Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Updates: 2595, 3207, 3501, 5804 (if                       March 5, 23, 2015
         approved)
Intended status: Standards Track
Expires: September 6, 24, 2015

Updated TLS Server Identity Check Procedure for Email Related Protocols
                   draft-ietf-uta-email-tls-certs-01
                   draft-ietf-uta-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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  Email Server Certificate Verification Rules . . . . . . . . .   3
   4.  Compliance Checklist for Certificate Authorities  . . . . . .   4
   5.  Compliance Checklist for Mail Service Providers and
       Certificate Signing Request generation tools  . . . . . . . .   4
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Changes since draft-ietf-uta-email-tls-certs-00  . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Use of TLS by SMTP Submission, IMAP, POP and ManageSieve clients is
   described in [RFC3207], [RFC3501], [RFC2595] and [RFC5804]
   respectively.  Each of the documents describes slightly different
   rules for server certificate identity verification (or doesn't define
   any rules at all).  In reality, email client and server developers
   implement many of these protocols at the same time, so it would be
   good to define modern and consistent rules for verifying email server
   identities using TLS.

   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
       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.

   2.  Support for the SRV-ID identifier type (subjectAltName of SRVName
       type [RFC4985]) is REQUIRED for email client software
       implementations.  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, as URI-ID were not historically
       used for email.

   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.  Compliance Checklist for Certificate Authorities

   1.  CA MUST support issuance of server certificates with DNS-ID
       identifier type (subjectAltName of dNSName type [RFC5280]).

   2.  CA MUST support issuance of server certificates with SRV-ID
       identifier type (subjectAltName of SRVName type [RFC4985]) for
       each type of email service.

   3.  For backward compatibility with deployed client base, CA MUST
       support issuance of server certificates with CN-ID identifier
       type (CN attribute from the subject name, see [RFC6125]).

   4.  CA MAY allow "*" (wildcard) as the left-most name component of
       DNS-ID or CN-ID in server certificates it issues.

5.  Compliance Checklist for Mail Service Providers and Certificate
    Signing Request generation tools

   1.  SHOULD include the DNS-ID identifier type (subjectAltName of
       dNSName type [RFC5280]) in Certificate Signing Requests for both
       the right hand side of served email addresses, as well as for the
       host name where the email server(s) are running.

   2.  If the email services provided are discoverable using DNS SRV as
       specified in [RFC6186], MSP MUST include the SRV-ID identifier
       type (subjectAltName of SRVName type [RFC4985]) for each type of
       email service in Certificate Signing Requests.

   3.  SHOULD include CN-ID identifier type (CN attribute from the
       subject name, see [RFC6125]) for the host name where the email
       server(s) is running in Certificate Signing Requests for backward
       compatibility with deployed email clients.  (Note, a certificate
       can only include a single CN-ID, so if a mail service is running
       on multiple hosts, either each host has to use different
       certificate with its own CN-ID, a single certificate with
       multiple DNS-IDs, or a single certificate with wildcard in CN-ID
       can be used).

   4.  MAY include "*" (wildcard) as the left-most name component of
       DNS-ID or CN-ID in Certificate Signing Requests.

6.  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 in domain "example.net" (DNS SRV records
   "_imap._tcp.example.net" and "_imaps._tcp.example.net").  A
   certificate for this service needs to include SRV-IDs of
   "_imap.example.net" and "_imaps.example.net" (see [RFC6186].  Note
   that unlike DNS SRV there is no "_tcp" component in SRV-IDs) along
   with DNS-IDs of "example.net" and "mail.example.net".  It might also
   include CN-IDs of "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 in domain "example.net" (DNS SRV
   records "_submission._tcp.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
   "submit.example.net" for backward compatibility with deployed
   infrastructure.

   Consider a host "mail.example.net" servicing email addresses of the
   form "user@example.net" and discoverable via DNS SRV lookups in
   domain "example.net", which runs SMTP Submission, IMAPS and POP3S
   (POP3-over-TLS) and ManageSieve services.  Each of the servers can
   use their own certificate specific to their service (see examples
   above).  Alternatively they can all share a single certificate that
   would include SRV-IDs of "_submission.example.net",
   "_imaps.example.net", "_pop3s.example.net" and "_sieve.example.net"
   along with DNS-IDs of "example.net" and "mail.example.net".  It might
   also include CN-IDs of "mail.example.net" for backward compatibility
   with deployed infrastructure.

7.  IANA Considerations

   This document doesn't require any action from IANA.

8.  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.

9.  References
9.1.  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.

   [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.

9.2.  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.

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.

Appendix B.  Changes since draft-ietf-uta-email-tls-certs-00

   [[Note to RFC Editor: Please delete this section before publication]]

   Added another example, clarified that subjectAltName and DNS SRV are
   using slightly different syntax.

   As any certificate can only include one CN-ID, corrected examples.

   Split rules to talk seperately about requirements on MUAs, CAs and
   MSPs/CSR generation tools.

   Updated Introduction section.

Author's Address

   Alexey Melnikov
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex  TW12 2NP
   UK

   EMail: Alexey.Melnikov@isode.com