Rotor Machines

Rotor machines implement complex polyalphabetic ciphers

• Each rotor contains a permutation.

  • Same as a set of substitutions.

• The position of a rotor implements a substitution alphabet.

• Spinning of a rotor implements a polyalphabetic cipher.

• Stacking several rotors and spinning them at different times adds complexity to the cipher.

The cipher key is:

• The set of rotors used.

• The relative order of the rotors.

• The position of the spinning ring.

• The original position of all the rotors.

Symmetrical (two-way) rotors allow decryption by "double encryption".

• Using a reflection disk (half-rotor)

Reciprocal operation with reflector

• Sending operator types "A" as plaintext and gets "Z" as ciphertext, which is transmitted.

• Receiving operator types the received "Z" and gets the plaintext "A".

• No letter could encrypt to itself!

Enigma

• WWII German rotor machine

  • Many models used.

• Initially presented in 1919.

  • Enigma I, with 3 rotors.

• Several variants where used.

  • With different number of rotors.

  • With patch cord to permute alphabets.

• Key settings distributed in codebooks.