Cryptographic Algorithm and Key Usage Update to DomainKeys Identified Mail (DKIM)
RFC 8301

Internet Engineering Task Force (IETF)                      S. Kitterman
Request for Comments: 8301                  Kitterman Technical Services
Updates: 6376                                               January 2018
Category: Standards Track
ISSN: 2070-1721

            Cryptographic Algorithm and Key Usage Update to
                   DomainKeys 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 in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://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 to BCP 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.

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RFC 8301                DKIM Crypto Usage Update            January 2018

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  Updates to DKIM Signing and Verification Requirements . . . .   3
     3.1.  Signing and Verification Algorithms . . . . . . . . . . .   3
     3.2.  Key Sizes . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  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 in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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3.  Updates to DKIM Signing and Verification Requirements

   This document updates [RFC6376] as follows:

   o  Section 3.1 of this document updates Section 3.3 of [RFC6376].

   o  Section 3.2 of this document updates Section 3.3.3 of [RFC6376].

   o  The algorithm described in Section 3.3.1 of [RFC6376] is now
      historic and no longer used by DKIM.

   Sections 3.3.2 and 3.3.4 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 in Section 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 in Section 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 in Section 3 of [RFC6194]
   are resolved due to rsa-sha1 no longer being used by DKIM.

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RFC 8301                DKIM Crypto Usage Update            January 2018

5.  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.  References

6.1.  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 in RFC
              2119 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>.

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RFC 8301                DKIM Crypto Usage Update            January 2018

Acknowledgements

   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.com

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