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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