Cipher disk

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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."