Using Secure DNS to Associate Certificates with Domain Names for S/MIME
RFC 8162

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Internet Engineering Task Force (IETF)                        P. Hoffman
Request for Comments: 8162                                         ICANN
Category: Experimental                                       J. Schlyter
ISSN: 2070-1721                                                 Kirei AB
                                                                May 2017

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

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 DNS-Based Authentication of Named Entities (DANE), RFC 6698,
   does for TLS.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc8162.

Copyright Notice

   Copyright (c) 2017 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.

Hoffman & Schlyter            Experimental                      [Page 1]
RFC 8162           DNS-Based Authentication for S/MIME          May 2017

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  . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     9.1.  Response Size . . . . . . . . . . . . . . . . . . . . . .   8
     9.2.  Email Address Information Leak  . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   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
   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