Break the cipher:-

Alice has published her RSA public key as (n, b)=(91, 5).

Accordingly, Bob sent her the cipher text 71. What letter of the alphabet was Bob's message? (A card experiment) Take a packet of p playing cards where p is prime, for example, seven cards. Have a friend select one of these cards, noting its value. Pretend to mix the cards, but arrange that the chosen card ends on top of the face-down packet. Hand the packet to your friend and ask him or her to select any number less than the number of cards in the packet. For example, with seven cards the friend can select any number from one to six. Suppose the number four is selected. Have the friend deal, one at a time, four cards from the top of the packet to the bottom.

Then turn the next card face up on top. The process is to be repeated p - 1 times, dealing four to the bottom and turning the next card face up on top. After p - 1 times, all but one card will be face up, and that one will be the originally selected card. This is true no matter what number between 1 and p - 1 the friend uses in the deal and turnover process. Try it yourself. Why does this work?

(In card magic literature, this is termed George Sands' prime number principle.)