What is a Mechanical Cipher?

A mechanical cipher is a cipher requiring some kind of mechanical device either to make it function or in order to make it manageable in the field. It does not include code-wheels, which are simply a system to make monoalphabetic or Vigenere type ciphers easier to handle.

There are a number of examples but by far the most interesting and long-lasting of all the types was designed by the brilliant American Thomas Jefferson.

The Jefferson Wheel Cipher

Take a cylinder of wood, slice it into twenty-five discs and mark the alphabet around the perimeter of each disc in a random order, different for each disc. To encrypt a message rotate the discs relative to each other until the message is displayed in a single row. The ciphertext is read off any of the other rows on the device.

This deceptively simple idea produces a cipher which is really quite secure and which operates on the autokey principle, in which the plaintext effectively encrypts itself. It is easy to use and fast in operation. The device remained in use with field units of the US military for over 100 years and is still a good field cipher today.

The Statistics of the Jefferson Wheel Cipher

• Alphabets: 25
• Blocklength: 25
• Keyspace: 2.4x10³⁵