Cryptography Reference
In-Depth Information
limit the damage caused by compromise at the hand of the enemy, code-
books usually contained key material for a single month. U-boats' code-
books were printed using water-soluble ink, and if compromise was
suspected, operators switched to the following month's keys or followed
special procedures known only to officers.
For all its theoretical security, the Enigma was broken even before the
start of the war by three mathematicians working for the Polish intelli-
gence service. Not only did they succeed in fully cryptanalyzing the
Enigma, they also designed and built
bombes
, special-purpose electrome-
chanical devices that could decrypt Enigma-secured communications in
real time. A replica of the Enigma as well as plans for the bombes were
eventually transmitted to French and English intelligence services in
August 1939, providing the initial impetus for work at Bletchley Park, the
British headquarters of the Allied cryptanalytic effort.
Ironically, the machine's user-friendliness proved a liability. Once daily
procedures for key agreement were performed, messages could be
encrypted and decrypted as fast as they could be typed and read off the
“screen”—three rows of letters above the keyboard, backlit by light bulbs.
Secret military communications had never been so easy: “Wehrmacht
officers found the machines so convenient, so portable, that they could
not bring themselves to reserve the machine for only high-grade (secret)
traffic. Instead Enigma carried all grades of messages, from New Year's
greetings and routine supply reports and requests to virtually all U-boat
orders and Herr commands' daily situation reports.”
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Even if the Germans
were aware of the importance of limiting the amount of signals sent,
“they could not bring themselves to observe it rigorously.”
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Routine
signals, in particular, created ideal conditions for cryptanalysis, carrying
repetitive information, with repetitive traffic patterns. Stereotyped lan-
guage, such as forms of salutation, address, and signature also strongly
patterned the content of the messages, providing a foothold for Allied
cryptanalysis efforts.
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But mostly, cryptanalysis thrived on procedural error, both systematic
and random. The practice of double encipherment of the indicator at the
beginning of each message provided the Poles with their initial break. The
operator's “random” choice of rotor positions would often in practice
involve adjacent letters on the keyboard, or meaningful initials. As the war
dragged on, cryptographic fatigue led clerks to reuse portions of codebooks.
