Making the Enigma ciphers for the film "Enigma"
Codes and Ciphers
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What Tony had to do was to recreate the short signals sent from the
U-boats on making contact with the Convoy. Since the scripted position
of the convoy was known in the North Atlantic and its speed and direction,
this could be used as the contents of the Short Signal.
Since Bletchley Park had captured a copy of the Short Signal Code Book,
Bletchley Park could also recreate the codes used to describe the position
and direction and speed of the convoy, this gave what Bletchley Park called
a "crib", the guessed plain text of the intercepted cipher message.
Tony then had to decide on a setting for the Enigma machine for that day
and use a real machine to encipher the Short Signal texts just as the
U-boat operator would have done. Then by simulating the interception of
these signals at a "Y" station the teleprinter output could be created
which would have arrived in Bletchley Park.
The essence of breaking the day's settings from these short signals was
that in order for the messages to be sent quickly, the Germans used a
simple system for the message setting. This message setting is the
positions to which the Enigma wheels are to be turned to decrypt the
message and the Enigma operator just looked up an indicator/message setting
pair in his K Book (Kenngruppenheft Nr 1), included the indicator in the
message heading and enciphered the short signal starting with the wheels
at the messages setting.
Bletchley Park had captured a K Book so from the indicator in the message
header could just look up the corresponding message setting.
Thus from the intercepted short signals the code-breakers knew the relative
start positions of the messages but not the base configuration of the Enigma
machine. But they did know that all the messages were sent on the same
base configuration. This allows the messages to be written out on squared
paper offset by the message settings.
When sufficient messages had been received and set out in this way it was
possible to construct a menu to be run and on the Turing Bombe. This device
then found the base settings of the Enigma machine for that day.
What Tony had to do was to arrange that the set of messages generated did in
fact produce a working menu and that when it ran on the Bombe that this did
give the settings for the day.
From the script for the film Tony knew where the convoy was in the North
Atlantic when it was intercepted by the U-boats. The script also gave the
speed and direction of the convoy. Thus Tony was able to generate successive
messages as if they had come from the U boats at various points along the
path of the convoy.
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|This page was originally created by the late Tony Sale
the original curator of the Bletchley Park Museum