A challenge-response user-authentication procedure masks an authentication center's expected response (XRES) utilizing a masking function (f) and by transmitting a random challenge and a masked expected response (XRES') instead of the XRES to an intermediate party where the actual user authentication takes place. The intermediate party sends the random challenge to the user and recieves a user response (RES) from the user. The intermediate party then generates a masked user response (RES') using the same masking function (f) and verifies that RES' corresponds to XRES'.