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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 (if approved)                                June 15, 2011
Intended status: Standards Track
Expires: December 17, 2011


Updated TLS Server Identity Check Procedure for Email Related Protocols
                   draft-melnikov-email-tls-certs-00

Abstract

   This document describes TLS server identity verification procedure
   for SMTP, 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
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   This Internet-Draft will expire on December 17, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

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







































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1.  Introduction

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

   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
       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 [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 [RFC4985] is REQUIRED for
       email client software implementations.  Certification authorities
       that issue email-specific certificates MUST support the SRV-ID



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       identifier type.  Service providers SHOULD include the SRV-ID
       identifier type in Certificate Signing Requests.

   3.  URI-ID identifier type SHOULD NOT be used by clients for server
       verification.

   4.  For backward compatibility with deployed software CN-ID
       identifier type 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 might include SRV-IDs of
   "_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.

   TBD: Include an example certificate as shown by OpenSSL or similar.

   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 might 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.

   TBD: Add an example of a single certificate that can be shared
   between multiple email services.

5.  IANA Considerations

   This document doesn't require any action from IANA.




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6.  Security Considerations

   TBD

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.

   [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.

   [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

   TBD

   The editor of this document copied lots of text from RFC 2595 and RFC
   6125.

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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