Cipher Disk

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

Books

Deavours, Cipher, et al. Cryptology: Machines, History & Methods. Norwood, MA: Artech House, 1989.

Singh, Simon. The Code Book. New York: Doubleday, 1999.

The Union Cipher Disk from the American Civil War was 3.75 inches (95 mm) in diameter and made of light yellow heavy card stock. It consisted of two concentric disks of unequal size revolving on a central pivot. The disks were divided along their outer edges into 30 equal compartments. The smaller inner disk contained letters, terminations and word pauses, while the outer disk contained groups of signal numbers. For easier recognition, the number eight represented two. The initials A.J.M. represent the Chief Signal Officer General Albert J. Myer. Each disk had a control number used for accountability.

A cipher disk is an enciphering and deciphering tool developed in the 15th century by Leon Battista Alberti. Rather than constructing a table with the regular and cipher alphabets on it, he created two circular scales, one smaller and on a disk that he mounted concentric to the larger circle. This enabled him to move the two alphabet scales relative to each other.

The cipher disk provided an easy way to change ciphers: merely moving the scales provided 26 ways to represent a letter, depending entirely on the position of the inner disk. The sender and the person receiving the messages would agree on a cipher key setting (e.g., the "G" in the regular alphabet would be positioned next to the "Q" in the cipher alphabet).

In addition to simple substitution ciphers, the cipher disk opened the way for easy polyalphabetic ciphers. An easy way to do this is for the sender and the recipient to agree that a certain number of characters into the message, the scales would be shifted one character to the right, repeating the procedure every tenth letter. This would make it more difficult to crack, using statistical methods.

The cipher disk and its variants have been used since that time. Recently, they have been labeled "decoders" and have been used for novelties. Many of the cipher disks that were radio premiums were called "secret decoder rings."

Cipher machines
Rotor machines: CCM | Enigma | Fialka | Hebern | HX-63 | KL-7 | Lacida | M-325 | Mercury | NEMA | OMI | Portex | SIGABA | SIGCUM | Singlet | Typex
Mechanical: Bazeries cylinder | C-36 | C-52 | CD-57 | Cipher disk | HC-9 | Kryha | Jefferson disk | M-94 | M-209 | Reihenschieber | Scytale
Teleprinter: 5-UCO | BID 770 | KW-26 | KW-37 | Lorenz SZ 40/42 | Siemens and Halske T52
Secure voice: BID 150 | KY-3 | KY-57 | KY-58 | KY-68 | OMNI | SIGSALY | STE | STU-II | STU-III | VINSON | SCIP | Sectéra Secure Module
Miscellaneous: Cryptex | JADE | KG-84 | KL-43 | Noreen | PURPLE | Pinwheel | Rockex
Cryptography
History of cryptography | Cryptanalysis | Cryptography portal | Topics in cryptography
Symmetric-key algorithm | Block cipher | Stream cipher | Public-key cryptography | Cryptographic hash function | Message authentication code | Random numbers

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