DKIM glossary
What does a=rsa-sha256 mean in a DKIM signature?
By Samuel Chenard · CEO & Co-Founder, Palisade · Reviewed July 15, 2026
a=rsa-sha256 is the signing algorithm in a DKIM-Signature: an RSA key with SHA-256 hashing. It's the modern default. The older rsa-sha1 is deprecated and treated as insecure by today's receivers, and ed25519-sha256 exists but has only partial support — so RSA-SHA256 stays the safe choice.
a=rsa-sha256 at a glance | |
|---|---|
| Tag | a (signing algorithm) |
| Valid values | rsa-sha256 (standard) · ed25519-sha256 (RFC 8463) · rsa-sha1 (deprecated) |
| Default | No universal default in DNS; the signer chooses. rsa-sha256 is the accepted norm. |
| Where it goes | In the DKIM-Signature header, near v= and c=. |
How a=rsa-sha256 works
a= states how the message was signed: the key algorithm and the hash. rsa-sha256 — an RSA key with SHA-256 hashing — is the modern standard, and it's what you should see on essentially all current mail.
The older rsa-sha1 is deprecated. SHA-1 is broken for security use, and today's receivers treat rsa-sha1 signatures as insecure or simply fail them. If you find a sender still using it, that's a key and configuration to update, not a curiosity.
ed25519-sha256 (RFC 8463) is a newer option with smaller signatures, but receiver support is only partial. Because of that, most senders that adopt it publish an RSA key too and dual-sign — so rsa-sha256 remains the safe default you can rely on everywhere.
Correct record vs common mistake
Correct
DKIM-Signature: v=1; a=rsa-sha256; d=example.com; s=s1; ...The current standard: an RSA key with SHA-256 hashing, verified by essentially every modern receiver.
Common mistake
DKIM-Signature: v=1; a=rsa-sha1; d=example.com; s=s1; ...rsa-sha1 relies on the broken SHA-1 hash. Modern receivers treat it as insecure and many fail it outright — move the sender to rsa-sha256.
Troubleshooting a=rsa-sha256
| Issue | Likely cause | Fix |
|---|---|---|
| Receiver flags the signature as insecure | The sender still signs with rsa-sha1 | Reconfigure the sender to sign with rsa-sha256 and rotate to a modern key |
| ed25519 signatures failing at some receivers | Partial receiver support for ed25519-sha256 | Dual-sign: keep an rsa-sha256 signature alongside the ed25519 one |
| Weak key even with rsa-sha256 | SHA-256 is fine but the RSA key is only 1024-bit | Rotate to a 2048-bit RSA key while keeping a=rsa-sha256 |
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Why it matters for MSPs
A single legacy sender still signing with rsa-sha1 can drag a client's authentication results down and put mail at risk of being failed. Across a fleet, these stragglers hide in older appliances and forgotten integrations — worth sweeping for before you push any domain to reject.
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