Cryptography Reference
In-Depth Information
keys adjacent in one row on the keyboard and similar things. Also, the
rotors' positions after the last ciphering session often served as key. Since
the Poles had constantly listened in on them, they knew these positions.
Bauer [BauerMM] provides a table with 40 message keys used; only
three of them were not stereotypical.
As we know, the purpose of the message key was to prevent too much
material encrypted with the same key from falling into the adversary's
hands, and to increase the security. The outcome was the exact opposite.
They hadn't thought of the human insecurity factor.
Which takes us to the next vulnerability: the best method is worth nothing
when the keys are drawn from a small value space. We are talking of
a
reduced key space
. Prehistoric stuff? Nope. That was an illness that
transpired when transmitting credit card numbers via Netscape — a highly
topical issue. Once upon a time, people died because codes were broken.
Today, you might find your accounts cleared out, though people still die.
•
Next ciphering error: the message key was an extremely important piece
of information. As mentioned earlier, polyalphabetic methods (and the
Enigma implemented one) have the benefit that brief transmission inter-
ferences cause little damage. Except for the message key, because if this
key is lost the entire message is indecipherable. The receiver's request,
'again please', would naturally also be encrypted — and here you had
a neat vulnerability for a plaintext attack. Such a request would often
have been out of the question for military reasons: for example, radio
silence was ordered for submarines to prevent others from direction-
finding their positions. A command sent from the headquarters simply
had
to be decipherable!
Such mishaps caused by transmission interferences were prevented by
typing the
message key twice in a row
at the beginning. This became
apparent statistically, and the Polish genius Marian Rejewski seemed
to have guessed quickly that a string occurred twice at the beginning,
and that it was probably a key. Since this concerned only six encrypted
characters, it moved only the first rotor in 20 out of 26 cases. So they
found the inner wiring of the first rotor. By 1936, the drum position was
changed every three months so that each rotor enjoyed once the honor of
working at the very front — and the Poles enjoyed analyzing it. Poland
built a copy of the Enigma with five possible drums and passed it on to
both France and Great Britain.
