]> DKIM Hash Algorithm Adaptivity
steffen@sdaoden.eu
 General Internet Engineering Task Force DKIM DKIM (RFC 6376, section 3.7) defines how "data-hash" is generated as input to a RSA (RFC 8017) "sig-alg". Modern signature algorithms (for example EdDSA, RFC 8032) include extensive data hashing as part of the signing process. This memo allows DKIM signing algorithm "data-hash" adaptivity, taking advantage of algorithm progress and digital signature API reality.
Introduction In section 3.7 DKIM specifies the algorithm how "Computing the Message Hashes" for IMF messages has to be performed, and notes that real life digital signature APIs often combine hashing and signing into a single call that performs both, "hash-alg" and "sig-alg". Different to the RSA algorithm solely supported by DKIM at the time of creation, modern algorithms like for example EdDSA include extensive data hashing as part of digital signature creation.
INFORMATIVE NOTE: EdDSA was adapted to DKIM as Ed25519-SHA256 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 its current form DKIM defines the generated "data-hash" as the sole input of "sig-alg". But modern signing algorithms then perform "prehashing", applying hashing to the "data-hash". With the mentioned Ed25519-SHA256, for example, a 64-byte SHA-256 input is prehashed with SHA-512 to a 128-byte output. The conclusion is that currently the standard complicates implementations, fosters data processing redundancy, and potentially weakens security attributes of algorithms by feeding in only data subsets, prefiltered by potentially weak(er) algorithms.
Algorithm Adaptivity The computation described in DKIM section 3.7 is modified so that the described input to "sig-alg", the "data-hash", can adapt to standardized algorithms as appropriate. If an algorithm chooses adaption, "hash-alg" is only used to produce the "body-hash", whereas the input formerly used to create the "data-hash" is fed into "sig-alg" instead of to "hash-alg". More formally, the new pseudo-code for the signature algorithm is:
IANA Considerations This memo includes no request to IANA.
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.
References Normative References Informative References