An unbreakable cipher when used properly. Used improperly by the Soviet NKVD during World War II, it opened much of the story of Soviet espionage efforts against the United States.

The one-time pad contains thousands of groups, usually five digits. Each group represents a single word or phrase. Then, to ensure security, the person using the pad next, transposes or converts the numbers of his enciphered message to another set of digits by using a specific, but randomly chosen page of the pad. The key to the pages and lines used on the pad would precede the encrypted message.

The one-time pad takes on various forms, from a thick booklet the size of a postage stamp to a scroll about the size of a cigarette butt. The important feature is that the pad must be small, so that it can be easily concealed. The booklets can be very thick, several hundred pages, and sometimes in two colors, to distinguish enciphering and deciphering sections. The "printing" is often a form of reduced photography.

Further, the "paper" of the one-time pad can be made of cellulose nitrate or some other highly flammable material that enables its rapid destruction.