Editing Lorenz cipher (section)

==Cryptanalysis==
{{Main|Cryptanalysis of the Lorenz cipher}}
[[File:British Tunny Rebuild.jpg|right|thumbnail|upright=1.35|A rebuilt British Tunny at [[The National Museum of Computing]], [[Bletchley Park]]. It emulated the functions of the Lorenz SZ40/42, producing printed cleartext from ciphertext input.]]
British cryptographers at [[Bletchley Park]] had deduced the operation of the machine by January 1942 without ever having seen a Lorenz machine, a feat made possible thanks to mistakes made by German operators.

===Interception===
Tunny traffic was known by [[Y Service|Y Station]] operators used to listening to [[Morse code]] transmission as "new music". Its interception was originally concentrated at the Foreign Office Y Station operated by the [[Metropolitan Police]] at [[Denmark Hill]] in [[Camberwell]], London. But due to lack of resources at this time (around 1941), it was given a low priority. A new Y Station, [[Knockholt]] in [[Kent]], was later constructed specifically to intercept Tunny traffic so that the messages could be efficiently recorded and sent to Bletchley Park.<ref>{{Harvnb|Good|Michie|Timms|1945|p=281}} in ''Knockholt''</ref> The head of Y station, [[Harold Kenworthy]], moved to head up Knockholt. He was later promoted to head the Foreign Office Research and Development Establishment (F.O.R.D.E).

===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>

===Decryption machines===
Several complex machines were built by the British to aid the attack on Tunny. The first was the [[British Tunny]].<ref>{{Harvnb|Halton|1993}}</ref><ref>[https://www.theregister.co.uk/2011/05/26/bletchley_park_tunny_rebuild_project/ Bletchley Park completes epic Tunny machine] The Register,  [https://www.theregister.co.uk/2011/05/26/ 26 May 2011], Accessed May 2011</ref> This machine was designed by Bletchley Park, based on the [[reverse engineering]] work done by Tiltman's team in the Testery, to emulate the Lorenz Cipher Machine. When the pin wheel settings were found by the Testery, the Tunny machine was set up and run so that the messages could be printed.

A family of machines known as "[[Heath Robinson (codebreaking machine)|Robinsons]]" were built for the Newmanry. These used two [[paper tape]]s, along with logic circuitry, to find the settings of the ''χ'' pin wheels of the Lorenz machine.{{sfn|Copeland|2006|p=66|loc="Machine against Machine"}} The Robinsons had major problems keeping the two paper tapes synchronized and were relatively slow, reading only 2,000 characters per second.
[[File:ColossusRebuild 11.jpg|thumb|A team led by [[Anthony Sale|Tony Sale]] (right) reconstructed a Colossus (Mark II) at Bletchley Park. Here, in 2006, Sale supervises the breaking of an enciphered message with the completed machine.]]
The most important machine was the [[Colossus computer|Colossus]] of which ten were in use by the war's end, the first becoming operational in December 1943. Although not fully programmable, they were far more efficient than their predecessors, representing advances in electronic digital [[computer]]s. The [[Colossus computer|Colossus]] computers were developed and built by [[Tommy Flowers]], of the [[Dollis Hill]] [[Post Office Research Station]], using algorithms developed by [[W.T. Tutte|Bill Tutte]] and his team of mathematicians.<ref>{{cite web|url=https://uwaterloo.ca/combinatorics-and-optimization/about/professor-william-t-tutte/biography-professor-tutte#bletchley|title=Biography of Professor Tutte - Combinatorics and Optimization|date=13 March 2015|access-date=2017-05-13 |archive-date=2019-08-19 |archive-url=https://web.archive.org/web/20190819115149/https://uwaterloo.ca/combinatorics-and-optimization/about/professor-william-t-tutte/biography-professor-tutte#bletchley|url-status=dead}}</ref> Colossus proved to be efficient and quick against the twelve-rotor Lorenz SZ42 on-line teleprinter cipher machine.

Some influential figures had doubts about his proposed design for the decryption machine, and Flowers  proceeded with the project while partly funding it himself.<ref>{{cite book|last=Boden|first=Margaret Ann|title=Mind as Machine: A History of Cognitive Science|url=https://books.google.com/books?id=yRyETy43AdQC&q=TOMMY+FLOWERS+built+at+his+own+expense+colossus&pg=PA159|publisher=Clarendon Press |location=Oxford|date=2006|page=159|isbn=9780199543168}}</ref><ref>{{cite book |last=Atkinson|first=Paul |date=2010|title=Computer|location=UK|publisher=Reaktion Books|isbn=9781861897374 |url=https://books.google.com/books?id=D5H_OsxEywwC |page=29}}</ref> Like the later [[ENIAC]] of 1946, Colossus did not have a [[stored program]], and was programmed through plugboards and jumper cables. It was faster, more reliable and more capable than the Robinsons, so speeding up the process of finding the Lorenz ''χ'' pin wheel settings. Since Colossus generated the putative keys electronically, it only had to read one tape. It did so with an optical reader which, at 5,000 characters per second, was driven much faster than the Robinsons' and meant that the tape travelled at almost 30 miles per hour (48&nbsp;km/h).<ref>{{Harvnb|Flowers|2006|p=100}}</ref> This, and the clocking of the electronics from the optically read paper tape sprocket holes, completely eliminated the Robinsons' synchronisation problems. Bletchley Park management, which had been sceptical of Flowers's ability to make a workable device, immediately began pressuring him to construct another. After the end of the war, Colossus machines were dismantled on the orders of Winston Churchill,<ref>Verdict of Peace: Britain Between Her Yesterday and the  future, Correlli Barnett, 2002</ref> but GCHQ retained two of them.{{sfn|Copeland|2006|p=173}}

===Testery executives and Tunny codebreakers===
* [[Ralph Tester]]: linguist and head of Testery
* [[Jerry Roberts]]: shift-leader, linguist and senior codebreaker
* Peter Ericsson: shift-leader, linguist and senior codebreaker
* Victor Masters: shift-leader
* Denis Oswald: linguist and senior codebreaker
* [[Peter Hilton]]: codebreaker and mathematician
* [[Peter Benenson]]: codebreaker
* Peter Edgerley: codebreaker
* John Christie: codebreaker
* John Thompson: codebreaker
* [[Roy Jenkins]]: codebreaker
* [[Shaun Wylie]]: codebreaker
* Tom Colvill: general manager

By the end of the war, the Testery had grown to nine cryptographers and 24 [[Auxiliary Territorial Service|ATS]] girls (as the women serving that role were then called), with a total staff of 118, organised in three shifts working round the clock.