Ed25519 for DKIM
The elliptic-curve signing algorithm added to DKIM by RFC 8463 in 2018, identified as a=ed25519-sha256 with a key record of k=ed25519.
What it means
RFC 8463 added Ed25519-SHA256 as a second DKIM algorithm. Its public keys are 256 bits, so the base64 key in the DNS record is only 44 characters and always fits in one TXT string, unlike a 2048-bit RSA key.
The RFC says verifiers must implement it and signers should. Receivers built before 2018 cannot verify it, so RFC 8463 describes dual signing for the transition: add both an RSA and an Ed25519 signature, under different selectors (one key record per selector), so a receiver that cannot verify one still verifies the other.
Never sign with Ed25519 alone unless every receiver you care about verifies it.
Example
ed2026._domainkey.example.com. TXT "v=DKIM1; k=ed25519; p=11qYAYKxCrfVS/7TyWQHOg7hcvPapiMlrwIaaPcHURo="Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
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