Any Guesses to how this crypto-algorithm works? (Score 1)

One possible guess is: x = credit card number y = passcode t = time (random number generator) z = f(x,y,t) z is then passed to the store t is passed to AMEX (?would this be recorded by the store) AMEX can calculate x from y, z, and t and then send the verification check to the store. What does the store record in their database? Is it z and t? or only z? The next question would be: what is f(x,y,t)?