draft-ietf-uta-email-tls-certs-06.txt		draft-ietf-uta-email-tls-certs-07.txt
	Network Working Group A. Melnikov		Network Working Group A. Melnikov
	Internet-Draft Isode Ltd		Internet-Draft Isode Ltd
	Updates: 2595, 3207, 3501, 5804 (if December 4, 2015		Updates: 2595, 3207, 3501, 5804 (if December 9, 2015
	approved)		approved)
	Intended status: Standards Track		Intended status: Standards Track
	Expires: June 6, 2016		Expires: June 11, 2016
	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-06		draft-ietf-uta-email-tls-certs-07
	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, updates Section 4.1 of RFC 3207, updates		Section 2.4 of RFC 2595, updates Section 4.1 of RFC 3207, updates
	Section 11.1 of RFC 3501, updates Section 2.2.1 of RFC 5804.		Section 11.1 of RFC 3501, updates Section 2.2.1 of RFC 5804.
	Status of This Memo		Status of This Memo
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	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on June 6, 2016.		This Internet-Draft will expire on June 11, 2016.
	Copyright Notice		Copyright Notice
	Copyright (c) 2015 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.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	skipping to change at page 2, line 13		skipping to change at page 2, line 13
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	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 . . . . . . . . . . . . . . 3		2. Conventions Used in This Document . . . . . . . . . . . . . . 3
	3. Email Server Certificate Verification Rules . . . . . . . . . 3		3. Email Server Certificate Verification Rules . . . . . . . . . 3
	4. Compliance Checklist for Certification Authorities . . . . . 5		4. Compliance Checklist for Certification Authorities . . . . . 5
	4.1. Notes on handling of SRV-ID by Certificate Authorities . 5		4.1. Notes on handling of SRV-ID by Certification Authorities 5
	5. Compliance Checklist for Mail Service Providers and		5. Compliance Checklist for Mail Service Providers and
	Certificate Signing Request generation tools . . . . . . . . 5		Certificate Signing Request generation tools . . . . . . . . 6
	5.1. Notes on hosted domains . . . . . . . . . . . . . . . . . 6		5.1. Notes on hosting multiple domains . . . . . . . . . . . . 6
	6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6		6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 7		8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
	9.1. Normative References . . . . . . . . . . . . . . . . . . 7		9.1. Normative References . . . . . . . . . . . . . . . . . . 9
	9.2. Informative References . . . . . . . . . . . . . . . . . 8		9.2. Informative References . . . . . . . . . . . . . . . . . 10
	Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10		Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 11
	Appendix B. Changes since draft-ietf-uta-email-tls-certs-00 . . 10		Appendix B. Changes since draft-ietf-uta-email-tls-certs-00 . . 11
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10		Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
	1. Introduction		1. Introduction
	Use of TLS by SMTP Submission, IMAP, POP and ManageSieve clients is		Use of TLS by SMTP Submission, IMAP, POP and ManageSieve clients is
	described in [RFC3207], [RFC3501], [RFC2595] and [RFC5804]		described in [RFC3207], [RFC3501], [RFC2595] and [RFC5804]
	respectively. Each of the documents describes slightly different		respectively. Each of the documents describes slightly different
	rules for server certificate identity verification (or doesn't define		rules for server certificate identity verification (or doesn't define
	any rules at all). In reality, email client and server developers		any rules at all). In reality, email client and server developers
	implement many of these protocols at the same time, so it would be		implement many of these protocols at the same time, so it would be
	good to define modern and consistent rules for verifying email server		good to define modern and consistent rules for verifying email server
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	Matching is performed according to the rules specified in Section 6		Matching is performed according to the rules specified in Section 6
	of [RFC6125], including "certificate pinning" and the procedure on		of [RFC6125], including "certificate pinning" and the procedure on
	failure to match. The following inputs are used by the verification		failure to match. The following inputs are used by the verification
	procedure used in [RFC6125]:		procedure used in [RFC6125]:
	1. For DNS-ID and CN-ID identifier types the client MUST use one or		1. For DNS-ID and CN-ID identifier types the client MUST use one or
	more of the following as "reference identifiers": (a) the domain		more of the following as "reference identifiers": (a) the domain
	portion of the user's email address, (b) the hostname it used to		portion of the user's email address, (b) the hostname it used to
	open the connection (without CNAME canonicalization). The client		open the connection (without CNAME canonicalization). The client
	MAY also use (c) a value securely derived from (a) or (b), such		MAY also use (c) a value securely derived from (a) or (b), such
	as using "secure" DNSSEC validated lookup.		as using "secure" DNSSEC [RFC4033][RFC4034][RFC4035] validated
			lookup.
	2. When using email service discovery procedure specified in		2. When using email service discovery procedure specified in
	[RFC6186] the client MUST also use the domain portion of the		[RFC6186] the client MUST also use the domain portion of the
	user's email address as another "reference identifier" to compare		user's email address as another "reference identifier" to compare
	against SRV-ID identifier in the server certificate.		against SRV-ID identifier in the server certificate.
	The rules and guidelines defined in [RFC6125] apply to an email		The rules and guidelines defined in [RFC6125] apply to an email
	server certificate, with the following supplemental rules:		server certificate, 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
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	each type of email service. See Section 4.1 for more discussion		each type of email service. See Section 4.1 for more discussion
	on what this means for Certification Authorities.		on what this means for Certification Authorities.
	3. For backward compatibility with deployed client base, CA MUST		3. For backward compatibility with deployed client base, CA MUST
	support issuance of server certificates with CN-ID identifier		support issuance of server certificates with CN-ID identifier
	type (CN attribute from the subject name, see [RFC6125]).		type (CN attribute from the subject name, see [RFC6125]).
	4. CA MAY allow "*" (wildcard) as the left-most name component of		4. CA MAY allow "*" (wildcard) as the left-most name component of
	DNS-ID or CN-ID in server certificates it issues.		DNS-ID or CN-ID in server certificates it issues.
	4.1. Notes on handling of SRV-ID by Certificate Authorities		4.1. Notes on handling of SRV-ID by Certification Authorities
	TBD. List some possible recommendations and limitations of different		[RFC6186] provides an easy way for organizations to autoconfigure
	approaches.		email clients. It also allows for delegation of email services to an
			email hosting provider. When connecting to such delegated hosting
			service an email client that attempts to verify TLS server identity
			needs to know that if it connects to imap.hosting.example.net that
			such server is authorized to provide email access for an email such
			as alice@example.org. In absence of SRV-IDs, users of compliant
			email clients would be forced to manual confirm exception, because
			TLS server certificate verification procedures specified in this
			document would result in failure to match TLS server certificate
			against the expected domains. One way to provide such authorization
			is for the TLS certificate for imap.hosting.example.net to include
			SRV-ID(s) (or DNS-ID) for example.org domain. (Another way is for
			SRV lookups to be protected by DNSSEC, but this solution depends
			reliance of DNSSEC and thus is not discussed in this document. A
			future update to this document might rectify this.)
			The ability of issuing certificates that contain SRV-ID implies the
			ability to verify that entities requesting them are authorized to run
			email service for these SRV-IDs. In particular, certification
			authorities that can't verify such authorization MUST NOT include
			email SRV-IDs in certificates they issue. This document doesn't
			specify exact mechanism(s) that can be used to achieve this.
			However, a few special case recommendations are listed below.
			A certification authority willing to sign a certificate containing a
			particular DNS-ID SHOULD also support signing a certificate
			containing one or more of email SRV-IDs for the same domain, because
			the SRV-ID effectively provides more restricted access to an email
			service for the domain (as opposed to unrestricted use of any
			services for the same domain, as specified by DNS-ID).
			A certification authority which also provides DNS service for a
			domain can use DNS information to validate SRV-IDs for the domain.
			A certification authority MAY treat a certificate for a subdomain of
			example.com (e.g. imap.sub1.example.com or imap.example.com) that
			contains one or more SRV-ID for example.com as validated.
	5. Compliance Checklist for Mail Service Providers and Certificate		5. Compliance Checklist for Mail Service Providers and Certificate
	Signing Request generation tools		Signing Request generation tools
	1. MUST include the DNS-ID identifier type in Certificate Signing		1. MUST include the DNS-ID identifier type in Certificate Signing
	Requests for the host name(s) where the email server(s) are		Requests for the host name(s) where the email server(s) are
	running. SHOULD include the DNS-ID identifier type in		running. SHOULD include the DNS-ID identifier type in
	Certificate Signing Requests for the domain portion of served		Certificate Signing Requests for the domain portion of served
	email addresses.		email addresses.
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	backward compatibility with deployed email clients. (Note, a		backward compatibility with deployed email clients. (Note, a
	certificate can only include a single CN-ID, so if a mail service		certificate can only include a single CN-ID, so if a mail service
	is running on multiple hosts, either each host has to use		is running on multiple hosts, either each host has to use
	different certificate with its own CN-ID, a single certificate		different certificate with its own CN-ID, a single certificate
	with multiple DNS-IDs, or a single certificate with wildcard in		with multiple DNS-IDs, or a single certificate with wildcard in
	CN-ID can be used).		CN-ID can be used).
	4. MAY include "*" (wildcard) as the left-most name component of		4. MAY include "*" (wildcard) as the left-most name component of
	DNS-ID or CN-ID in Certificate Signing Requests.		DNS-ID or CN-ID in Certificate Signing Requests.
	5.1. Notes on hosted domains		5.1. Notes on hosting multiple domains
	TBD. Compare one certificate that contains all hosted domains versa		A server that hosts multiple domains needs to do one of the following
	use of SNI or separate IPs for each hosted domain with its own		(or some combination thereof):
	certificate.
			1. Use a single TLS certificate that includes a complete list of all
			the domains it is serving.
			2. Serve each domain on its own IP/port, using separate TLS
			certificates on each IP/port.
			3. Use Server Name Indication (SNI) TLS extension [RFC6066] to
			select the right certificate to return during TLS negotiation.
			Each domain has its own TLS certificate in this case.
			Each of these deployment choices have their scaling disadvantages
			when the list of domains changes. A single certificate (the first
			choice) requires that when a domain is added, then a new Certificate
			Signing Request that includes a complete list of all the domains
			needs to be issued and passed to a CA in order to generate a new
			certificate. Separate IP/port can avoid regenerating the
			certificate, but requires more transport layer resources. Use of TLS
			SNI requires each email client to support it.
			Several Mail Service Providers host hundreds and even thousands of
			domain. This document, as well as its predecessors RFC 2595, RFC
			3207, RFC 3501 and RFC 5804 don't address scaling issues caused by
			use of TLS in multi-tenanted environments. Further work is needed to
			address this issue, possibly using DNSSEC or something like POSH
			[RFC7711].
	6. Examples		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". A certificate for this		addresses of the form "user@example.net". A certificate for this
	service needs to include DNS-IDs of "example.net" (because it is the		service needs to include DNS-IDs of "example.net" (because it is the
	domain portion of emails) and "mail.example.net" (this is what a user		domain portion of emails) and "mail.example.net" (this is what a user
	of this server enters manually, if not using [RFC6186]). It might		of this server enters manually, if not using [RFC6186]). It might
	also include CN-ID of "mail.example.net" for backward compatibility		also include CN-ID of "mail.example.net" for backward compatibility
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	[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",		[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP",
	RFC 2595, DOI 10.17487/RFC2595, June 1999,		RFC 2595, DOI 10.17487/RFC2595, June 1999,
	<http://www.rfc-editor.org/info/rfc2595>.		<http://www.rfc-editor.org/info/rfc2595>.
	[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform		[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
	Resource Identifier (URI): Generic Syntax", STD 66,		Resource Identifier (URI): Generic Syntax", STD 66,
	RFC 3986, DOI 10.17487/RFC3986, January 2005,		RFC 3986, DOI 10.17487/RFC3986, January 2005,
	<http://www.rfc-editor.org/info/rfc3986>.		<http://www.rfc-editor.org/info/rfc3986>.
			[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
			Rose, "DNS Security Introduction and Requirements",
			RFC 4033, DOI 10.17487/RFC4033, March 2005,
			<http://www.rfc-editor.org/info/rfc4033>.
			[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
			Rose, "Resource Records for the DNS Security Extensions",
			RFC 4034, DOI 10.17487/RFC4034, March 2005,
			<http://www.rfc-editor.org/info/rfc4034>.
			[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
			Rose, "Protocol Modifications for the DNS Security
			Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
			<http://www.rfc-editor.org/info/rfc4035>.
	[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax		[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
	Specifications: ABNF", STD 68, RFC 5234,		Specifications: ABNF", STD 68, RFC 5234,
	DOI 10.17487/RFC5234, January 2008,		DOI 10.17487/RFC5234, January 2008,
	<http://www.rfc-editor.org/info/rfc5234>.		<http://www.rfc-editor.org/info/rfc5234>.
			[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
			Extensions: Extension Definitions", RFC 6066,
			DOI 10.17487/RFC6066, January 2011,
			<http://www.rfc-editor.org/info/rfc6066>.
	[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication		[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
	of Named Entities (DANE) Transport Layer Security (TLS)		of Named Entities (DANE) Transport Layer Security (TLS)
	Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August		Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
	2012, <http://www.rfc-editor.org/info/rfc6698>.		2012, <http://www.rfc-editor.org/info/rfc6698>.
			[RFC7711] Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
			(POSH)", RFC 7711, DOI 10.17487/RFC7711, November 2015,
			<http://www.rfc-editor.org/info/rfc7711>.
	Appendix A. Acknowledgements		Appendix A. Acknowledgements
	Thank you to Chris Newman, Viktor Dukhovni, Sean Turner, Russ Housley		Thank you to Chris Newman, Viktor Dukhovni, Sean Turner, Russ
	and Alessandro Vesely for comments on this document.		Housley, Alessandro Vesely, Harald Alvestrand and John Levine 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		Appendix B. Changes since draft-ietf-uta-email-tls-certs-00
	[[Note to RFC Editor: Please delete this section before publication]]		[[Note to RFC Editor: Please delete this section before publication]]
	Added another example, clarified that subjectAltName and DNS SRV are		Added another example, clarified that subjectAltName and DNS SRV are
	using slightly different syntax.		using slightly different syntax.
End of changes. 15 change blocks.
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