OPRF

An OPRF is a two-party protocol where a server holding a secret key helps a client evaluate a pseudorandom function on the client's input, without the server learning the input or the client learning the key. This building block enables privacy-preserving systems like compromised-credential checking, where a browser can ask whether a password appears in a breach database without revealing the password. The CFRG standardized OPRF constructions over elliptic curves in RFC 9497, and they are core components of the OPAQUE PAKE and Privacy Pass.