Hebern rotor machine
Similar to other cryptographic rotor machines, the Hebern machine used a disk with electrical wires to encode and decode characters. Each rotor contained 26 electrical contacts on either side. While rotating, the contacts changed the connections to the wires on both sides of the disc. The wires on both sides of the disc were connected to input and output characters, so the rotating disc acted as a simple substitution cipher.
The rotor installed in the Hebern machine rotated a gear each time a key was pressed. The secret key in this case might be presented as an internal wiring of the rotor. Because the rotor had 26 connections, the key settings would be reused after 26 characters. Attacking such a cipher would be a relatively easy task, and the amount of work would be comparable to attacking old polyalphabetic substitution ciphers.
Over time, to make the key size longer, Hebern added additional rotors to the machine. All input letters were passing through all rotors, which means that every letter was changed several times, before it was delivered as the output ciphertext character.
The first rotor moved after a keystroke, while each next rotor turned once after the previous one rotated a full turn.
Security of the Hebern rotor machine
The machine was proved to be unsecure by an American cryptographer, William Frederick Friedman. He proved that due to the fact that the rotors moved only when the previous disc had rotated a full turn, the whole algorithm might be divided into a number of single substitution ciphers, each one with 26-letter long texts. This mean that the encryption was easy to break by using common frequency analysis methods.