The Tunny machine was an emulator of the German Lorenz SZ42 cipher machine built at the Post Office Research Labs at Dollis Hill in 1943. It was used to decipher intercepted cipher messages enciphered on the SZ42 once the wheel patterns and start positions had been found by Colossus.
Although the rebuild of Colossus is now working in The National Museum of Computing in Bletchley Park, the Tunny rebuild is not yet completed. This virtual Tunny enables SZ42 messages to be deciphered. Because it is reciprocal, just like the original SZ42, it can also encipher texts on given patterns and wheel starts.
Virtual tunny is supplied as a zip file, tunny.zip. It needs to be extracted into an empty folder. This gives tunny.htm, output.htm and a folder, tunny_files.
Or you can just run Tunny from the web.
There are problems in loading a text file into an Html page. This has been solved in virtual Tunny by having a text area in a frame in the header. The text file can be copied from Notepad and pasted into this text area.
The file to be deciphered or enciphered needs to be loaded into a Notepad window. (Notepad is in the Windows directory). First check that the text file has been terminated by ****. If it hasn't add this in and save it. Now click onto the Notepad window and "select all" under Edit in the top line of the window.
Now click on "copy" under Edit. This copies your text into the Clipboard and it can now be "past"ed into the text Area in virtual Tunny header.
Start Tunny.htm. Scroll down the header, data, window until the row of wheel start position boxes appear. Going slightly further on reveals the top of the large text Area box. Click on the very top left hand corner of this box and then click on "Past" under Edit in the top line of the Tunny window. This will copy the saved text from the Clipboard into the text Area.
Go back up to the top of the header data window and come down to the row of wheel start position boxes. Click on the left hand box, K1, enter its start postion and then use tab to proceed to the next wheel start, K2. When all 12 starts have been entered click on "pattern" drop down list and select the pattern to be used. This will be plugged up on the main window. Then click on the "cipher" drop down list and select "textin". The wheel starts will be plugged up on the main window and the input text will be displayed on the right hand window.
Now you need to operate Tunny. Scrolling up the main window reveals the small grey panel with a row of toggle switches. These are, from left to right, SU, which resets the wheel start positions to the ones you have entered. Next comes R, to run Tunny, followed by T to test, or single step Tunny. Then comes the Limitation switches, K2 for K2 one back and P5 for the P5 limitation using bit 5 of the plain language charater two back.
All transmissions will use K2 one back so put this switch down. Now putting down the R switch starts Tunny working on the given text. After a short while the result of Tunny's actions will appear on the right hand output window.
If you wish to save this output the click on the top left hand corner of the output window and with the right hand mouse button down, select (blue over) all the text. Use "copy" under Edit in the top line of the Tunny window to copy the output to the Clipboard. Now open an empty Notepad window and "past" the selected text into it from the Clipboard. This can now be saved to disc as required, preferably adding **** at the end of the text before doing so.
Because of the reciprocal action of the SZ42, emulated in the Tunny machine, if this output text is now re-entered as textin, with the same pattern and wheel starts, the output from Tunny will be the original text input. (The reciprocity of a Vernam additive cipher!)
Tony Sale, Leader, the Colossus Rebuild Project and Trustee/Director of the National Museum of Computing