[dave1010uk]

This is my understanding of how it works, without knowing the actual maths behind the functions:

    # client
    r = random_blinding_factor()
    x = client_secret_input()
    x_blinded = blind(x, r)

    # Server
    y_blinded = OPRF(k, x_blinded)

    # Client
    y = unblind(y_blinded, r)

So you end up with y = OPRF(k, x). But the server never saw x and the client never saw k.

This feels like the same kind of unintuitive cryptography as homomorphic encryption.