Join us as we look at breaking Hitler's top-secret cipher.


Andy Clark, Director and Founding Trustee, National Museum of Computing


18:00 - Arrive, network, refreshments & snacks in the concourse
18:15 - AGM of the London South Branch

18:30 - Andy Clark's highly entertaining and informative presentation
19:40 - Questions and answers, further networking
20:00 - Close


The story of the British and Allied codebreaking efforts against the German Enigma machine in World War 2 has been told through films and TV documentaries and is widely known by the general public.

While Enigma was used for tactical communications, less well known is the story of codebreaking efforts against the German strategic communications that were enciphered by machine and transmitted by teleprinter.

These teleprinter signals were first heard in early 1940 by a group of policemen on the South Coast who were listening out for possible German spy transmissions from inside the UK. Although unidentified at the time, the transmissions were from the "Oberkommando der Wehrmacht".

Brigadier John Tiltman, one of the top codebreakers in Bletchley Park, took a particular interest in these enciphered teleprinter messages. They were given the code name "Fish". The messages which (as was later found out) were enciphered using a Lorenz machine, were known by the British as "Tunny". The effort to successfully break Tunny traffic was many orders of magnitude greater than the Enigma and resulted in the design and construction of the Colossus computers.

In this talk we will discuss the process by which Tunny was broken, acknowledging the role of all those engaged in the process from interception to the production of intelligence.

We will also describe how a team led by Tony Sale rebuilt a Colossus Mk2 over 13 years and how we celebrated its completion in 2007 by holding "The Cipher Challenge", open to all to see if they could beat Colossus in a real-world codebreaking race.

If time permits, we will discuss the untold story of the successes made by German codebreakers against the British.

About the speaker

Andy Clark is a cyber security consultant as well as a Director and Founding Trustee of the National Museum of Computing located on the Bletchley Park campus (

The museum is home to the world's largest collection of working historic computers, it's exhibits show the development of computing from the Turing-Welchman Bombe and Colossus of the 1940s through the large systems and mainframes of the 1950s, 60s and 70s, to the rise of personal computing and the rise of mobile computing and the internet.

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This event is brought to you by: BCS London South branch

Breaking Hitler's top-secret cipher (AGM)
Date and time
Tuesday 18 October, 6:15pm - 8:00pm
BCS, The Chartered Institute for IT
25 Copthall Ave
This event is sold out