The Pyry Forest Meeting

The UK was represented by Alastair Denniston, the Head of the Government Code and Cypher School (GC&CS — GCHQ’s name until 1946) and Dilly Knox, the cryptanalyst who had led work on breaking traffic encrypted on Enigma.

At an earlier meeting in Paris in January 1939, the British and Polish sides had not been prepared to share information fully, but in July the situation was very different: the British and French governments had given formal undertakings to support Poland if it were invaded and in return, on the cryptanalytic side the Poles were prepared to explain the progress they had made against Enigma. 

Even though Polish success against Enigma had been much reduced in the preceding months by measures taken by the Germans to increase Enigma security, the British and French representatives learned that the Poles were much further forward than they were, both quantitatively, in terms of the amount of material the Poles had already read, and qualitatively, in terms of the Polish mathematician’s design and construction of the bomba kryptologiczna — the cryptologic Bomb — which was the first electromechanical device used to support cryptanalysis.

The knowledge gained in Poland transformed GC&CS’s approach to Enigma cryptanalysis, and in January 1940, Alan Turing, who was by then working with Dilly Knox at Bletchley Park, travelled to Paris to meet the Polish cryptanalysts who had been evacuated to France after the fall of Poland.  Turing’s conversations with Marian Rejewski, the Polish mathematician and cryptanalyst were fundamental to developing the thinking which led to the design and installation of the first British "Bombe" which was working at Bletchley Park in time for the German Blitzkrieg in the West.