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Versions: 00 01 02 draft-ietf-uta-email-tls-certs

Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Updates: 2595, 3207 (if approved)                           July 1, 2014
Intended status: Standards Track
Expires: January 2, 2015


Updated TLS Server Identity Check Procedure for Email Related Protocols
                   draft-melnikov-email-tls-certs-02

Abstract

   This document describes TLS server identity verification procedure
   for SMTP Submission, IMAP, POP and ManageSieve clients.  It replaces
   Section 2.4 of RFC 2595.

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 January 2, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  Email Server Certificate Verification Rules . . . . . . . . .   2
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   This document describes the updated TLS server identity verification
   procedure for SMTP Submission [RFC4409] [RFC3207], IMAP [RFC3501],
   POP [RFC1939] and ManageSieve [RFC5804] clients.  It replaces
   Section 2.4 of RFC 2595.

   Note that this document doesn't apply to use of TLS in MTA-to-MTA
   SMTP.

   The main goal of the document is to provide consistent TLS server
   identity verification procedure across multiple email related
   protocols.  This should make it easier for Certificate Authorities
   and ISPs to deploy TLS for email use, and would enable email client
   developers to write more secure code.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Email Server Certificate Verification Rules

   During a TLS negotiation, an email client (i.e., an SMTP, IMAP, POP3
   or ManageSieve client) MUST check its understanding of the server
   hostname against the server's identity as presented in the server
   Certificate message, in order to prevent man-in-the-middle attacks.
   Matching is performed according to the rules specified in Section 6
   of [RFC6125], including "certificate pinning" and the procedure on
   failure to match.  The following inputs are used by the verification
   procedure used in [RFC6125]:

   1.  The client MUST use the server hostname it used to open the
       connection as the value to compare against the server name as



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       expressed in the server certificate (the reference identity).
       The client MUST NOT use any form of the server hostname derived
       from an insecure remote source (e.g., insecure DNS lookup).
       CNAME canonicalization is not done.

   The rules and guidelines defined in [RFC6125] apply to an email
   server certificates, with the following supplemental rules:

   1.  Support for the DNS-ID identifier type (subjectAltName of dNSName
       type [RFC5280]) is REQUIRED in Email client software
       implementations.  Certification authorities that issue Email-
       specific certificates MUST support the DNS-ID identifier type.
       Service providers SHOULD include the DNS-ID identifier type in
       Certificate Signing Requests.

   2.  Support for the SRV-ID identifier type (subjectAltName of SRVName
       type [RFC4985]) is REQUIRED for email client software
       implementations.  Certification authorities that issue email-
       specific certificates MUST support the SRV-ID identifier type.
       Service providers SHOULD include the SRV-ID identifier type in
       Certificate Signing Requests.  List of SRV-ID types for email
       services is specified in [RFC6186].  For ManageSieve the value
       "sieve" is used.

   3.  URI-ID identifier type (subjectAltName of
       uniformResourceIdentifier type [RFC5280]) MUST NOT be used by
       clients for server verification.

   4.  For backward compatibility with deployed software CN-ID
       identifier type (CN attribute from the subject name, see
       [RFC6125]) MAY be used for server identity verification.

   5.  Email protocols allow use of certain wilcards in identifiers
       presented by email servers.  The "*" wildcard character MAY be
       used as the left-most name component of DNS-ID or CN-ID in the
       certificate.  For example, a DNS-ID of *.example.com would match
       a.example.com, foo.example.com, etc. but would not match
       example.com.  Note that the wildcard character MUST NOT be used
       as a fragment of the left-most name component (e.g.,
       *oo.example.com, f*o.example.com, or foo*.example.com).

4.  Examples

   Consider an IMAP-accessible email server which supports both IMAP and
   IMAPS (IMAP-over-TLS) at the host "mail.example.net" servicing email
   addresses of the form "user@example.net" and discoverable via DNS SRV
   lookups on the application service name of "example.net".  A
   certificate for this service needs to include SRV-IDs of



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   "_imap.example.net" and "_imaps.example.net" (see [RFC6186]) along
   with DNS-IDs of "example.net" and "mail.example.net".  It might also
   include CN-IDs of "example.net" and "mail.example.net" for backward
   compatibility with deployed infrastructure.

   Consider an SMTP Submission server at the host "submit.example.net"
   servicing email addresses of the form "user@example.net" and
   discoverable via DNS SRV lookups on the application service name of
   "example.net".  A certificate for this service needs to include SRV-
   IDs of "_submission.example.net" (see [RFC6186]) along with DNS-IDs
   of "example.net" and "submit.example.net".  It might also include CN-
   IDs of "example.net" and "submit.example.net" for backward
   compatibility with deployed infrastructure.

5.  IANA Considerations

   This document doesn't require any action from IANA.

6.  Security Considerations

   The goal of this document is to improve interoperability and thus
   security of email clients wishing to access email servers over TLS
   protected email protocols, by specifying a consistent set of rules
   that email service providers, email client writers and certificate
   authorities can use when creating server certificates.

7.  References

7.1.  Normative References

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

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

   [RFC4409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              RFC 4409, April 2006.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.




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   [RFC5804]  Melnikov, A. and T. Martin, "A Protocol for Remotely
              Managing Sieve Scripts", RFC 5804, July 2010.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

   [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.

   [RFC4985]  Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, August 2007.

7.2.  Informative References

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
              2595, June 1999.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186, March 2011.


























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Appendix A.  Acknowledgements

   Thank you to Chris Newman for comments on this document.

   The editor of this document copied lots of text from RFC 2595 and RFC
   6125, so the hard work of editors of these document is appreciated.

Author's Address

   Alexey Melnikov
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   EMail: Alexey.Melnikov@isode.com


































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