My purpose in writing this paper is to present the basic principles and workings of cryptography systems and to point out reasons why I believe cryptography is so necessary to modern data communications. I first became interested in the subject while taking an operating systems course. The topic was covered briefly near the end of the term, when we were running short on time. The information I gained was enough to whet my appetite and prompt me to see what I could uncover on this interesting subject. This paper is the end result of that research.