# ECDSA, EdDSA and ed25519 relationship / compatibility

I'm trying to understand the relationship between those three signature schemes (ECDSA, EdDSA and ed25519) and mainly, to what degree are they mutually compatible in the sense of key pair derivation, signing and signature verification, but I was not able to find any conclusive information. I mean, for example, can you verify ed25519 signatures with EdDSA and/or viceversa?

The best info I found was: https://askubuntu.com/questions/363207/what-is-the-difference-between-the-rsa-dsa-and-ecdsa-keys-that-ssh-uses from which I got the suspicion that there might be some compatibility between those schemes but I could not find any source to prove or disprove it.

Could somebody shed some light on this?

On a practical level, what a user might need to know is that Ed25519 keys are not compatible in any meaningful sense with keys in any instance of ECDSA. So, e.g., in the ssh protocol, an ssh-ed25519 key is not compatible with an ecdsa-sha2-nistp521 key, which is why they are marked with different types. Similarly, an ssh-ed448 key, for Ed448, is incompatible, which is why it is also marked with a different type.