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Initial Knowledge

Finally a stage where it is obvious what crypto was used to generate the ciphertext. In principle, we could have attacked this stage immediately, but being placed as the 8th problem we thought that it would be too difficult. After some skirmishes with Stage 6 without any success, some team members turned their attention to this problem, and it was pretty much solved in parallel with Stages 6 and 7.

So what do we know, and what information do we need? It is clear from the illustration in Stage 8 that the ciphertext was encrypted with a three-wheel Enigma for which the contents of the three wheels as well as the reflector are known. This means that there are four classes of unknown parameters:

The direction the wheels rotate

It is not clear from the figure in what direction the wheels rotate.

The initial wheel positions

For each of the three wheels, there are 26 initial rotations possible.

The ring settings

The first two wheels contain two rings. Remember that the wheels are rotated after each encrypted character in an odometer-like fashion: The first wheel is always rotated, the second wheel every 26 rotations of the first wheel, and the third wheel every 26 rotations of the second wheel. The rings determine the point of the first rotation of a wheel: If the ring on the first wheel is reached after 10 rotations, then the second wheel is rotated after 10, 36, 62, ...rotations. For the first two wheels, there are 26 possible ring settings, but we can actually disregard some placements on the second wheel as the ciphertext is shorter than 676 characters long.

The plugboard connections

The plugboard contains a set of pairs of letters which are exchanged for each other: If E and Z are connected, then every in- or out-going signal which is an E is mapped to a Z, and vice versa. According to Singh, there were at most six pairs of exchanged letters.

This is what we deduced from the problem formulation. In fact, we also introduced another degree of freedom as one team member interpreted the wheel transpositions differently. This later doubled the CPU time needed to find the solution.

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