Bob is stationed as a spy in Cyberia for a week and wants to prove everyday of this week that he is alive and has not been captured. He has chosen a secret random number, x, which he memorized and told to no one. But he did tell his boss the value y = h(h(h(h(h(h(h(x))))))), where h is a one-way function. Unfortunately, he knows that the Cyberian Intelligence Agency (CIA) was able to listen in on that message; hence, they also know the value of y. Explain how he can send a single message every day that proves he is still alive and has not been captured. Your solution should not allow anyone to replay any previous message from Bob as a (false) proof he is still alive.