draft-ietf-dcrup-dkim-usage-00.txt		draft-ietf-dcrup-dkim-usage.txt
	Network Working Group S. Kitterman		Network Working Group S. Kitterman
	Internet-Draft Kitterman Technical Services		Internet-Draft Kitterman Technical Services
	Updates: 6376 (if approved) May 30, 2017		Updates: 6376 (if approved) June 3, 2017
	Intended status: Standards Track		Intended status: Standards Track
	Expires: December 1, 2017		Expires: December 5, 2017
	Cryptographic Algorithm and Key Usage to DKIM		Cryptographic Algorithm and Key Usage to DKIM
	draft-ietf-dcrup-dkim-usage-00		draft-ietf-dcrup-dkim-usage-01
	Abstract		Abstract
	The cryptographic algorithm and key size requirements included when		The cryptographic algorithm and key size requirements included when
	DKIM was designed in the last decade are functionally obsolete and in		DKIM was designed in the last decade are functionally obsolete and in
	need of immediate revision. This document updates DKIM requirements		need of immediate revision. This document updates DKIM requirements
	to those minimaly suitable for operation with currently specified		to those minimaly suitable for operation with currently specified
	algorithms. This document updates RFC 6376.		algorithms. This document updates RFC 6376.
	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 December 1, 2017.		This Internet-Draft will expire on December 5, 2017.
	Copyright Notice		Copyright Notice
	Copyright (c) 2017 IETF Trust and the persons identified as the		Copyright (c) 2017 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 24		skipping to change at page 2, line 24
	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. DKIM Signing and Verification Algorithms . . . . . . . . . . 3		3. DKIM Signing and Verification Algorithms . . . . . . . . . . 3
	3.1. The rsa-sha1 Signing Algorithm . . . . . . . . . . . . . 3		3.1. The rsa-sha1 Signing Algorithm . . . . . . . . . . . . . 3
	3.2. The rsa-sha256 Signing Algorithm . . . . . . . . . . . . 3		3.2. The rsa-sha256 Signing Algorithm . . . . . . . . . . . . 3
	3.3. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . 3		3.3. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . 3
	3.4. Other Algorithms . . . . . . . . . . . . . . . . . . . . 4		3.4. Other Algorithms . . . . . . . . . . . . . . . . . . . . 4
	4. Security Considerations . . . . . . . . . . . . . . . . . . . 4		4. The DKIM-Signature Header Field . . . . . . . . . . . . . . . 4
	5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4		5. Key Management and Representation . . . . . . . . . . . . . . 4
	5.1. DKIM Hash Algorithms . . . . . . . . . . . . . . . . . . 4		6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
	6. Normative References . . . . . . . . . . . . . . . . . . . . 4		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
	Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 5		7.1. DKIM Hash Algorithms . . . . . . . . . . . . . . . . . . 5
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5		8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
			8.1. Normative References . . . . . . . . . . . . . . . . . . 5
			8.2. Informative References . . . . . . . . . . . . . . . . . 5
			Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6
			Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
	1. Introduction		1. Introduction
	Discussion Venue: Discussion about this draft is directed to the		Discussion Venue: Discussion about this draft is directed to the
	dcrup@ietf.org [1] mailing list.		dcrup@ietf.org [1] mailing list.
	DKIM [RFC6376] signs e-mail messages, by creating hashes of the		DKIM [RFC6376] signs e-mail messages, by creating hashes of the
	message headers and content and signing the header hash with a		message headers and content and signing the header hash with a
	digital signature. Message recipients fetch the signature		digital signature. Message recipients fetch the signature
	verification key from the DNS where it is stored in a TXT record.		verification key from the DNS where it is stored in a TXT record.
	The defining documents specify a single signing algorithm, RSA		The defining documents specify a single signing algorithm, RSA
	[RFC8017], and recommends key sizes of 1024 to 2048 bits (but require		[RFC8017], and recommends key sizes of 1024 to 2048 bits (but require
	verification of 512 bit keys). While 1024 bit signatures are common,		verification of 512 bit keys). As discussed in US-CERT VU#268267
	stronger signatures are not. Widely used DNS configuration software		[VULNOTE], the operational community has recognized that shorter keys
	places a practical limit on key sizes, because the software only		compromise the effectiveness of DKIM. While 1024 bit signatures are
	handles a single 256 octet string in a TXT record, and RSA keys		common, stronger signatures are not. Widely used DNS configuration
			software places a practical limit on key sizes, because the software
			only handles a single 256 octet string in a TXT record, and RSA keys
	longer than 1024 bits don't fit in 256 octets.		longer than 1024 bits don't fit in 256 octets.
	2. Conventions Used in This Document		2. Conventions Used in This Document
	The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",		The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
	"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and		"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in		"OPTIONAL" in this document are to be interpreted as described in
	[RFC2119].		[RFC2119].
	3. DKIM Signing and Verification Algorithms		3. DKIM Signing and Verification Algorithms
	skipping to change at page 4, line 23		skipping to change at page 4, line 23
	o The security goals of DKIM,[RFC6376], are modest compared to		o The security goals of DKIM,[RFC6376], are modest compared to
	typical goals of other systems that employ digital signatures		typical goals of other systems that employ digital signatures
	See [RFC3766] for further discussion on selecting key sizes.		See [RFC3766] for further discussion on selecting key sizes.
	3.4. Other Algorithms		3.4. Other Algorithms
	Other algorithms will be defined in the future. Verifiers MUST		Other algorithms will be defined in the future. Verifiers MUST
	ignore any signatures using algorithms that they do not implement.		ignore any signatures using algorithms that they do not implement.
	4. Security Considerations		4. The DKIM-Signature Header Field
			This section updates the a= tag in [RFC6376] Section 3.5.
			The text description of the tag is now:
			a= The algorithm used to generate the signature (plain-text;
			REQUIRED). Verifiers MUST support "rsa-sha256"; Signers MUST
			sign using "rsa-sha256". See [RFC6376] Section 3.3 (as updated
			by this document) for a description of the algorithms.
			The following ABNF element is updated:
			ABNF:
			sig-a-tag-h = "sha256" / x-sig-a-tag-h
			5. Key Management and Representation
			This section updates the h= tag in [RFC6376] Section 3.6.1.
			The following ABNF element is updated:
			ABNF:
			key-h-tag-alg = "sha256" / x-key-h-tag-alg
			6. Security Considerations
	This document does not change the Security Considerations of		This document does not change the Security Considerations of
	[RFC6376]. It reduces the risk of signature compromise due to weak		[RFC6376]. It reduces the risk of signature compromise due to weak
	cryptography.		cryptography.
	5. IANA Considerations		7. IANA Considerations
	IANA is requested to update registries as follows.		IANA is requested to update registries as follows.
	5.1. DKIM Hash Algorithms		7.1. DKIM Hash Algorithms
	The following value is changed in the DKIM Hash Algorithms		The following value is changed in the DKIM Hash Algorithms
	+------+-----------------+----------+		+------+-----------------+----------+
	| TYPE | REFERENCE | STATUS |		| TYPE | REFERENCE | STATUS |
	+------+-----------------+----------+		+------+-----------------+----------+
	| sha1 | (this document) | obsolete |		| sha1 | (this document) | obsolete |
	+------+-----------------+----------+		+------+-----------------+----------+
	Table 1: DKIM Hash Algorithms Changed Value		Table 1: DKIM Hash Algorithms Changed Value
	6. Normative References		8. References
			8.1. 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, DOI 10.17487/		Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
	RFC2119, March 1997,		RFC2119, March 1997,
	<http://www.rfc-editor.org/info/rfc2119>.		<http://www.rfc-editor.org/info/rfc2119>.
	[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For		[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
	Public Keys Used For Exchanging Symmetric Keys", BCP 86,		Public Keys Used For Exchanging Symmetric Keys", BCP 86,
	RFC 3766, DOI 10.17487/RFC3766, April 2004,		RFC 3766, DOI 10.17487/RFC3766, April 2004,
	<http://www.rfc-editor.org/info/rfc3766>.		<http://www.rfc-editor.org/info/rfc3766>.
	skipping to change at page 5, line 20		skipping to change at page 5, line 48
	[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,		[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
	"DomainKeys Identified Mail (DKIM) Signatures", STD 76,		"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
	RFC 6376, DOI 10.17487/RFC6376, September 2011,		RFC 6376, DOI 10.17487/RFC6376, September 2011,
	<http://www.rfc-editor.org/info/rfc6376>.		<http://www.rfc-editor.org/info/rfc6376>.
	[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,		[RFC8017] Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
	"PKCS #1: RSA Cryptography Specifications Version 2.2",		"PKCS #1: RSA Cryptography Specifications Version 2.2",
	RFC 8017, DOI 10.17487/RFC8017, November 2016,		RFC 8017, DOI 10.17487/RFC8017, November 2016,
	<http://www.rfc-editor.org/info/rfc8017>.		<http://www.rfc-editor.org/info/rfc8017>.
			8.2. Informative References
			[VULNOTE] US-CERT, "Vulnerability Note VU#268267, DomainKeys
			Identified Mail (DKIM) Verifiers may inappropriately
			convey message trust", October 2012.
	Appendix A. Acknowledgements		Appendix A. Acknowledgements
	The author wishes to acknowledge the following for their review and		The author wishes to acknowledge the following for their review and
	constructive criticism of this proposal: TBD (surely there will be		constructive criticism of this proposal: TBD (surely there will be
	someone).		someone).
	Thanks to John Levine for draft-ietf-dcrup-dkim-crypto-00, which was		Thanks to John Levine for draft-ietf-dcrup-dkim-crypto-00, which was
	the source for much of the introductory material in this draft.		the source for much of the introductory material in this draft.
	Author's Address		Author's Address
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