SMTP security via opportunistic DANE TLS
draft-ietf-dane-smtp-with-dane-13

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DANE                                                         V. Dukhovni
Internet-Draft                                                 Two Sigma
Intended status: Standards Track                             W. Hardaker
Expires: April 29, 2015                                          Parsons
                                                        October 26, 2014

                SMTP security via opportunistic DANE TLS
                   draft-ietf-dane-smtp-with-dane-13

Abstract

   This memo describes a downgrade-resistant protocol for SMTP transport
   security between Mail Transfer Agents (MTAs) based on the DNS-Based
   Authentication of Named Entities (DANE) TLSA DNS record.  Adoption of
   this protocol enables an incremental transition of the Internet email
   backbone to one using encrypted and authenticated Transport Layer
   Security (TLS).

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
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   This Internet-Draft will expire on April 29, 2015.

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   Copyright (c) 2014 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
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Dukhovni & Hardaker      Expires April 29, 2015                 [Page 1]
Internet-Draft  SMTP security via opportunistic DANE TLS    October 2014

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Background  . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  SMTP channel security . . . . . . . . . . . . . . . . . .   6
       1.3.1.  STARTTLS downgrade attack . . . . . . . . . . . . . .   6
       1.3.2.  Insecure server name without DNSSEC . . . . . . . . .   7
       1.3.3.  Sender policy does not scale  . . . . . . . . . . . .   8
       1.3.4.  Too many certification authorities  . . . . . . . . .   8
   2.  Identifying applicable TLSA records . . . . . . . . . . . . .   9
     2.1.  DNS considerations  . . . . . . . . . . . . . . . . . . .   9
       2.1.1.  DNS errors, bogus and indeterminate responses . . . .   9
       2.1.2.  DNS error handling  . . . . . . . . . . . . . . . . .  11
       2.1.3.  Stub resolver considerations  . . . . . . . . . . . .  12
     2.2.  TLS discovery . . . . . . . . . . . . . . . . . . . . . .  13
       2.2.1.  MX resolution . . . . . . . . . . . . . . . . . . . .  14
       2.2.2.  Non-MX destinations . . . . . . . . . . . . . . . . .  15
       2.2.3.  TLSA record lookup  . . . . . . . . . . . . . . . . .  17
   3.  DANE authentication . . . . . . . . . . . . . . . . . . . . .  19
     3.1.  TLSA certificate usages . . . . . . . . . . . . . . . . .  19
       3.1.1.  Certificate usage DANE-EE(3)  . . . . . . . . . . . .  21
       3.1.2.  Certificate usage DANE-TA(2)  . . . . . . . . . . . .  22
       3.1.3.  Certificate usages PKIX-TA(0) and PKIX-EE(1)  . . . .  23
     3.2.  Certificate matching  . . . . . . . . . . . . . . . . . .  24
       3.2.1.  DANE-EE(3) name checks  . . . . . . . . . . . . . . .  24
       3.2.2.  DANE-TA(2) name checks  . . . . . . . . . . . . . . .  24
       3.2.3.  Reference identifier matching . . . . . . . . . . . .  25
   4.  Server key management . . . . . . . . . . . . . . . . . . . .  26
   5.  Digest algorithm agility  . . . . . . . . . . . . . . . . . .  26
   6.  Mandatory TLS Security  . . . . . . . . . . . . . . . . . . .  27
   7.  Note on DANE for Message User Agents  . . . . . . . . . . . .  27
   8.  Interoperability considerations . . . . . . . . . . . . . . .  28
     8.1.  SNI support . . . . . . . . . . . . . . . . . . . . . . .  28
     8.2.  Anonymous TLS cipher suites . . . . . . . . . . . . . . .  28
   9.  Operational Considerations  . . . . . . . . . . . . . . . . .  29
     9.1.  Client Operational Considerations . . . . . . . . . . . .  29
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