<- Virtual Exhibitions in Informatics

Alberti's Cipher Disk


Cipher Disk

Leone Battista Alberti also left his marks by employing the first mechanic device facilitating the encryption process. His cipher disk is made up of two circles out of copper plates. The larger one is called stationary, the smaller is called movable. The circumference of each plate is divided into equal cells that contain the letters of the alphabet in their usual order. To encipher a message according to the Caesar Cipher the plaintext letter 'A' of the outer plate is moved above the letter 'D' of the inner plate, so that 'A' is being replaced by 'D'. With the help of this disk the letters of the cipher text can be easily read off. The special advantage of the cipher disk actually has an effect on the usage of the Vigenére cipher. So the position of the plates only have to be changed for each letter with respect to the secret keyword. The impractical and complex Vigenére table is therefore fallen into disuse and gave way to the cipher disks which made the encryption process more simple, quicker and less error-prone. Messages encrypted with the Vigenére cipher had been absolutely secure until it was broken in the middle of the 19th century by Charles Babbage.