Post by Jim H.I was re-watching 'The Imitation Game' this afternoon
(yep, I know it's a very simplified Hollywooded version
of the real story), but questions arose.
AIUI, there were several Enigma systems in use.
There was one Enigma system. (Though from 1942,
the U-boats used Enigmas modified for an additional
scrambler wheel. One might call that a different
"system".)
However, there were many Enigma "keys". Each key
was a suite of daily settings, issued monthly to
all users on a particular key. There were separate
keys for the Army, Navy, Air Force, and military
supply service (initially). As the war continued,
additional keys proliferated - for the SS, for the
Abwehr, for different theaters, and for different
categories of units and operations within each
service.
Towards the end of the war, for instance, there
were separate Navy keys for
- general operations
- Mediterranean operations
- U-boat operations
- U-boat training
- large surface warships
- commerce raiders
- weather reports
and probably others. There were over 50 for all
services and branches.
Post by Jim H.For the various systems, who in Germany decided what
the machine's settings were to be on a daily basis...
Some clerk in the central office of the Signals
division of OKW.
Post by Jim H.how was this setting selection made...
Randomly, as far as anyone knows.
A key specified for each day:
Scrambler wheel selection and order
(The Enigma scrambler wheels were swappable;
they could be inserted in any order in the
three slots, and as the war continued additional
wheels were issued with each machine, allowing
for the use of any three of up to seven wheels.)
Position of the outer indicator ring on each wheel
(Each wheel had an A-Z ring on its outside to
indicate position through a window in the machine
cover. But the ring rotated relative to the actual
scramble wiring; it was secured with a clip in
operation. Thus if scramble wheel #1 was in slot A,
setting it to show X could still have 26 possible
positions.)
Plugboard connections
(The Enigma had a set of 26 sockets, which could
be cross-connected by double-ended cables. By
cross-connecting 4 to 10 pairs of sockets, another
layer of scramble was introduced. Though unlike
the effect of the scrambler wheels, the plugboard
was constant for the whole day, it was completely
variable from day to day.)
Discriminants
(These were three-letter tags sent in clear to
indicate what key the message was in. Each key had
a set of four; all were used. For multipart messages,
successive parts were tagged with different
discriminants.)
That clerk had to generate all those settings for
all the different Enigma keys.
But see below.
Post by Jim H.and how was it sent out to the end users in the field?
In printed booklets, I'd presume. For a month at
a time, except naval units on long deployments.
----------------------------------------------
Gordon Welchman was one of the top people at "Station
X", the British cryptanalysis center at Bletchley
Park. His field was mathematical attacks on Enigma,
including the development of mechanical systems (the
famous "bombes", which could test tens of thousands of
possible settings against a cipher message).
He wrote a memoir called _The Hut Six Story_, in which
he discussed the problems faced by the cryptanalysts
and how they overcame them. He noted that the Allies
would not have been able to break Enigma without major
German mistakes. The story below is taken from his
book.
As the war progressed, and the keys proliferated, that
clerk in Berlin (Welchman dubbed him "Herr X") had
more and more work to do, and one day he said "the
hell with it." He adopted a clever labor-saving trick:
recycling elements of old keys. He would take the
wheel order from an old Navy key, the ring settings
from an old Army key, the plugboard connection from an
old SS key, and issue the result to the Luftwaffe. Who
would ever know?
As it happened, Reg Parker, an analyst at Bletchley
Park, was keeping a log of all solved Enigma keys,
just to see if any patterns appeared. When Herr X said
"the hell with it", Reg Parker caught him.
Thus for much of 1942, Bletchley Park often had parts
or all of many Enigma keys for up to a month in
advance.
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