Virtual Wartime Bletchley Park
by Tony Sale

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More runs on the Bombe

The next set of menus, fig 10 in the US Report on the British Bombe, illustrate a very real problem in constructing and running menus. Middle wheel turnover on the original Enigma machine.

Depending on which wheels are in the original Enigma and their start positions for enciphering the message, the right hand wheel will at some point through the message pass its turnover letter and the middle wheel will advance one letter, it is even possible that the left hand wheel will turn over as well. Any turnover within the span covered by a menu will invalidate the menu and it is very difficult to find strong menus within a short length, say 13 letters, of the cipher text/crib overlap.

The set of menus in fig 10, which I have labelled 101, 102, 103 and 104 show a method used to defeat turnover. It involves assuming and allowing for 4 different possible turnover points within 26 letters of the cipher/crib distance by incrementing the middle offset letter.

The correct answer is given as w/o 186 stop GZT so just to keep things within a reasonable search time I have restricted the wheel order search to 185-187. (you can try the whole set if you like).

So now run bombe.exe and load text data file brfig101.txt. Click the 'OK' button and click 'run' on the Setup window menu bar. The simulation will start running at full speed and you will see first the wheel starts going from AAA to ZZZ then the wheel orders going from 185 to 187. It will reach a loop stop at 187 ZZZ and no valid stops. (If you want to convince yourself that it is working, click Finish, set machine-gun: to 'N' and then OK, 'run' again. A whole load of invalid stops will come out).

Now try the next menu, brfig102.txt. By clicking 'Finish' you can click 'Setup' again and load this data without coming out of the Bombe program. Just click 'OK' and 'run' again. This also gives no stops.

Next comes brfig103.txt. This gives 13 valid stops amongst which is 186 GZT and the Steckers of this stop agree with the Checking Machine derivation shown in fig 10.

The last menu of fig 10, brfig104.txt yields 11 stops none of which is the correct one.

Just thank your lucky stars you weren't doing this for real. You would have had to check every possible stop against Enigma messages to find the correct one.

Now that we are getting nearer to real Bombe stops the Ringstellung problem needs to be explained. See the next page.

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This page was originally created by the late Tony Sale
the original curator of the Bletchley Park Museum

Virtual Wartime Bletchley Park by Tony Sale

More runs on the Bombe

Virtual Wartime Bletchley Park
by Tony Sale