Lorenz Cipher - The Vernam Cipher

The Vernam Cipher

Gilbert Vernam was an AT&T Bell Labs research engineer who, in 1917, invented a cipher system that used the Boolean "exclusive or" (XOR) function, symbolised by ⊕. This is represented by the following "truth table", where 1 represents "true" and 0 represents "false".

Other names for this function are: Not equal (NEQ), modulo 2 addition (without 'carry') and modulo 2 subtraction (without 'borrow').

Vernam's cipher is a Symmetric-key algorithm, i.e. the same key is used both to encipher plaintext to produce the ciphertext and to decipher ciphertext to yield the original plaintext:

Plaintext ⊕ Key = Ciphertext

and:

Ciphertext ⊕ Key = Plaintext

This produces the essential reciprocity that allows the same machine with the same settings to be used for both enciphering and deciphering.

Vernam's idea was to use conventional telegraphy practice with a paper tape of the plaintext combined with a paper tape of the key. Each key tape would have been unique (a one-time tape), but generating and distributing such tapes presented considerable practical difficulties. In the 1920s four men in different countries invented rotor cipher machines to produce a key stream to act instead of a tape. The 1940 Lorenz SZ40/42 was one of these.