The Rebuild of Heath Robinson

Finding the Chi wheel pattern start positions.




At Bletchley Park, Bill Tutte had proposed the "Double Delta" method for finding the Lorenz machine wheel start positions to which the German operator had set his Lorenz machine. (different wheel start positions for each message).

The Tutte algorithm required adding modulo two (XORing) the current and previous character bits on both the cipher text (Z) and the Chi wheel patterns (X) being tested to get the Deltas and then XORing these together for two of the five channels (tracks) on the paper tape. Then counting, down the whole length of the tape the number of times that this result equated to zero, trying various pattern start positions to find the maximum score.
i.e. if channels 1 and 2 are being used then count when;

DeltaZ1 + DeltaX1 + DeltaZ2 +DeltaX2 = 0

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