Network Working Group                                       S. Kitterman
Internet-Draft                              Kitterman Technical Services
Updates: 6376 (if approved)                                July 31, 2017
Intended status: Standards Track
Expires: February 1, 2018


          Cryptographic Algorithm and Key Usage Update to DKIM
                     draft-ietf-dcrup-dkim-usage-03

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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 1, 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.  The rsa-sha256 Signing Algorithm  . . . . . . . . . . . .   3
     4.2.  Key Sizes . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.3.  Other Algorithms  . . . . . . . . . . . . . . . . . . . .   4
   5.  The DKIM-Signature Header Field . . . . . . . . . . . . . . .   4
   6.  Key Management and Representation . . . . . . . . . . . . . .   4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   6
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

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.





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   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 is removed from the
   protocol.  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

   This section replaces [RFC6376] Section 3.3 in its entirety.

   Generally, DKIM supports multiple digital signature algorithms.  One
   algorithm, rsa-sha256, is currenlty defined.  Signers MUST implement
   and sign using rsa-sha256.  Verifiers MUST implement and verify using
   rsa-sha256.

4.1.  The rsa-sha256 Signing Algorithm

   The rsa-sha256 Signing Algorithm computes a message hash as described
   in [RFC6376], Section 3.7 using SHA-256 [FIPS-180-3-2008] as the
   hash-alg.  That hash is then signed by the Signer using the RSA
   algorithm (defined in PKCS#1 version 1.5 [RFC8017]) as the crypt-alg
   and the Signer's private key.  The hash MUST NOT be truncated or
   converted into any form other than the native binary form before
   being signed.  The signing algorithm SHOULD use a public exponent of
   65537.

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.  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.

   Factors that should influence the key size choice include the
   following:




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   o  The practical constraint that large (e.g., 4096-bit) keys might
      not fit within a 512-byte DNS UDP response packet

   o  The security constraint that keys smaller than 2048 bits may be
      subject to off-line attacks

   o  Larger keys impose higher CPU costs to verify and sign email

   o  Keys can be replaced on a regular basis; thus, their lifetime can
      be relatively short

   o  The security goals of DKIM [RFC6376] are modest compared to
      typical goals of other systems that employ digital signatures

   See [RFC3766] for further discussion on selecting key sizes.

4.3.  Other Algorithms

   The rsa-sha1 was formerly used by DKIM [RFC6376].  Signers MUST NOT
   sign with rsa-sha1 and verifiers MUST NOT verify using rsa-sha1.

   Other algorithms will be defined in the future.  Verifiers MUST
   ignore any signatures using algorithms that they do not implement.

5.  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

6.  Key Management and Representation

   This section updates the h= tag in [RFC6376] Section 3.6.1.

   The following ABNF element is updated:





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         ABNF:

         key-h-tag-alg   = "sha256" / x-key-h-tag-alg

7.  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 the removal of rsa-sha1 from DKIM.

8.  IANA Considerations

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

                   +------+-----------------+----------+
                   | TYPE | REFERENCE       | STATUS   |
                   +------+-----------------+----------+
                   | sha1 | (this document) | obsolete |
                   +------+-----------------+----------+

                Table 1: DKIM Hash Algorithms Changed Value

9.  References

9.1.  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>.

   [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
              Public Keys Used For Exchanging Symmetric Keys", BCP 86,
              RFC 3766, DOI 10.17487/RFC3766, April 2004,
              <http://www.rfc-editor.org/info/rfc3766>.

   [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>.




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9.2.  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,
              <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.

9.3.  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 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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