# General Report on Tunny

11B Page 10

It follows at once from (A1) that

 K = P + Z P = Z + K (A3) and Z + P + Χ + .Ψ' = / (A4)

This shows that ciphering and deciphering both involve adding the key, and are in fact the same process, as long as P5 is not involved in the limitation. When P5 is involved, the limitation must be taken from the output when enciphering and the input when deciphering.

(j) Mechanical Aspects.

Three models of the Tunny machine are known:

 SZ 40 (1940) with no limitation SZ 42A (1942) with Χ2 or Χ2P5 lim. SZ 42B (1942) with Χ2Ψ1 or Χ2Ψ1P5 lim.

Apart from the limitation difference the models differ very little in construction.

Fig. 11 (II) shows a photograph of a German Tunny machine captured after the surrender. The machine is shown without its metal covering, stands on a metal base of dimensions 19 inches x 15½ inches, and has an overall height of 17 inches.

The 12 wheels appear in the picture with their German number painted above them. From left to right these wheels are

 German name 1 2 3 4 5 6 7 8 9 10 11 12 British name Ψ1 Ψ2 Ψ3 Ψ4 Ψ5 μ37 μ61 Χ1 Χ2 Χ3 Χ4 Χ5

The patterns of dots and crosses on each wheel is set up by means of a series of cams which may be either operative or non-operative, according to whether they are placed in a vertical position (NOCKE) or an oblique position (KEINE).

In the photograph, the cams are most easily seen on Χ2 (Wheel 9) and Χ4 (Wheel 11).

On the front of each wheel can be seen a series of numbers, one of which (seen through a window which is not shown) denotes the wheel position. German wheel numbering and British wheel numbering are arranged in opposite direction so that successive active positions are numbered by the Germans in reverse order.

The addition of chis and psis is arranged electrically. The motorising is mechanical.

(k) Switching on and Switching off.

The machine can be switched in and out of the circuit by moving the switch at the bottom to EIN or AUS. When the switch is at AUS, the teleprinter mechanism is wired direct to the W/T transmitter or receiver. When the machine is switched on, the wheels are reset at positions which are used for ciphering the first letter of the transmission. Before the second letter is ciphered all wheels move on once (irrespective of motor and limitation) and between the ciphering of the second and the third letters the psis move normally but μ37 always moves. After that, the machine moves in the normal way.

When the machine is switched off, no further letters are enciphered and the wheels stop two places ahead of the last ciphering position.