Bill Tutte's algorithm runs again

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For the first time since the end of World War II, the 1+2 Double Delta algorithm was successfully performed on the balanced modulator logic circuits of the Robinson machine -- but on the ongoing reconstruction this time.

The Double Delta algorithm was a statistical attack devised by Bill Tutte in 1942 to find the Lorenz wheel start positions. Bill Tutte had previously deduced how the Lorenz machine worked without ever having seen it. Tutte's feat is often given the accolade of being the greatest intellectual achievement of World War II.

Max Newman, head of the Newmanry, which aimed to industrialise code-breaking, approached TRE at Malvern to design an electronic machine to implement Tutte's Double-Delta method of finding wheel start positions. The resulting machine was built at Dollis Hill and subsequently used at Bletchley Park. It was known as Heath Robinson after the cartoonist designer of fantastic machines and was the predecessor of Colossus.

For the Robinson to perform the Double-Delta attack, one tape had punched on to it the pure Lorenz wheel patterns that the manual code breakers had laboriously worked out. The other tape was the intercepted enciphered message tape. The Double-Delta cross-correlation measurement was then made for the whole length of the message tape. The relative positions then moved one character and the correlation measurement repeated. The code-breaker was looking for the relative position which gave the highest cross-correlation score — which hopefully would correspond to the correct Lorenz wheel start position.

For details of the logic test, see here.

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