zero-knowledge proof
Prove you know.
There is a passphrase hidden in the book. Enter it and you will prove that you hold it — without sending it. Your words never leave this page; only a proof does. The server learns that you know, and nothing about what you know.
Schnorr proof over secp256k1. Open the network tab — you will see a point, a scalar, and a nonce leave your browser. You will not see the passphrase, because it is never sent.