I dare disagreeing with the approved answer.

Cryptographic signatures are absolutely forms of zk-proofs.

Why?

Because by providing a signature, the signer reveals absolutely nothing, not a pico part of clue, of what it knows. Yet absolutely proved, without a pico chance of error, that he knows.

Granted the scheme is secure. But it is implied in the question. Plus, zk-proof technology could also have flaws that reveal what is known, so the "private key could be recovered" argument is moot.

Cryptographic signing is however an incomplete and an unwise illustration of zk proof. Serves different purpose. Designed to guarantee entirely different properties.

I dare disagreeing with the approved answer.

Semantics makes the question difficult to answer.

Cryptographic signatures are absolutely forms of zk-proofs.

Why?

Because by providing a signature, the signer reveals absolutely nothing, not a pico part of clue, of what it knows. Yet absolutely proved, without a pico chance of error, that he knows.

Granted the scheme is secure. But it is implied in the question. Plus, zk-proof systens could as easily have flaws that reveal what is known if exploited, so the "private key could be reconstructed by other parties" argument is moot. And granted that we accept all messsages can only be of the size and format of a private key. And granted we assume that such constraints giving away the size of the secret doesn't qualify as giving away information about the message. That is all messages shared using the system are always ever and forever of the same length.

Cryptographic signing is however an incomplete and unwise illustration of zk proof. Serves different purpose. Designed to guarantee entirely different properties.

Are cars forms of trains? Yes they are. Because they can be. If they can fit on rails they would accomplish about the same thing. They are a sort of train, and poor at that. But they aren't trains.