TNMOC

There is so much for me to see at The National Museum of Computing in Bletchley that I can go there again and again.  I treated myself to a couple of hours there on Thursday afternoon.
The story of the decoding of "Fish"/"Tunny" traffic created on German Lorentz cypher machines amazed me as it has done before.    Whereas the more famous Enigma traffic was for highly encrypted for short mobile / battlefield communications Tunny was used by top commanders, including Hitler, in Berlin to send strategic information to army commanders in the different theatres of war.  The Enigma machine was known to codebreakers as simpler variants had been available before the war.  The Lorentz machine automated the coding and decoding process using teleprinters which produced 5 bit baudot codes.  The codes were encrypted using a machine with 12 rotors (enigma used 3) and sent out via radio to other european destinations.  Communications speed was too high for an intercept operator to reliably note down the results so they were recorded on ululating tape and transcribed afterwards.
The big break in understanding traffic came when a message was sent with the same encryption settings twice, but with some small variations.  This allowed Ralph Tiltman at BP after considerable effort to decode it.  Some time later a young Cambridge mathematician Bill Tutte analysed the message in great depth over a 12 month period and managed to deduce from patterns in the coded message how the Lorentz machine was constructed, that it had 12 wheels and how many teeth / positions were on each, a phenomenal achievement based on the contents of a single message.
From this point BP code breakers were able to manually break Tunny traffic manually although it could take weeks to decode messages.
A machine called the Heath Robinson was designed to help the codes breakers analyse messages.  It had a paper tape containing the message which was repeatedly scanned to do a statistical analysis.  However it wasn't fast enough and prone to breaking.
Max Newman persuaded BP management that a better solution was required and Tommy Flowers at the Post Office research in Dollis Hill was approached.  BP didn't feel that his solution using over a thousand valves and optical readers would work but he had such faith that he was able to build a machine, called Colossus and deliver it early in 1944 just in time to decode vital German communications in advance of D day.  Using colossus BP were able to decode Tunny messages until the end of the war.