Cryptographic Algorithm and Key Usage Update to DomainKeys Identified Mail (DKIM)
draft-ietf-dcrup-dkim-usage-06

Network Working Group                                       S. Kitterman
Internet-Draft                              Kitterman Technical Services
Updates: 6376 (if approved)                             November 3, 2017
Intended status: Standards Track
Expires: May 7, 2018

          Cryptographic Algorithm and Key Usage Update to DKIM
                     draft-ietf-dcrup-dkim-usage-06

Abstract

   The cryptographic algorithm and key size requirements included when
   DKIM was designed in the last decade are functionally obsolete and in
   need of immediate revision.  This document updates DKIM requirements
   to those minimaly suitable for operation with currently specified
   algorithms.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on May 7, 2018.

Copyright Notice

   Copyright (c) 2017 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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Discussion Venue  . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   4.  DKIM Signing and Verification Algorithms  . . . . . . . . . .   3
     4.1.  DKIM Signing and Verification Algorithms  . . . . . . . .   3
     4.2.  Key Sizes . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   4
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Discussion Venue

   RFC EDITOR: Please remove this section before publication.

   Discussion about this draft is directed to the dcrup@ietf.org [1]
   mailing list.

2.  Introduction

   DKIM [RFC6376] signs e-mail 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 specify a single signing algorithm, RSA
   [RFC8017], and recommends key sizes of 1024 to 2048 bits (but require
   verification of 512 bit keys).  As discussed in US-CERT 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 sha1 hash algorithm, see
   [RFC6194], and the wide availability of the sha256 hash algorithm (it
   has been a required part of DKIM [RFC6376] since it was originally
   standardized in 2007), the sha1 hash algorithm MUST NOT be used.

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   This is being done now to allow the operational community time to
   fully shift to sha256 in advance of any sha1 related crisis.

3.  Conventions Used in This Document

   The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

4.  DKIM Signing and Verification Algorithms

   Section 4.1 updates [RFC6376] Section 3.3.

   Section 4.2 updates [RFC6376] Section 3.3.3.

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

   [RFC6376] Sections 3.3.2 and 3.3.4 are not affected.

4.1.  DKIM 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 [RFC6376] Section 3.9.

4.2.  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.

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   DKIM signatures with insufficient key sizes (currently rsa-sha256
   with less than 1024 bits) have permanently failed evaluation as
   discussed in [RFC6376] Section 3.9.

5.  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 [RFC6194] Section 3 are
   resolved due to rsa-sha1 no longer being used by DKIM.

6.  IANA Considerations

   IANA is requested to update the "sha1" registration in the "DKIM Hash
   Algorithms" as follows:

                      +------+-----------+----------+
                      | TYPE | REFERENCE | STATUS   |
                      +------+-----------+----------+
                      | sha1 | [RFC6376] | historic |
                      +------+-----------+----------+

                Table 1: DKIM Hash Algorithms Changed Value

7.  References

7.1.  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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc8017>.

7.2.  Informative References

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   [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,
              <http://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.

7.3.  URIs

   [1] mailto:dcrup@ietf.org

Appendix A.  Acknowledgements

   The author wishes to acknowledge the following for their review and
   comment on this proposal: Kurt Andersen, Murray S.  Kucherawy, Martin
   Thomson, John Levine, Russ Housley, and Jim Fenton.

   Thanks to John Levine for his DCRUP work that was the source for much
   of the introductory material in this draft.

Author's Address

   Scott Kitterman
   Kitterman Technical Services
   3611 Scheel Dr
   Ellicott City, MD  21042

   Phone: +1 301 325-5475
   Email: scott@kitterman.com

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