I have a public/private key pair of Curve25519 keys used by Wireguard.
How can I use this keypair to generate/verify digital signatures?
Preferrably, I would like to use EdDSA/Ed25519 but I struggle to derive a Ed25519 keypair from the Curve25519 keys used by Wireguard.
I don't think it's possible to do what you are asking because of the way keys are generated for use in ECDH versus Ed25519.
Consider a Wireguard key pair (sk1, pk1). Note that sk1 is just 32 random bytes with the appropriate bits set/cleared (source) and that pk1 is derived from sk1 in the typical ECDH manner (source).
For Ed25519 signatures, you need to hash the 32 random bytes to produce 64 bytes, half of which are used to derive the public key the other half of which are used when creating signatures.
So you could derive a private key sk2 from sk1 and use sk2 to create Ed25519 signatures. But based on the discussion in the comments, it sounds like you want to verify these signatures without access to sk1 (presumably at the other end of the Wireguard tunnel, which only has pk1). And since you can't derive sk1 from pk1, you can't calculate SHA512(sk1) from pk1. Therefore you can't derive pk2 from pk1 alone.
