A cipher disk is a handheld coding device for generating a limited number of substitution ciphers, that is, ciphers in which each letter of the regular alphabet is enciphered as a single character from a cipher alphabet. A typical cipher disk consists of an inner ring with the characters of the regular alphabet printed around its outer edge, and an outer ring that fits snugly around the inner ring and can be rotated. Around the outer ring is printed a cipher alphabet that has the same number of characters as the regular alphabet. This cipher alphabet may consist of a scrambled regular alphabet or of other symbols. To encipher a message, the user of the cipher disk first chooses some particular alignment of the outer ring with the inner ring. For example, if the cipher alphabet consists of the numbers 1 through 26 (in order), the user may align the number 10 on the outer ring with the letter A on the inner ring. The letter A will then encipher as 10, the letter C as 12, the letter Z as 9, and so forth. By shifting the outer ring one or more letter-positions, the user obtains a different substitution cipher. Some cipher disks have an internal mechanism that advances the outer ring by one step after the encipherment of each letter; this prevents a given plaintext letter from always enciphering as the same ciphertext letter.
The earliest known description of the cipher disk was penned by Italian artist Leon Battista Alberti (1404–1472) in 1470. Cipher disks produce ciphers that are too simple for practical use in the modern world, but were used in the field by Confederate forces during the United States Civil War (1861–1865). Union cryptographers, however, often had no problem reading the Confederacy's encrypted messages. Cipher disks were also widely distributed in the U.S. in the 1940s as marketing giveaways for radio adventure programs such as Captain Midnight . These programs were popular even with adults, including active air crews during World War II, and stories—possibly apocryphal—have circulated claiming that combat forces occasionally put the toy cipher disks to real-life use. More complex ciphering systems based fundamentally on the cipher disk concept, such as Enigma, have seen extensive real-world service.
█ FURTHER READING:
Deavours, Cipher, et al. Cryptology: Machines, History & Methods. Norwood, MA: Artech House, 1989.
Singh, Simon. The Code Book. New York: Doubleday, 1999.