Cryptography Reference
In-Depth Information
in information technology, for example, for the Turing machine, a basic
model for a computer.)
•
In 1940, the Germans corrected their ciphering error, i.e., the leading dou-
ble message key. By then it was too late. The British knew many details of
the machine. And they could mount plaintext attacks often enough — recall
the HEILHITLER at the end of a command, or ANX at the beginning of a
text, or the popular KEINEBESONDERENVOKOMMNISSE.
•
Another opportunity for recovering plaintext was supplied by floating
mines. The ship that discovered the mines had to issue an encrypted warn-
ing to the other ships and submarines very quickly. While the Enigma-
encrypted messages to the submarines flowed over the air, they used sim-
pler methods that had already been broken to warn the other units. Usually
there was no time to rewrite the messages using a different encryption
method and so the adversary also learned the plaintext of the messages
to the submarines. Bauer [BauerMM] refers to this as a
compromised-
ciphertext-ciphertext attack
. The British called it '
kiss
' — they could
have kissed the operators for their ciphering errors.
When the Germans did not make errors, the British were very
inventive. For example, they bombed a light buoy with the
sole purpose of making a German observer send the encrypted
radiogram 'ERLOSCHENISTLEUCHTTONNE' (FLAREWENTOUT),
which he promptly did [BauerMM]. '
Gartenpflege
' (gardening) was the
watchword for mining port entries or previously cleared areas, which
triggered similar stereotype messages, or supplied a kiss. This is how
things can look in practice when one foists plaintext on an adversary
(which is a 'chosen-plaintext attack'; see Chapter 3).
•
In 1941, the poorly armed trawler
Krebs
(cancer) fell into the hands of
the British when they attacked an industrial site on the Lofoten Islands
off the western coast of Norway, and the trawler's crew didn't have
time to destroy all secret documents before the British came aboard.
The British found two rotors they knew about already. Most importantly,
however, they found the basic key for February. They could use it to
finally decipher many unknown messages in arrears for the first time.
Among other things, they learned that the German weather ships also used
Enigmas to encrypt their information, and that they had used special code
books that contained what they called a
weather key
for their weather
reports since October 1940. Such weather messages represented strategic
data, so their encryption was justified.
