possible.

   In theory, two different local-parts could hash to the same value.
   This document assumes that such a hash collision has a negliable
   chance of happening.

   If an obtained S/MIME certificate is revoked or expired, that
   certificate MUST NOT be used, even if that would result in sending a
   message in plaintext.

   Anyone who can obtain a DNSSEC private key of a domain name via
   coercion, theft or brute force calculations, can replace any SMIMEA
   record in that zone and all of the delegated child zones.  Any future
   messages encrypted with the malicious SMIMEA key could then be read.
   Therefore, an certificate or key obtained from a DNSSEC validated

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   SMIMEA record can only be trusted as much as the DNS domain can be

   Organisations that are required to be able to read everyone's
   encrypted email should publish the escrow key as the SMIMEA record.
   Mail servers of such organizations MAY optionally re-encrypt the
   message to the individual's S/MIME key.  This case can be considered
   a special case of the key-replacement attack described above.

9.1.  Response Size

   To prevent amplification attacks, an Authoritative DNS server MAY
   wish to prevent returning SMIMEA records over UDP unless the source
   IP address has been confirmed with [RFC7873].  If a query is received
   via UDP without source IP address verification, the server MUST NOT
   return REFUSED, but answer the query with an empty answer section and
   the truncation flag set ("TC=1").

9.2.  Email Address Information Leak

   The hashing of the local-part in this document is not a security
   feature.  Publishing SMIMEA records will create a list of hashes of
   valid email addresses, which could simplify obtaining a list of valid
   email addresses for a particular domain.  It is desirable to not ease
   the harvesting of email addresses where possible.

   The domain name part of the email address is not used as part of the
   hash so that hashes can be used in multiple zones deployed using
   DNAME [RFC6672].  This makes it slightly easier and cheaper to brute-
   force the SHA2-256 hashes into common and short local-parts, as
   single rainbow tables [Rainbow] can be re-used across domains.  This
   can be somewhat countered by using NSEC3 [RFC5155].

   DNS zones that are signed with DNSSEC using NSEC [RFC4033] for denial
   of existence are susceptible to zone-walking, a mechanism that allows
   someone to enumerate all the SMIMEA hashes in a zone.  This can be
   used in combination with previously hashed common or short local-
   parts (in rainbow tables) to deduce valid email addresses.  DNSSEC-
   signed zones using NSEC3 for denial of existence instead of NSEC are
   significantly harder to brute-force after performing a zone-walk.

10.  Acknowledgements

   A great deal of material in this document is copied from [RFC7929].
   That material was created by Paul Wouters and other participants in
   the IETF DANE WG.

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

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, 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, DOI 10.17487/RFC5280, May 2008,

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <>.

   [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
              Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
              2010, <>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <>.

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   [RFC7671]  Dukhovni, V. and W. Hardaker, "The DNS-Based
              Authentication of Named Entities (DANE) Protocol: Updates
              and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
              October 2015, <>.

11.2.  Informative References

   [Rainbow]  Oechslin, P., "Making a Faster Cryptanalytic Time-Memory
              Trade-Off", 2003,

   [RFC4262]  Santesson, S., "X.509 Certificate Extension for Secure/
              Multipurpose Internet Mail Extensions (S/MIME)
              Capabilities", RFC 4262, DOI 10.17487/RFC4262, December
              2005, <>.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,

   [RFC6530]  Klensin, J. and Y. Ko, "Overview and Framework for
              Internationalized Email", RFC 6530, DOI 10.17487/RFC6530,
              February 2012, <>.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,

   [RFC7218]  Gudmundsson, O., "Adding Acronyms to Simplify
              Conversations about DNS-Based Authentication of Named
              Entities (DANE)", RFC 7218, DOI 10.17487/RFC7218, April
              2014, <>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <>.

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   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,

   [RFC7929]  Wouters, P., "DNS-Based Authentication of Named Entities
              (DANE) Bindings for OpenPGP", RFC 7929,
              DOI 10.17487/RFC7929, August 2016,

              The Unicode Consortium, "The Unicode Standard, Version
              6.1.0, defined by: "The Unicode Standard, Version 6.1.0",
              (Mountain View, CA: The Unicode Consortium, 2009. ISBN
              978-1-936213-02-3).", 2012.

Authors' Addresses

   Paul Hoffman


   Jakob Schlyter
   Kirei AB


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