This pamphlet is designed for use as enrichment material for able junior and senior high school students who are interested in mathematics. No more than a clear understanding of basic arithmetic is expected. Students are introduced to ideas from number theory and modern algebra by learning mathematical ways of coding and decoding secret messages. Modular arithmetic matrix multiplication, and finite number systems are discussed. Applications of some of the ideas presented in this booklet are presented in order to stimulate further study in these areas. (FL)