The art or process of writing in or deciphering secret code. A Sherlock Holmes story tells how Professor Moriarty, despite his superior intelligence and mathematical skills, is unable to break a simple code, one that uses a particular edition of a rare book as the key. The story does not reveal the secret of the code until the end. There is no repetition, no underlying formula that Moriarty could derive from the seemingly random numbers in the ciphertext. Unbreakable code, according to the artistic license exercised by Conan Doyle.
Cryptography is nearly as old as the written word.
Surreptitious communication has been a requirement of governments and their armies since the beginning of organized society. Some of the first applications for digital computers were computations to assist codebreakers. The subject intrigues us because it involves complex puzzles and their solutions and because the solutions reveal the locked-away secrets of others. We like to use our wits to get into locked places, to outsmart those who contrive the locks, to contrive locks of our own that others cannot pick. Applied Cryptography , by Bruce Schneier, is a comprehensive treatment of the subject, describing its concepts and demonstrating its processes.