Colossus

How the Lorenz SZ 42 Teleprinter Cipher Machine Worked: Nazi High Command Encryption and Its 1941 Breakthrough

The Lorenz SZ 42 was a teleprinter attachment used by the German high command for encrypting top-secret communications during World War II, operating on the Vernam cipher principle with twelve wheels generating a keystream. Unlike the portable Enigma, Lorenz integrated directly between teletypes for automatic five-bit ITA2 encryption. British interceptors at Knockholt first encountered its signals in 1940, later named Tunny. A critical operator error on 30 August 1941 allowed cryptanalysts at Bletchley Park to deduce the machine's structure. This led to the development of the Colossus computer in 1944 for automated decryption. The article details the χ, ψ, and μ wheel groups, the stuttering psi mechanism, and Python implementations of ITA2 encoding and XOR operations.