I'm running into a problem where I generate signatures using a library like pynacl's (https://pynacl.readthedocs.io/en/stable/signing/) ed25519 implementation and then when I verify it using xedDSA's verify function (https://github.com/signalapp/curve25519-java/blob/master/android/jni/ed25519/additions/xeddsa.c#L45) it passes sometimes and fails other times.
This seems to be related to the priv/pub key pair that generates the signatures. Once a keypair generates a signature that the xedDSA function can verify, all subsequent signatures are also validated correctly from the same keypair. I'm converting the ed25519 public key that is given by pynacl into a curve25519 pub key (
verify_key.to_curve25519_public_key()), which I then pass into the xedDSA function.
In the intro here: https://signal.org/docs/specifications/xeddsa/#curve25519
It is stated that an ed25519 signature will be correctly verified by a xedDSA verify function. But this is not the case here.
Can anyone tell me exactly why this is happening? I'm quite confident it has something to do with the key generation and the subsequent conversion of the ed pubkey to a curve pubkey. But I don't know enough about crypto to pinpoint the issue.