B-21 cipher machine
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1st problem
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We know everything (the internal key, the message key, the 
wiring of the half-rotors, the structure of the keyboard, ...).
If, moreover, the working of the machine B-21 is known, the 
message can easily be decrypted.

2nd problem
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The internal key is known. If you want to decipher the message
just try all the possible external keys.

3rd problem
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We know the beginning of the message. That should be enough 
to find the key. 

4th problem
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We know several probable words and in addition we know a part 
of the key. If we can position the probable words, we can 
deduce the pieces of the missing key.

The problems from 5th to 8th are in English.
They use the machine already used in the 1st problem.

5th problem
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We know the beginning of the message. There is no plugboard
That should be enough to find the key.

6th problem
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There is a unknown plugboard, then the problem is (slighty)
more difficult.

7th problem
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We know several probable words. If we can position the 
probable words, we can deduce pieces of the key.

8th problem
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The problem is hard, indeed we know nothing (just the Language).