17 May 2013

Lorenz & Colossus

At Bletchley Park is displayed a working replica of the Bombe machines used in the decryption of WWII Germany's signals, encoded through Enigma machines. On the same site is the National Museum of Computing, home to a working replica of Colossus, the world's first electronic computer, used to break Lorenz-enciphered messages. Lorenz was used by Germany's Army High Command to communicate with its Group Commanders, as it provided greater security than Enigma.

Lorenz, ultimately emulated by a machine largely made of telephone exchange components and known as Tunny by the Bletchley code-breakers, used twelve rotors, compared with the three of a standard Enigma machine. It was brought into service in 1941. A Lorenz attachment fitted to a conventional teleprinter added, to the standard five impulses used for each letter - the equivalent of today's 1s and 0s - the five impulses for each letter of the key, automatically generated by the machine. There was a pair of wheels for each impulse, plus two motor wheels that added a degree of randomness.

The coded messages, known as fish at Bletchley, each fish name being one of the communication routes, were transmitted by radio and fed straight into a receiving Lorenz machine. By addition of the same key, this produced teleprinter output in plain text. The key changed with each message, as opposed to Enigma's diurnal key changes. There were 16,000,000,000,000,000,000 possible initial wheel settings. The British could intercept the coded messages, but not break them.

It was not until August 1941, when a long message was sent twice, using the same initial wheel settings, by a Lorenz operator, who in the second message also utilized various abbreviations, that a way into the fish was found. Cryptanalyst John Tiltman took ten days to decipher both messages, and Bill Tutte deduced mathematically how Lorenz worked. From mid-1942 Lorenz-encoded messages could be decoded, but an automated solution was essential to speed the process of determining the ever-changing key. 

A machine - known as Heath Robinson - was designed by Dr Max Newman to determine the Lorenz starting rotor positions, but this was slow. Post Office engineer Tommy Flowers was brought in. Within just nine months he had designed and built the Colossus prototype, which made use of punched tape and optical reader input, and 1,500 thermionic valves. This was operational by December 1943. The coded message was run again and again, each time against a different starting position for a pair of wheels, searching for an output pattern that, based on carefully calculated probabilities, indicated when the correct positions had been determined.

By the end of the war there were ten Colossus Mark IIs at Bletchley, each using 2,500 valves. Colossus was kept secret until 1975, the mechanical assembly drawings having been deliberately destroyed in 1960. Between 1993 and 2008 Tony Sale, ex-MI5 principal scientific officer, led a project to rebuild a Mark II Colossus, with just eight photographs of 1945 and a few fragmentary circuit diagrams, thankfully kept illegally by some engineers, as source material. The rebuilt machine stands where Colossus No. 9 stood during the war.

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