Stan Isaacs wrote: << I had a student in my computer class ask me a mathematics question: she asked what was the use of prime numbers . . . . . .

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A lot of cryptography is based on finite fields, and there is a one-to-one correspondence between finite fields (up to isomorphism) and prime powers p^n, where n is any prime 2,3,5,... and n >= 1. (Note: For n = 1 the addition and multiplication are just the usual ones mod p; but for n > 1 they are more subtle. See, e.g., <http://en.wikipedia.org/wiki/Finite_field>.) --Dan