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15 October 2014
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Wartime Code Breaking

by actiondesksheffield

Contributed by 
People in story: 
Oliver Hugh Lawn and Sheila Isabelle (nee Mackenzie) Lawn
Location of story: 
Bletchley Park, Buckinghamshire
Background to story: 
Civilian Force
Article ID: 
Contributed on: 
31 August 2005

This story was submitted to the People’s War site by Roger Marsh of the ‘Action Desk — Sheffield’ Team on behalf of Oliver Hugh Lawn and Sheila Isabelle (nee Mackenzie) Lawn, and has been added to the site with both the author’s permission. The authors fully understand the site's terms and conditions.

Wartime Code Breaking
Oliver Hugh Lawn and Sheila Isabelle (nee Mackenzie) Lawn

Oliver and Sheila Lawn both worked at Bletchley Park in Buckinghamshire, the very secret wartime code breaking establishment. It was called the Government Communications Headquarters (GCHQ). All the work done at Bletchley Park remained Top Secret for some 30 years after the war, and only then were Oliver and Sheila able to talk about their work there.

Oliver Lawn was recruited for Bletchley Park by Gordon Welchman in July 1940. He had just completed a Mathematics Degree at Jesus College, Cambridge, and had expected then to be called up into the Army, as many of his contemporaries were being. Gordon Welchman, a Cambridge Mathematics Don, had been recruited for Bletchley at the beginning of the War in September 1939, along with other Oxford and Cambridge Dons, who included Alan Turing. In July 1940 Welchman was recruiting other Mathematicians, and Oliver was one of these.

He joined a team with Welchman in Hut 6 at Bletchley Park, which was concerned with breaking the Enigma Codes used by the German Army and Air Force. ['The Enigma Codes used by the German Navy were different, and were broken by a quite different group of people, in Hut 5.] He remained in Hut 6 for 5 years, until September 1945.

The methods mostly used for breaking these Enigma Codes were by guessing `cribs' - that is, guessing what some part of an encoded message actually said, in German. This was of course only possible for `routine' messages such as daily weather reports or forecasts, which often began, or ended with a standard phrase, or a record of the time of sending (e.g. `Wettervorhersage' or `nullsechsnullnull' _ `0600' hours). By aligning such phrases with the encoded text which was received over the radio in Morse code, pairs of text letters and encoded letters were thrown up, and these were grouped into `menus' looking rather like diagrams of Euclidean Geometry. The menus were then tested by large machines called `Bombes', seeking the correct setting of the Enigma machine at which the message had been encoded - the daily `key'. When this `key' had been found, all the messages on that key, and on that day, could be decoded. Keys changed daily at midnight, and each day's key had to be separately discovered. There were separate daily keys for different parts of the German Services. The total number of possible Keys was 150 million million million.

Success in breaking the keys varied, but generally most of the keys were broken for most days.

Sheila was recruited for Bletchley Park in July 1943 when she was in the middle of a Modern Languages Honours Degree Course (French and German) at Aberdeen University. She had earlier been `reserved' from call-up as a potential teacher, but decided that she must abandon this reservation and join the War effort. She worked in a part of the Naval Section at Bletchley Park, in Block B. Her main work was to decode and then translate 3-letter codes used by German Coastal Batteries and Radar Stations located around the Channel coasts. She continued at Bletchley until the end of the War in September 1945.

Early in his time at Bletchley, Oliver had joined the Scottish Reels Club, run by Hugh Foss, who was Head of the BP Japanese Section ( though, of course, at the time he did not know this). In due course, after she arrived in 1943, Sheila joined this Club. She was first attracted by Oliver's excellent dancing, and then by Oliver himself. The attraction was mutual, and they soon started going out together, as far as their different shift patterns permitted.

After the War, when they both left Bletchley Park, Oliver had to find a job, and he joined the Administrative Home Civil Service in 1946. Sheila, meanwhile, had to complete her interrupted studies. Since there was at that time no scope for travelling abroad (and what use would a Language Degree be without it?), she changed her course to a General Degree at Aberdeen followed by a Social Science Diploma at Birmingham University.

Then, in May 1948, Oliver and Sheila got married. They lived in London. Oliver worked in several Government Departments, including latterly the Department of the Environment. Sheila initially had a Personnel job with London Transport.

They had 2 sons, David and Richard, and now have 4 grandchildren. On retirement in 1978 they moved to Sheffield, and have been involved there with a number of voluntary and charitable activities.

Oliver and Sheila first revisited Bletchley Park in 1996, 50 years after they had left, and they have made many subsequent visits. They have also, since secrecy was lifted, given many talks to groups, and also interviews on TV, Radio and for Newspapers, about their wartime work. They are delighted to see the continuing public interest in Wartime code breaking - `Top Secret' for so long - and to see the large numbers of people now visiting Bletchley Park.


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