RSA is the cryptographic algorithm most commonly associated with DKIM signatures. In a typical DKIM exchange, the sending domain uses an RSA private key to sign selected message headers and the body hash, while the recipient retrieves the matching public key from DNS and verifies the signature.
The algorithm name in a DKIM-Signature header is usually written as “rsa-sha256.” That label combines two different functions: RSA performs the public-key signature operation, while SHA-256 hashes the message data before it is signed. This is why SHA-256 is a tempting but incomplete answer to the question. It is a hash algorithm, not the public-key signing algorithm itself.
DKIM originally permitted RSA with SHA-1 or SHA-256, but modern standards prohibit rsa-sha1 for new use because SHA-1 is cryptographically weak. Ed25519-SHA256 was added as an alternative, yet RSA remains the dominant and most broadly deployed choice because of long-standing implementation and infrastructure support. DKIM authenticates domain-level signing, not the message’s encryption or the sender’s personal identity.