OPRF (oblivious pseudorandom function)
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 output, and without the client learning the key.
Encryption and sign-in
Think of it as a blind keyed hash: the client wants F(key, password) but must not reveal the password; the server owns the key but must not learn what it was applied to. The client blinds its input, the server evaluates on the blinded value, and the client unblinds the result.
OPRFs are the engine inside OPAQUE: they let a client stretch its password into a strong encryption key with the server’s participation. The password never reaches the server, and without the server’s OPRF key a stolen database leaves nothing to test password guesses against. OPRFs also appear in private set intersection and privacy-preserving credential checkers (the "has this password been breached" lookups that never send your password).