Return-Path: X-Original-To: dcrup@ietfa.amsl.com Delivered-To: dcrup@ietfa.amsl.com Received: from localhost (localhost [127.0.0.1]) by ietfa.amsl.com (Postfix) with ESMTP id E98EF12EB5B for ; Sat, 3 Jun 2017 11:02:18 -0700 (PDT) X-Virus-Scanned: amavisd-new at amsl.com X-Spam-Flag: NO X-Spam-Score: -1.99 X-Spam-Level: X-Spam-Status: No, score=-1.99 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, HTML_MESSAGE=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001, T_HTML_ATTACH=0.01, URIBL_BLOCKED=0.001] autolearn=ham autolearn_force=no Authentication-Results: ietfa.amsl.com (amavisd-new); dkim=pass (1024-bit key) header.d=kitterman.com Received: from mail.ietf.org ([4.31.198.44]) by localhost (ietfa.amsl.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id ffkushCWzSTi for ; Sat, 3 Jun 2017 11:02:16 -0700 (PDT) Received: from mailout03.controlledmail.com (mailout03.controlledmail.com [IPv6:2607:f0d0:3001:aa::2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by ietfa.amsl.com (Postfix) with ESMTPS id 2EA9912EB55 for ; Sat, 3 Jun 2017 11:02:16 -0700 (PDT) Received: from kitterma-e6430.localnet (static-72-81-252-22.bltmmd.fios.verizon.net [72.81.252.22]) (using TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by mailout03.controlledmail.com (Postfix) with ESMTPSA id 44D4DC4010C for ; Sat, 3 Jun 2017 13:02:14 -0500 (CDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=kitterman.com; s=201409; t=1496512934; bh=qyu+TOYZMslF5m9MOTVcZCBe+OpoujYB0+2J2kZbE80=; h=From:To:Subject:Date:From; b=D0SfxJPz0beBfglczZpP4B6wmv0dTEQKxtZbXjfPbQ8qwE4DSRK3mpUnm1qB/dMhB 1KIGet2E28lp+hl50Y2OgO98UMQimavoQYv0h2ZFpkSj61VLQYJOMGmDGzOZULiUuO W4jbasHA+245ORLOt+4dYvZwS8swAKFTcqIIixos= From: Scott Kitterman To: dcrup@ietf.org Date: Sat, 03 Jun 2017 14:02:13 -0400 Message-ID: <1870784.2tGGxD3xed@kitterma-e6430> User-Agent: KMail/4.13.3 (Linux/3.13.0-119-generic; KDE/4.13.3; x86_64; ; ) MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="nextPart1965194.WfSBeShprZ" Content-Transfer-Encoding: 7Bit Archived-At: Subject: [Dcrup] Work on draft-ietf-dcrup-dkim-usage X-BeenThere: dcrup@ietf.org X-Mailman-Version: 2.1.22 Precedence: list List-Id: DKIM Crypto Update List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 03 Jun 2017 18:02:19 -0000 This is a multi-part message in MIME format. --nextPart1965194.WfSBeShprZ Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" I have locally updated the draft based on my interpretation of the discussion on the list. Rfcdiff attached. Added an informative reference to the US-CERT vulnerability report on this issue. Added updates to header field and key records that I missed in my first draft. Also, I forgot to mention before that, since all the cool kids are doing it, I put this up on my github account. This is, of course, in no way officially anything at all. It's only a way for me to keep track of changes. https://github.com/kitterma/draft-ietf-dcrup-dkim-usage I did not take on the question of adding key sizes to the IANA registry for several reasons: 1. Adding it at all is somewhat controversial and I'm hoping to limit the controversy in this particular document so we can get it done. I view this document as the minimum update to RFC 6376 required to update DKIM to remove known insecure things. An IANA registry doesn't affect that. 2. Different types of algorithms have widely different key size requirements, so I don't believe we will really know what such a registry would need to look like until after we've finished working through questions about what is the next generation DKIM algorithm going to be. Making a registry now would mean we might need to redo it later. Let's do it once, if we are going to to it at all. 3. There is some nuance about key size rules that I'm not sure how to fit into an IANA registry. One other thing that perhaps should go into this draft is more on Security Considerations. If you look at the Security Considerations in RFC 6376, it does not mention risks associated with using an obsolete hash algorithm or a key that is too small. Adding a new security consideration as an "update" to RFC 6376 that describes the problem this draft is solving might be a good thing? As revisions are cheap, unless someone objects, I'll go ahead and post this (or something similar based on feedback) soon. Scott K --nextPart1965194.WfSBeShprZ Content-Disposition: attachment; filename="draft-ietf-dcrup-dkim-usage-from--00.diff.html" Content-Transfer-Encoding: 7Bit Content-Type: text/html; charset="UTF-8"; name="draft-ietf-dcrup-dkim-usage-from--00.diff.html" Diff: draft-ietf-dcrup-dkim-usage-00.txt - draft-ietf-dcrup-dkim-usage.txt
 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
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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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