Lorenz cipher Totally Explained

Everything about Tunny totally explained

ť "Tunny" redirects here. For the fish, see Tuna.

The Lorenz SZ 40 and SZ 42 (Schlüsselzusatz, meaning "cipher attachment") were German cipher machines used during World War II for teleprinter circuits. British codebreakers, who referred to encrypted German teleprinter traffic as "Fish", termed the machine and its traffic "Tunny". While the well-known Enigma machine was generally used by field units, the Lorenz machine was used for high-level communications which could support the heavy machine, teletypewriter and attendant fixed circuits. The machine itself measured 51cm × 46cm × 46cm (20in × 18in × 18in), and served as an attachment to a standard Lorenz teleprinter. The machines implemented a stream cipher.

Operation

The teleprinters of the day output each character as five parallel bits on five lines, typically encoded in the Baudot code or something similar. The Lorenz machine output groups of five pseudorandom bits to be XORed with the plaintext. The pseudorandom bits were generated by ten pinwheels, five of which stepped regularly, termed the

chi

("chi") wheels, and five of which were stepped irregularly, termed the

psi

("psi") wheels. The stepping of the

psi

wheels was determined by two more pinwheels, termed the "motor wheels". Apart from the stepping of the five irregular pinwheels (which either all stepped together, or all stayed together), the Lorenz machine is actually five parallel pseudorandom generators; there's no other interaction between the five lines. The numbers of pins on all the wheels were relatively prime.
Colonel Parker Hitt of the US Army first proposed a very similar device in 1914. . He explained its use in his Manual for Military Ciphers published in 1916. But Col. Hitt's design was without the feature that allowed the stepping of five wheels to be irregular.

Cryptanalysis

British cryptographers at Bletchley Park had deduced the operation of the machine by January 1942 without ever having seen a Lorenz machine. This was made possible because of a mistake made by a German operator. On 30 August, 1941, a 4,000 character message was transmitted; however, the message wasn't received correctly at the other end, so (after the recipient sent an unencoded request for retransmission, which let the codebreakers know what was happening) the message was retransmitted with the same key settings (HQIBPEXEZMUG); a forbidden practice. Moreover, the second time the operator made a number of small alterations to the message, such as using abbreviations. From these two related ciphertexts, John Tiltman was able to recover both the plaintext and the keystream. From the keystream, the entire structure of the machine was reconstructed by W. T. Tutte.
   Tunny traffic was intercepted at Knockholt in Kent, before being sent to Bletchley Park.
   Several complex machines were built by the British to attack Tunny. The first was a family of machines known as "Heath Robinsons", which used several high-speed paper tapes, along with electronic logic circuitry, to help break into Tunny.
   The next was the Colossus, the world's first electronic digital computer. This was developed by the British engineer Tommy Flowers at Bletchley Park. Like ENIAC, it didn't have a stored program, and was programmed through plugboards and jumper cables. It was both faster and more reliable than the Heath Robinsons; using it, the British were able to read a large proportion of Tunny traffic.
   The Swedish cryptanalytic service, the FRA (Försvarets Radioanstalt), also broke into an early version of the Lorenz system; their break occurred in April, 1943. They tapped cables carrying traffic between Germany and Norway. The work was led by mathematician Arne Beurling.

Further Information

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