As far as I know, the part of computer that involves information security needs some knowledge of "number theory" and "abstract algebra". You can look through these two books. Classical number theory is enough for the less in-depth part, and modern number theory is needed for learning algorithms and encryption in the future: analytic number theory, algebraic number theory and other disciplines. Abstract algebra is the successor subject of advanced algebra. Abstraction is her characteristic, but it is the basis for you to continue your study, so work hard ~