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rfc:rfc8301

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.

Kitterman Standards Track [Page 1] 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.

Kitterman Standards Track [Page 2] RFC 8301 DKIM Crypto Usage Update January 2018

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.

Kitterman Standards Track [Page 3] 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>.

Kitterman Standards Track [Page 4] 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

Kitterman Standards Track [Page 5]

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