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 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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Nurpmeso Expires 7 August 2025 [Page 1] Internet-Draft DKIM Signing Algorithm AdaEd25519-SHA256 February 2025 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. Nurpmeso Expires 7 August 2025 [Page 2] Internet-Draft DKIM Signing Algorithm AdaEd25519-SHA256 February 2025 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, . [RFC8032] Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital Signature Algorithm (EdDSA)", RFC 8032, DOI 10.17487/RFC8032, January 2017, . [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, . 5.2. Informative References [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008, . [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, . [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, . [RFC8463] Levine, J., "A New Cryptographic Signature Method for DomainKeys Identified Mail (DKIM)", RFC 8463, DOI 10.17487/RFC8463, September 2018, . Nurpmeso Expires 7 August 2025 [Page 3] Internet-Draft DKIM Signing Algorithm AdaEd25519-SHA256 February 2025 Author's Address Steffen Nurpmeso (editor) Email: steffen@sdaoden.eu Nurpmeso Expires 7 August 2025 [Page 4]