Internet DRAFT - draft-melnikov-email-tls-certs
draft-melnikov-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.
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 January 2, 2015.
Copyright Notice
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
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.
Melnikov Expires January 2, 2015 [Page 1]
Internet-Draft TLS Server Identity Check for Email July 2014
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
Melnikov Expires January 2, 2015 [Page 2]
Internet-Draft TLS Server Identity Check for Email July 2014
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
Melnikov Expires January 2, 2015 [Page 3]
Internet-Draft TLS Server Identity Check for Email July 2014
"_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.
Melnikov Expires January 2, 2015 [Page 4]
Internet-Draft TLS Server Identity Check for Email July 2014
[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.
Melnikov Expires January 2, 2015 [Page 5]
Internet-Draft TLS Server Identity Check for Email July 2014
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
Melnikov Expires January 2, 2015 [Page 6]
