A revised and updated translation of a work originally published in Portuguese in 1997. It introduces the algorithmic aspects of number theory and its applications to the study of codes, and of code-breaking. The work ends with study of the Rivest, Shamir, and Aldeman (RSA) public key cryptosystem, focusing more on the mathematics of the system than on the details of implementing the system. Very little previous knowledge of mathematics is required, but computer literacy is assumed. ^^^^ Annotation c. by Book News, Inc., Portland, Or.