Editing Lorenz cipher (section)

===Code breaking===
On 30 August 1941, a message of some 4,000 characters was transmitted from [[Athens]] to [[Vienna]]. However, the message was not received correctly at the other end. The receiving operator then sent an uncoded request back to the sender asking for the message to be retransmitted. This let the codebreakers know what was happening.

The sender then retransmitted the message but, critically, did not change the key settings from the original "HQIBPEXEZMUG".  This was a forbidden practice; using a different key for every different message is critical to any stream cipher's security.  This would not have mattered had the two messages been identical, however the second time the operator made a number of small alterations to the message, such as using abbreviations, making the second message somewhat shorter.

From these two related ciphertexts, known to cryptanalysts as a [[Cryptanalysis#Depth|depth]], the veteran cryptanalyst [[John Tiltman|Brigadier John Tiltman]] in the Research Section teased out the two plaintexts and hence the [[keystream]]. But even almost 4,000 characters of key was not enough for the team to figure out how the stream was being generated; it was just too complex and seemingly random.

After three months, the Research Section handed the task to mathematician [[W. T. Tutte|Bill Tutte]]. He applied a technique that he had been taught in his cryptographic training, of writing out the key by hand and looking for repetitions. Tutte did this with the original teleprinter 5-bit [[Baudot code#ITA2|International Telegraph Alphabet No. 2 (ITA2)]] (which was a development of the [[Baudot code#Baudot code (ITA1)|Baudot code (ITA1)]]), which led him to his initial breakthrough of recognising a 41-bit repetition.<ref name=Sale/><ref>{{Harvnb|Tutte|1998|pp=356, 357}}</ref> Over the following two months up to January 1942, Tutte and colleagues worked out the complete logical structure of the cipher machine. This remarkable piece of [[reverse engineering]] was later described as "one of the greatest intellectual feats of World War II".<ref name=Sale/>

After this cracking of Tunny, a special team of code breakers was set up under [[Ralph Tester]], most initially transferred from [[Alan Turing]]'s [[Hut 8]]. The team became known as the [[Testery]]. It performed the bulk of the subsequent work in breaking Tunny messages, but was aided by machines in the complementary section under [[Max Newman]] known as the [[Newmanry]].<ref>{{Harvnb|Roberts|2009}}</ref>