ed25519

ed25519 is a signature scheme on an elliptic curve - the algorithm whose public keys literally are v3 onion addresses.

ed25519 is a digital signature scheme (technically eddsa over the Edwards25519 curve) introduced in 2011 by daniel j. bernstein and colleagues. it produces compact 64-byte signatures with 32-byte keys and verifies fast, which is why it spread through ssh, tls, cryptocurrencies and pgp within a decade.

one standout property is deterministic signing: the signature depends on the message and the key, not on fresh randomness. that eliminates an entire class of disasters where a flawed random number generator leaks the private key through repeated signatures - the failure mode that drained some early cryptocurrency wallets.

its role in this world is foundational: a v3 onion address is the base32 encoding of an ed25519 public key. the self-authenticating property of onion hostnames rests entirely on this signature keypair, with the service proving ownership of the address on every connection.

in openpgp, modern keys can carry ed25519 signing subkeys alongside curve25519 encryption subkeys - the signing and encryption roles split across the two sibling algorithms. our key generator creates such keys, and the browser path signs and verifies them natively.