I think it’s the original quote that is unclear:

> a digital signature proves your possession of a private key without revealing that key.

Signatures do not themselves do this; but they can be used to construct a protocol that does (e.g. the provee provides a random challenge that the prover must sign). But still this is not AFAIU a zero-knowledge proof as the signature is itself “knowledge”.

I think a definition of the security of a signature scheme is that a computationally limited attacker should not have a non-negligibly better than chance guess of the secret key.

I think some of the “ZKP” techniques are supposed to only be “ZK” for a computationally limited observer? Though I may be mistaken, and maybe non-interactive ZKP schemes are only assuming that the prover has limited computational resources, not that the observer/attacker hoping to get information from them does?