Alexander I. Kheyfits 
Discrete Mathematics With Cryptographic Applications [PDF ebook] 
A Self-Teaching Introduction

This book covers discrete mathematics both as it has been established after its emergence since the middle of the last century and as its elementary applications to cryptography. It can be used by any individual studying discrete mathematics, finite mathematics, and similar subjects. Any necessary prerequisites are explained and illustrated in the book. As a background of cryptography, the textbook gives an introduction into number theory, coding theory, information theory, that obviously have discrete nature. 
FEATURES:

• Designed in a “self-teaching” format, the book includes about 600 problems (with and without solutions) and numerous examples of cryptography
• Covers cryptography topics such as CRT, affine ciphers, hashing functions, substitution ciphers, unbreakable ciphers, Discrete Logarithm Problem (DLP), and more.