Internet Engineering Task Force S. Nurpmeso, Ed. Internet-Draft 31 October 2024 Updates: 6376 (if approved) Intended status: Informational Expires: 4 May 2025 DKIM Signing Algorithm AdaEd25519-SHA256 draft-nurpmeso-dkim-algo-adaed25519-01 Abstract This memo 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 (draft-nurpmeso- dkim-hash-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 4 May 2025 [Page 1] Internet-Draft DKIM Signing Algorithm AdaEd25519-SHA256 October 2024 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. AdaEd25519-SHA256 . . . . . . . . . . . . . . . . . . . . . . 2 2.1. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 2 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 4. Security Considerations . . . . . . . . . . . . . . . . . . . 3 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 3 5.1. Normative References . . . . . . . . . . . . . . . . . . 3 5.2. Informative References . . . . . . . . . . . . . . . . . 3 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction Ed25519-SHA256[RFC8463], which defines a DKIM key type for Ed25519[RFC8032], became standardized in 2018, but has not gained much traction in the six years since its introduction. A survey of DKIM implementations revealed necessity for error prone extra code paths to implement it with existing APIs. In addition the DKIM[RFC6376] algorithm of how "Computing the Message Hashes" for IMF[RFC5322] messages causes Ed25519-SHA256 to create a 64-byte SHA- 256[RFC6234] "data-hash" input for Ed25519, which is then "prehashed" with the stronger SHA-512[RFC6234] that expands to 128 bytes. AdaEd25519-SHA256 bases upon DKIM Hash Algorithm Adaptivity[I-D.nurpmeso-dkim-hash-adaptivity] that takes advantage of algorithm progress and digital signature API reality. 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". Nurpmeso Expires 4 May 2025 [Page 2] Internet-Draft DKIM Signing Algorithm AdaEd25519-SHA256 October 2024 3. IANA Considerations This memo requests adding the new key type "adaed25519" to the "DKIM Key Type" registry. 4. Security Considerations This specification should reduce implementation burden and complexity, aids hash hardening of affected algorithms to a certain extend, and potentially increases, dependent upon algorithm, data volume and API optimization efforts, processing performance. 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-00, 4 October 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, . Nurpmeso Expires 4 May 2025 [Page 3] Internet-Draft DKIM Signing Algorithm AdaEd25519-SHA256 October 2024 [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, . Author's Address Steffen Nurpmeso (editor) Email: steffen@sdaoden.eu Nurpmeso Expires 4 May 2025 [Page 4]