Cipher disk

For thousands of years, people have used codes and ciphers to conceal the content of messages from those who weren't intended to read them. Diplomatic and military communications in particular needed to be kept secret from hostile eyes, however, private messages between others, including family members ane lovers, have used codes and ciphers.

Ciphers are communications where each character (letter or number) is replaced with another. Ciphers have been used for millenia. Julius Caesar created a cipher where he shifted every letter in his alphabet by 3 (e.g., "A" would equal "D", "B" would equal "E",etc.) and the recipient would reverse the process. This is now called a Caesar cipher in his honor.

Where all the same letters in a cipher are represented by the same cipher letter, such as in Julius Caesar's cipher, the cipher is termed simple substitution or monalphabetic substitution. Anolther form of cipher uses a scheme to have different letters appear for the same original letter, using some scheme thast both the sender and receiver underastand. These are called polyalphabetic substitution ciphers.

In the 15th Century, Leon Battista Alberti developed a tool to help encipher and decipher messages. 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 nect 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 ansd 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."