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Internet Engineering Task Force (IETF)                      S. KittermanRequest for Comments: 8301                  Kitterman Technical ServicesUpdates:6376                                               January 2018Category: Standards TrackISSN: 2070-1721Cryptographic Algorithm and Key Usage Update toDomainKeys Identified Mail (DKIM)Abstract   The cryptographic algorithm and key size requirements included when   DomainKeys Identified Mail (DKIM) was designed a decade ago are   functionally obsolete and in need of immediate revision.  This   document updates DKIM requirements to those minimally suitable for   operation with currently specified algorithms.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8301.Copyright Notice   Copyright (c) 2018 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://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.Kitterman                    Standards Track                    [Page 1]

RFC 8301                DKIM Crypto Usage Update            January 2018Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in This Document . . . . . . . . . . . . . .23.  Updates to DKIM Signing and Verification Requirements . . . .33.1.  Signing and Verification Algorithms . . . . . . . . . . .33.2.  Key Sizes . . . . . . . . . . . . . . . . . . . . . . . .34.  Security Considerations . . . . . . . . . . . . . . . . . . .35.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .46.  References  . . . . . . . . . . . . . . . . . . . . . . . . .46.1.  Normative References  . . . . . . . . . . . . . . . . . .46.2.  Informative References  . . . . . . . . . . . . . . . . .4   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .5   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .51.  Introduction   DKIM [RFC6376] signs email messages by creating hashes of the message   headers and content and signing the header hash with a digital   signature.  Message recipients fetch the signature verification key   from the DNS where it is stored in a TXT record.   The defining documents,RFC 6376 [RFC6376] and its predecessors,   specify a single signing algorithm, RSA [RFC8017], and recommend key   sizes of 1024 to 2048 bits (but require verification of 512-bit   keys).  As discussed in US-CERT Vulnerability Note VU#268267   [VULNOTE], the operational community has recognized that shorter keys   compromise the effectiveness of DKIM.  While 1024-bit signatures are   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   significantly longer than 1024 bits don't fit in 256 octets.   Due to the recognized weakness of the SHA-1 hash algorithm (see   [RFC6194]) and the wide availability of the SHA-256 hash algorithm   (it has been a required part of DKIM [RFC6376] since it was   originally standardized in 2007), the SHA-1 hash algorithm MUST NOT   be used.  This is being done now to allow the operational community   time to fully shift to SHA-256 in advance of any SHA-1-related   crisis.2.  Conventions Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.Kitterman                    Standards Track                    [Page 2]

RFC 8301                DKIM Crypto Usage Update            January 20183.  Updates to DKIM Signing and Verification Requirements   This document updates [RFC6376] as follows:   oSection 3.1 of this document updatesSection 3.3 of [RFC6376].   oSection 3.2 of this document updatesSection 3.3.3 of [RFC6376].   o  The algorithm described inSection 3.3.1 of [RFC6376] is now      historic and no longer used by DKIM.   Sections3.3.2 and3.3.4 of [RFC6376] are not affected.3.1.  Signing and Verification Algorithms   DKIM supports multiple digital signature algorithms.  Two algorithms   are defined by this specification at this time: rsa-sha1 and   rsa-sha256.  Signers MUST sign using rsa-sha256.  Verifiers MUST be   able to verify using rsa-sha256.  rsa-sha1 MUST NOT be used for   signing or verifying.   DKIM signatures identified as having been signed with historic   algorithms (currently, rsa-sha1) have permanently failed evaluation   as discussed inSection 3.9 of [RFC6376].3.2.  Key Sizes   Selecting appropriate key sizes is a trade-off between cost,   performance, and risk.  Since short RSA keys more easily succumb to   off-line attacks, Signers MUST use RSA keys of at least 1024 bits for   all keys.  Signers SHOULD use RSA keys of at least 2048 bits.   Verifiers MUST be able to validate signatures with keys ranging from   1024 bits to 4096 bits, and they MAY be able to validate signatures   with larger keys.  Verifier policies can use the length of the   signing key as one metric for determining whether a signature is   acceptable.  Verifiers MUST NOT consider signatures using RSA keys of   less than 1024 bits as valid signatures.   DKIM signatures with insufficient key sizes (currently, rsa-sha256   with less than 1024 bits) have permanently failed evaluation as   discussed inSection 3.9 of [RFC6376].4.  Security Considerations   This document does not change the Security Considerations of   [RFC6376].  It reduces the risk of signature compromise due to weak   cryptography.  The SHA-1 risks discussed inSection 3 of [RFC6194]   are resolved due to rsa-sha1 no longer being used by DKIM.Kitterman                    Standards Track                    [Page 3]

RFC 8301                DKIM Crypto Usage Update            January 20185.  IANA Considerations   IANA has updated the Reference and Status fields of the "sha1"   registration in the "DKIM Hash Algorithms" registry.  The   registration now appears as follows:                 +------+---------------------+----------+                 | Type | Reference           | Status   |                 +------+---------------------+----------+                 | sha1 | [RFC6376] [RFC8301] | historic |                 +------+---------------------+----------+6.  References6.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,RFC 6376, DOI 10.17487/RFC6376, September 2011,              <https://www.rfc-editor.org/info/rfc6376>.   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,              "PKCS #1: RSA Cryptography Specifications Version 2.2",RFC 8017, DOI 10.17487/RFC8017, November 2016,              <https://www.rfc-editor.org/info/rfc8017>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.6.2.  Informative References   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security              Considerations for the SHA-0 and SHA-1 Message-Digest              Algorithms",RFC 6194, DOI 10.17487/RFC6194, March 2011,              <https://www.rfc-editor.org/info/rfc6194>.   [VULNOTE]  US-CERT, "Vulnerability Note VU#268267: DomainKeys              Identified Mail (DKIM) Verifiers may inappropriately              convey message trust", October 2012,              <http://www.kb.cert.org/vuls/id/268267>.Kitterman                    Standards Track                    [Page 4]

RFC 8301                DKIM Crypto Usage Update            January 2018Acknowledgements   The author wishes to acknowledge the following individuals for their   review and comments on this proposal: Kurt Andersen, Murray   S. Kucherawy, Martin Thomson, John Levine, Russ Housley, and Jim   Fenton.   Thanks to John Levine for his DKIM Crypto Update (DCRUP) work that   was the source for much of the introductory material in this   document.Author's Address   Scott Kitterman   Kitterman Technical Services   3611 Scheel Dr   Ellicott City, MD  21042   United States of America   Phone: +1 301 325-5475   Email: scott@kitterman.comKitterman                    Standards Track                    [Page 5]