| Internet-Draft | DKIM Signing Algorithm AdaEd25519-SHA256 | February 2025 |
| Nurpmeso | Expires 7 August 2025 | [Page] |
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.¶
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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.¶
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.¶
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".¶
This memo requests adding the new key type "adaed25519" to the "DKIM Key Type" registry.¶
This specification should add all the gains of DKIM Hash Algorithm Adaptivity[I-D.nurpmeso-dkim-hash-adaptivity] to the Ed25519 DKIM key type.¶