Internet Engineering Task Force                         S. Nurpmeso, Ed.
Internet-Draft                                           3 February 2025
Updates: 6376 (if approved)                                             
Intended status: Informational                                          
Expires: 7 August 2025


                DKIM Signing Algorithm AdaEd25519-SHA256
                 draft-nurpmeso-dkim-algo-adaed25519-02

Abstract

   This specification adds the DKIM (RFC 6376) signing algorithm
   AdaEd25519-SHA256.  It is identical to Ed25519-SHA256 (RFC 8463)
   except for its use of DKIM hash algorithm adaptivity.  Private and
   public keys are identical, and can be used interchangeably.

Status of This Memo

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   This Internet-Draft will expire on 7 August 2025.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  AdaEd25519-SHA256 . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   2
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   3
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   3
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   3
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   The DKIM[RFC6376] algorithm of how "Computing the Message Hashes" for
   IMF[RFC5322] has to be performed does not adapt well to modern
   algorithms like Ed25519[RFC8032] which perform internal prehashing.
   The DKIM key type Ed25519-SHA256[RFC8463] as standardized in 2018,
   however, uses this algorithm.  It has not gained much traction since
   its introduction.  AdaEd25519-SHA256 instead uses the DKIM Hash
   Algorithm Adaptivity[I-D.nurpmeso-dkim-hash-adaptivity] algorithm.

2.  AdaEd25519-SHA256

   AdaEd25519-SHA256 is identical to Ed25519-SHA256[RFC8463] except that
   it uses DKIM Hash Algorithm
   Adaptivity[I-D.nurpmeso-dkim-hash-adaptivity]: the DKIM[RFC6376]
   (section 3.7) "hash-alg" SHA-256[RFC6234] is only used to produce the
   "body-hash", and "sig-alg" is fed in all the input used to create the
   "data-hash".  Private and public keys are identical, and can be used
   interchangeably.

   |  INFORMATIVE NOTE: Even though [RFC8410] defined ASN.1 Object
   |  Identifiers (OIDs) for Ed25519, it was considered more important
   |  to keep compatibility with actively used Ed25519-SHA256 keys, than
   |  to support ASN.1 standard means of inspecting key data blobs.

2.1.  Syntax

   The "sig-a-tag-k" ABNF definition of DKIM[RFC6376] (section 3.5), as
   well as the "key-k-tag-type" ABNF definition of DKIM[RFC6376]
   (section 3.6.1) are both extended by an entry "adaed25519".

3.  IANA Considerations

   This memo requests adding the new key type "adaed25519" to the "DKIM
   Key Type" registry.




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4.  Security Considerations

   This specification should add all the gains of DKIM Hash Algorithm
   Adaptivity[I-D.nurpmeso-dkim-hash-adaptivity] to the Ed25519 DKIM key
   type.

5.  References

5.1.  Normative References

   [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>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [I-D.nurpmeso-dkim-hash-adaptivity]
              Nurpmeso, S., "DKIM Hash Algorithm Adaptivity", Work in
              Progress, Internet-Draft, draft-nurpmeso-dkim-hash-
              adaptivity-01, 2 November 2024,
              <https://datatracker.ietf.org/doc/html/draft-nurpmeso-
              dkim-hash-adaptivity-01>.

5.2.  Informative References

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC8410]  Josefsson, S. and J. Schaad, "Algorithm Identifiers for
              Ed25519, Ed448, X25519, and X448 for Use in the Internet
              X.509 Public Key Infrastructure", RFC 8410,
              DOI 10.17487/RFC8410, August 2018,
              <https://www.rfc-editor.org/info/rfc8410>.

   [RFC8463]  Levine, J., "A New Cryptographic Signature Method for
              DomainKeys Identified Mail (DKIM)", RFC 8463,
              DOI 10.17487/RFC8463, September 2018,
              <https://www.rfc-editor.org/info/rfc8463>.



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Author's Address

   Steffen Nurpmeso (editor)
   Email: steffen@sdaoden.eu















































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