To a very great extent, mankind owes its progress to the ability to communicate, and a key aspect in this ability is the capability of communicating in writing. From the earliest days of writing, there have been occasions when individuals have desired to limit their information to a restricted group of people. They had secrets they wanted to keep. To this end, such individuals developed ideas by means of which their communications could be made unintelligible to those who had not been provided with the special information needed for decipherment. The general techniques used to accomplish such a purpose, i.e. the hiding of the meaning of messages, constitute the study known as cryptography.

Before the development of postal systems and electrical transmission of information, the usual manner of sending a communication was by private messenger. Even under these circumstances the use of the concealment tactics of cryptography was often advisable because of the possibility that the messenger might be apprehended or prove disloyal. In recent times a message transmitted by radio could be copied by anyone having appropriate equipment and listening to the right frequency at the right time. In such a case, a sender desiring privacy of communication would be required to employ some method of cryptographic concealment.

Now, just as the sender of the message attempted to conceal information from any but the desired recipient, there would be individuals very much interested in determining what the message said—most probably the very individuals from whom the sender was trying to keep the information.