draft-ietf-uta-email-tls-certs-00.txt   draft-ietf-uta-email-tls-certs-01.txt 
Network Working Group A. Melnikov Network Working Group A. Melnikov
Internet-Draft Isode Ltd Internet-Draft Isode Ltd
Updates: 2595, 3207 (if approved) September 11, 2014 Updates: 2595, 3207, 3501, 5804 (if March 5, 2015
approved)
Intended status: Standards Track Intended status: Standards Track
Expires: March 15, 2015 Expires: September 6, 2015
Updated TLS Server Identity Check Procedure for Email Related Protocols Updated TLS Server Identity Check Procedure for Email Related Protocols
draft-ietf-uta-email-tls-certs-00 draft-ietf-uta-email-tls-certs-01
Abstract Abstract
This document describes TLS server identity verification procedure This document describes TLS server identity verification procedure
for SMTP Submission, IMAP, POP and ManageSieve clients. It replaces for SMTP Submission, IMAP, POP and ManageSieve clients. It replaces
Section 2.4 of RFC 2595. Section 2.4 of RFC 2595.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on March 15, 2015. This Internet-Draft will expire on September 6, 2015.
Copyright Notice Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2 2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Email Server Certificate Verification Rules . . . . . . . . . 2 3. Email Server Certificate Verification Rules . . . . . . . . . 3
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Compliance Checklist for Certificate Authorities . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 5. Compliance Checklist for Mail Service Providers and
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4 Certificate Signing Request generation tools . . . . . . . . 4
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.1. Normative References . . . . . . . . . . . . . . . . . . 4 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5 8. Security Considerations . . . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6 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 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 This document describes the updated TLS server identity verification
procedure for SMTP Submission [RFC4409] [RFC3207], IMAP [RFC3501], procedure for SMTP Submission [RFC4409] [RFC3207], IMAP [RFC3501],
POP [RFC1939] and ManageSieve [RFC5804] clients. It replaces POP [RFC1939] and ManageSieve [RFC5804] clients. It replaces
Section 2.4 of RFC 2595. Section 2.4 of RFC 2595.
Note that this document doesn't apply to use of TLS in MTA-to-MTA Note that this document doesn't apply to use of TLS in MTA-to-MTA
SMTP. SMTP.
The main goal of the document is to provide consistent TLS server The main goal of the document is to provide consistent TLS server
identity verification procedure across multiple email related identity verification procedure across multiple email related
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expressed in the server certificate (the reference identity). expressed in the server certificate (the reference identity).
The client MUST NOT use any form of the server hostname derived The client MUST NOT use any form of the server hostname derived
from an insecure remote source (e.g., insecure DNS lookup). from an insecure remote source (e.g., insecure DNS lookup).
CNAME canonicalization is not done. CNAME canonicalization is not done.
The rules and guidelines defined in [RFC6125] apply to an email The rules and guidelines defined in [RFC6125] apply to an email
server certificates, with the following supplemental rules: server certificates, with the following supplemental rules:
1. Support for the DNS-ID identifier type (subjectAltName of dNSName 1. Support for the DNS-ID identifier type (subjectAltName of dNSName
type [RFC5280]) is REQUIRED in Email client software type [RFC5280]) is REQUIRED in Email client software
implementations. Certification authorities that issue Email- implementations.
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 2. Support for the SRV-ID identifier type (subjectAltName of SRVName
type [RFC4985]) is REQUIRED for email client software type [RFC4985]) is REQUIRED for email client software
implementations. Certification authorities that issue email- implementations. List of SRV-ID types for email services is
specific certificates MUST support the SRV-ID identifier type. specified in [RFC6186]. For ManageSieve the value "sieve" is
Service providers SHOULD include the SRV-ID identifier type in used.
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 3. URI-ID identifier type (subjectAltName of
uniformResourceIdentifier type [RFC5280]) MUST NOT be used by uniformResourceIdentifier type [RFC5280]) MUST NOT be used by
clients for server verification. clients for server verification, as
4. For backward compatibility with deployed software CN-ID 4. For backward compatibility with deployed software CN-ID
identifier type (CN attribute from the subject name, see identifier type (CN attribute from the subject name, see
[RFC6125]) MAY be used for server identity verification. [RFC6125]) MAY be used for server identity verification.
5. Email protocols allow use of certain wilcards in identifiers 5. Email protocols allow use of certain wilcards in identifiers
presented by email servers. The "*" wildcard character MAY be presented by email servers. The "*" wildcard character MAY be
used as the left-most name component of DNS-ID or CN-ID in the 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 certificate. For example, a DNS-ID of *.example.com would match
a.example.com, foo.example.com, etc. but would not match a.example.com, foo.example.com, etc. but would not match
example.com. Note that the wildcard character MUST NOT be used example.com. Note that the wildcard character MUST NOT be used
as a fragment of the left-most name component (e.g., as a fragment of the left-most name component (e.g.,
*oo.example.com, f*o.example.com, or foo*.example.com). *oo.example.com, f*o.example.com, or foo*.example.com).
4. Examples 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 Consider an IMAP-accessible email server which supports both IMAP and
IMAPS (IMAP-over-TLS) at the host "mail.example.net" servicing email IMAPS (IMAP-over-TLS) at the host "mail.example.net" servicing email
addresses of the form "user@example.net" and discoverable via DNS SRV addresses of the form "user@example.net" and discoverable via DNS SRV
lookups on the application service name of "example.net". A 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 certificate for this service needs to include SRV-IDs of
"_imap.example.net" and "_imaps.example.net" (see [RFC6186]) along "_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 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 include CN-IDs of "mail.example.net" for backward compatibility with
compatibility with deployed infrastructure. deployed infrastructure.
Consider an SMTP Submission server at the host "submit.example.net" Consider an SMTP Submission server at the host "submit.example.net"
servicing email addresses of the form "user@example.net" and servicing email addresses of the form "user@example.net" and
discoverable via DNS SRV lookups on the application service name of discoverable via DNS SRV lookups in domain "example.net" (DNS SRV
"example.net". A certificate for this service needs to include SRV- records "_submission._tcp.example.net"). A certificate for this
IDs of "_submission.example.net" (see [RFC6186]) along with DNS-IDs service needs to include SRV-IDs of "_submission.example.net" (see
of "example.net" and "submit.example.net". It might also include CN- [RFC6186]) along with DNS-IDs of "example.net" and
IDs of "example.net" and "submit.example.net" for backward "submit.example.net". It might also include CN-IDs of
compatibility with deployed infrastructure. "submit.example.net" for backward compatibility with deployed
infrastructure.
5. IANA Considerations 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. This document doesn't require any action from IANA.
6. Security Considerations 8. Security Considerations
The goal of this document is to improve interoperability and thus The goal of this document is to improve interoperability and thus
security of email clients wishing to access email servers over TLS security of email clients wishing to access email servers over TLS
protected email protocols, by specifying a consistent set of rules protected email protocols, by specifying a consistent set of rules
that email service providers, email client writers and certificate that email service providers, email client writers and certificate
authorities can use when creating server certificates. authorities can use when creating server certificates.
7. References 9. References
9.1. Normative References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008. October 2008.
[RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail", [RFC4409] Gellens, R. and J. Klensin, "Message Submission for Mail",
RFC 4409, April 2006. RFC 4409, April 2006.
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[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008. (CRL) Profile", RFC 5280, May 2008.
[RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure [RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name", Subject Alternative Name for Expression of Service Name",
RFC 4985, August 2007. RFC 4985, August 2007.
7.2. Informative References 9.2. Informative References
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC [RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999. 2595, June 1999.
[RFC6186] Daboo, C., "Use of SRV Records for Locating Email [RFC6186] Daboo, C., "Use of SRV Records for Locating Email
Submission/Access Services", RFC 6186, March 2011. Submission/Access Services", RFC 6186, March 2011.
Appendix A. Acknowledgements Appendix A. Acknowledgements
Thank you to Chris Newman for comments on this document. Thank you to Chris Newman for comments on this document.
The editor of this document copied lots of text from RFC 2595 and RFC 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. 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 Author's Address
Alexey Melnikov Alexey Melnikov
Isode Ltd Isode Ltd
14 Castle Mews 14 Castle Mews
Hampton, Middlesex TW12 2NP Hampton, Middlesex TW12 2NP
UK UK
EMail: Alexey.Melnikov@isode.com EMail: Alexey.Melnikov@isode.com
 End of changes. 20 change blocks. 
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