SMTP security via opportunistic DANE TLS
draft-ietf-dane-smtp-with-dane-03

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Last updated 2013-11-23
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DANE                                                         V. Dukhovni
Internet-Draft                                              Unaffiliated
Intended status: Standards Track                           W.H. Hardaker
Expires: May 28, 2014                                            Parsons
                                                       November 24, 2013

                SMTP security via opportunistic DANE TLS
                   draft-ietf-dane-smtp-with-dane-03

Abstract

   This memo describes a protocol for opportunistic TLS security based
   on the DANE TLSA DNS record.  The protocol is downgrade resistant
   when the SMTP client supports DANE TLSA and the server domain
   publishes TLSA records for its MX hosts.  This enables an incremental
   transition of the Internet email backbone (MTA to MTA SMTP traffic)
   to TLS encrypted and authenticated delivery.

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
   working documents as Internet-Drafts.  The list of current Internet-
   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 May 28, 2014.

Copyright Notice

   Copyright (c) 2013 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

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  SMTP Channel Security . . . . . . . . . . . . . . . . . .   3
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Hardening Opportunistic TLS . . . . . . . . . . . . . . . . .   5
     2.1.  TLS discovery . . . . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Non-MX destinations . . . . . . . . . . . . . . . . .   6
       2.1.2.  MX resolution . . . . . . . . . . . . . . . . . . . .   7
       2.1.3.  TLSA record lookup  . . . . . . . . . . . . . . . . .   9
     2.2.  DANE authentication . . . . . . . . . . . . . . . . . . .  11
       2.2.1.  TLSA certificate usages . . . . . . . . . . . . . . .  11
       2.2.2.  Certificate matching  . . . . . . . . . . . . . . . .  12
       2.2.3.  Digest algorithm agility  . . . . . . . . . . . . . .  14
   3.  Opportunistic TLS for Submission  . . . . . . . . . . . . . .  16
   4.  Mandatory TLS Security  . . . . . . . . . . . . . . . . . . .  17
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   Lacking verified DNS and "Server Name Indication" (SNI), there has
   historically been no scalable way for SMTP server operators to deploy
   certificates with a client-trusted subject name.  It's only with the
   deployment of DNSSEC and DANE that authenticated TLS for SMTP to MX
   becomes possible between parties that have not already established an
   identity convention out-of-band.

1.1.  Background

   The Domain Name System Security Extensions (DNSSEC) add data origin
   authentication and data integrity to the Domain Name System.  DNSSEC
   is defined in [RFC4033], [RFC4034] and [RFC4035].

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   As described in the introduction of [RFC6698], TLS authentication via
   the existing public Certificate Authority (CA) Public Key
   Infrastructure (PKI) suffers from an over-abundance of trusted
   certificate authorities capable of issuing certificates for any
   domain of their choice.  DNS-Based Authentication of Named Entities
   (DANE) leverages the DNSSEC infrastructure to publish trusted keys
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