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. This article is about algorithms for encryption and decryption. ... Decipherment is the analysis of documents written in ancient languages, where the language is unknown, or knowledge of the language has been lost. ... Leone Battista Alberti (February 1404 - 25th April 1472), Italian painter, poet, linguist, philosopher, cryptographer, musician, architect, and general Renaissance polymath . ...

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). A key is a piece of information that controls the operation of a cryptography algorithm. ...

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. In cryptography, a substitution cipher is a method of encryption by which units of plaintext are substituted with ciphertext according to a regular system; the units may be single letters (the most common), pairs of letters, triplets of letters, mixtures of the above, and so forth. ... A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. ... An example of statistics used in educational assessment. ...

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 "decoders."