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Problem with double stepping of the middle wheel
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Identified: 6 June 2006
Status: fixed in firmware 1.50 (22 June 2008)
Due to the way in which a real Enigma machine works, the middle wheel
may step twice on two successive key presses. This is sometimes referred
to as the double stepping anomaly. This feature is also implemented in
the Enigma-E, but due to a bug in the software it will not always work
as expected.
Whenever the middle wheel reaches the turnover point during a message,
all works fine. However, when the message starts with the middle wheel
already set at the turnover point, it will fail.
This is the case, for example, with the message
'The Detention Baron Gottfried von Bismarck-Schoenhausen'
on Frode Weierud's website.
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The problem can easily be demonstrated as follows:
- Set the wheel order to 123.
- Use any Ringstellung you want.
- Use any setting of the Steckerbrett.
- Use the Grundstellung AEA.
This puts wheel '2' at the turnover position, where the pawl (on a real Enigma)
would be engaged in the notch, such that the left wheel would turn over to 'B'
and the middle wheel would also step to 'F' on the next key press.
Now press a key.
The indicator should now read 'BFB'
whereas the Enigma-E will show 'AEB'.
The wheel turnover positions of the various wheels are as follows:
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| Wheel | Turnover position |
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| I | Q |
| II | E |
| III | V |
| IV | J |
| V | Z |
| VI | M and Z |
| VII | M and Z |
| VIII | M and Z |
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Workaround for users of firmware 1.00
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Enigma-E user Carl Claunch has found a rather simple workaround
which will overcome the described problem and allows you to read the
message.
Any time the indicators are set so that the middle wheel is at
the turnover letter for that wheel at the start of a message,
simply manually advance the
middle and left wheel one step. This produces correct encoding and decoding
operation, the same as if the step were done by the Enigma-E itself.
Please refer to the table above to see whether the middle wheel is
set to the turnover position.
Many thanks to Carl for spotting this problem and finding a suitable
workaround.
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This problem is fixed in version 1.50 of the firmware
that is supplied with the new UhrBox-E kit.
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© Crypto Museum. Last changed: Monday, 25 July 2016 - 08:47 CET.
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