The Lorenz cipher was a high-level German cipher machine used during World War II, developed by the Lorenz company around 1940. It was designed to encrypt teleprinter communications for the German Army, producing ciphertext in the Baudot code format. Unlike the simpler Enigma machine, the Lorenz cipher used twelve wheels with a complex system of rotations to generate a pseudo-random key stream, creating a stream cipher that combined plaintext bits with key bits via modulo-2 addition (XOR). Each wheel had a different number of positions, and five of the wheels—the “chi” wheels—advanced with every character, while the other wheels—the “psi” wheels—advanced irregularly according to the “motor” wheels. This irregular stepping and combination of multiple wheels produced a ciphertext stream that was extremely difficult to analyze manually.
To encrypt a message, the teleprinter plaintext in Baudot code was combined bitwise with the generated key stream. For example, if the plaintext character H corresponds to the Baudot bits 10100 and the key stream bits are 01101, each bit is XORed: 1⊕0=1, 0⊕1=1, 1⊕1=0, 0⊕0=0, 0⊕1=1, producing the ciphertext bits 11001. The receiving station, using an identical Lorenz machine with the same wheel settings, could perform the same XOR operation to recover the original plaintext. The cipher was symmetric and fully reversible, but relied on careful distribution of wheel settings, which acted as the key.
The Lorenz cipher became famous due to British cryptanalysis at Bletchley Park, where mathematicians including Bill Tutte deduced the logical structure of the machine without ever seeing it physically, ultimately enabling automated decryption using early computers such as Colossus. Its design highlights the early use of stream ciphers with irregular key generation and demonstrates how mechanical and later electronic systems advanced cryptography. The cipher’s security depended on key secrecy and wheel settings; any repetition or error could compromise communications. Today, the Lorenz cipher is studied as a historical example of complex rotor-based stream ciphers and a milestone in cryptanalytic achievement, illustrating both the ingenuity of wartime encryption and the power of mathematical deduction in breaking it.