Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Updates: 2595, 3207, 3501, 5804 (if                       August 6,                   September 20, 2015
         approved)
Intended status: Standards Track
Expires: February 7, March 23, 2016

Updated TLS Server Identity Check Procedure for Email Related Protocols
                   draft-ietf-uta-email-tls-certs-04
                   draft-ietf-uta-email-tls-certs-05

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. 2595, updates Section 4.1 of RFC 3207, updates
   Section 11.1 of RFC 3501, updates Section 2.2.1 of RFC 5804.

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 February 7, March 23, 2016.

Copyright Notice

   Copyright (c) 2015 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

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 [RFC6409] [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 Certification 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].

   The following terms or concepts are used through the document:

   reference identifier:  (as defined in [RFC6125]) One of the domain
      names associated by the email (i.e., an SMTP, IMAP, POP3 or
      ManageSieve) client with the destination email server and
      optionally an application service type for performing name checks
      on the server certificate.  When name checks are applicable, at
      least one of the reference identifiers MUST match an [RFC6125]
      DNS-ID or SRV-ID (or if none are present the [RFC6125] CN-ID) of
      the server certificate.

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.  For DNS-ID and CN-ID identifier types the client MUST use one or
       more of the following as "reference identifiers": (a) the right
       hand side of the email address, (b) the
       server hostname it used to open
       the connection as at least one of
       the values to compare against (*) in the server certificate. (without CNAME canonicalization).  The client MUST NOT MAY
       also use any form of the server hostname (c) a value securely derived from
       an insecure remote source (e.g., insecure DNS lookup).  CNAME
       canonicalization is not done. (a) or (b), such as
       using "secure" DNSSEC validated lookup.

   2.  When using email service discovery procedure specified in
       [RFC6186] the client MUST also use the right hand side of the
       email address as another "reference identifier" to compare
       against SRV-ID identifier in the server certificate.

   (*) - "reference identifier" (see the definition in [RFC6125]).

   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 that support [RFC6186].  List of SRV-ID types for
       email services is specified in [RFC6186].  For the ManageSieve
       protocol the service name "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 Certification 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], the Mail Service Provider 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".  A certificate for this
   service needs to include DNS-IDs of "example.net" (because it is the
   right hand side of emails) and "mail.example.net" (this is what a
   user of this server enters manually, if not using [RFC6186]).  It
   might also include CN-IDs of "mail.example.net" for backward
   compatibility with deployed infrastructure.

   Consider the IMAP-accessible email server from the previous paragraph
   which is additionally discoverable via DNS SRV lookups in domain
   "example.net" (DNS SRV records "_imap._tcp.example.net" and
   "_imaps._tcp.example.net").  In addition to DNS-ID/CN-ID identity
   types specified above, a certificate for this service also needs to
   include SRV-IDs of "_imap.example.net" (when STARTTLS is used on the
   IMAP port) and "_imaps.example.net" (when TLS is used on IMAPS port).
   See [RFC6186] for more details.  (Note that unlike DNS SRV there is
   no "_tcp" component in SRV-IDs).

   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 Certification
   Authorities can use when creating server certificates.

   TLS Server Identity Check for Email relies on use of trustworthy DNS
   hostnames when constructing "reference identifiers" that are checked
   against an email server certificate.  Such trustworthy names are
   either entered manually (for example if they are advertised on a Mail
   Service Provider's website), explicitly confirmed by the user (e.g.
   if they are a target of a DNS SRV lookup) or derived using a secure
   third party service (e.g.  DNSSEC-protected SRV records which are
   verified by the client or trusted local resolver).  Future work in
   this area might benefit from integration with DANE [RFC6698], but it
   is not covered by this document.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <http://www.rfc-editor.org/info/rfc5321>.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
              <http://www.rfc-editor.org/info/rfc6409>.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
              February 2002, <http://www.rfc-editor.org/info/rfc3207>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <http://www.rfc-editor.org/info/rfc3501>.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996,
              <http://www.rfc-editor.org/info/rfc1939>.

   [RFC5804]  Melnikov, A., Ed. and T. Martin, "A Protocol for Remotely
              Managing Sieve Scripts", RFC 5804, DOI 10.17487/RFC5804,
              July 2010, <http://www.rfc-editor.org/info/rfc5804>.

   [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, DOI 10.17487/RFC6125, March
              2011, <http://www.rfc-editor.org/info/rfc6125>.

   [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, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC4985]  Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, DOI 10.17487/RFC4985, August 2007,
              <http://www.rfc-editor.org/info/rfc4985>.

   [RFC6186]  Daboo, C., "Use of SRV Records for Locating Email
              Submission/Access Services", RFC 6186,
              DOI 10.17487/RFC6186, March 2011,
              <http://www.rfc-editor.org/info/rfc6186>.

9.2.  Informative References

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, DOI 10.17487/RFC2595, June 1999,
              <http://www.rfc-editor.org/info/rfc2595>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <http://www.rfc-editor.org/info/rfc6698>.

Appendix A.  Acknowledgements

   Thank you to Chris Newman, Viktor Dukhovni and Sean Turner 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