using a calculator!) A more interesting problem is that of trying to send a message secretly. What one wants are two black boxes - one that encrypts a message for sending and another that the message recipient uses for decrypting it. There is a very nice (easy-to-apply) Number Theoretic way of doing this (the RSA system) based on having an integer n with two very large prime divisors. (We tend to use a computer to perform the black box algorithms, but the ideas are quite elementary.) Some students may have heard of PGP (Pretty Good Privacy), a program commonly used to encrypt/decrypt email messages. PGP uses the RSA system.