A very introductory paper on
cryptography [pdf]
(covers substitution ciphers, the Vigenere cipher, one-time pads, the Hill
cipher), developed as a professional development workshop given to middle
school teachers as part of the
MSTP project I was involved in.
Further Travels with my Ant
(with D. Gale, J. Propp, and S. Troubetzkoy)
Mathematical Intelligencer, vol 17, no. 3 (1995), pp 48-56.
See also more about the ant.