Cryptology Rotor Machines

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General Description

Rotor machines are electromechanical devices that consist of several rotors in series connection.

The following picture gives an impression of the electric flow through such a machine.

Or look at John Savard's web page Rotor Machine Basics.

With each input letter the rotors move in individual ways, some by one position, some by several positions, some only after several steps.

The cryptographic security of a rotor machine depends on the number of rotors, the multitude of different settings, and, in a crucial way, on the complexity of the rotor movements.

Operating a Rotor Machine: The operator hits a key on the keyboard that corresponds to the next plaintext letter. This action closes an electric circuit powering a light-bulb that corresponds to the ciphertext letter. Or it powers a type bar that prints the ciphertext letter. The rotors move according to their control logic, in general before the circuit is closed. See the FAQ .

Rotor machines are the state of the art in encryption during the period from 1920 until 1970.

The mystic and irregularly rotating wheelwork that makes the desk tremble with each key hit is very attractive and impresses the general or diplomat who wants to buy security.

The mathematical description is in the PDF file.

Here are Perl programs for encryption and decryption by rotor machines.

The Key Space

A key of our idealized rotor machine consists of

• a choice of rotors,
• an initial state.

In a typical example with 5 rotors (Hebern machine) we have an effective key length of ≈ 29.4. That was good enough in 1920. Today, against an enemy with a computer, this is much too little.

In fact the Hebern machine was not good enough even in 1920 because it allowed attacks far more efficient than exhaustion.