Cryptography Reference
In-Depth Information
systems where the meaning of the message is concealed by cipher, code, etc.,
although its existence is not hidden, and the enemy is assumed to have any special
equipment necessary to intercept and record the transmitted signal. We consider
only the third type—concealment systems are primarily a psychological problem
and privacy systems a technological one.
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Yet although cryptography was a crucial element of military communi-
cations, espionage had few uses for it, as spies' main concern laid in the
concealment of communication itself. There were abundant methods to
choose from—some devised by the ancient Greeks and used by German
spies: prick holes in a book or a newspaper above the letters of the secret
message. Detecting such covert communication over multiple carriers—
newspapers, letters, movies, telegrams, and so on—became a major concern
of the United States after Pearl Harbor. The censorship service created in
aftermath of the attack employed close to 15,000 examiners who listened
to phone conversations; scanned movies, magazines, and radio scripts; and
opened up to a million pieces of overseas mail
each day
:
To plug up as many steganographic channels of communication as possible, the
Office of Censorship banned in advance the sending of whole classes of objects or
kinds of messages. International chess games by mail were stopped. Crossword
puzzles were extracted from letters, for the examiners did not have time to solve
them to see if they concealed a secret message, and so were newspaper clippings
which might have spelled out messages by dotting successive letters with secret ink.
. . . Listing of students' grades was tabooed. One letter containing knitting instruc-
tions was held up long enough for an examiner to knit a sweater to see if the given
sequence of knit two and cast off contained a hidden message. . . . A stamp bank
was maintained at each censorship station; examiners removed loose stamps, which
might spell out a code message, and replaced them with others of equal value, but
of different numbers and denomination. Blank paper, often sent from the United
States to relatives in paper-short countries, was similarly replaced from a paper bank
to obviate secret-ink transmissions. Childish scrawls, sent from proud parents to
proud grandparents, were removed because of the possibility of their covering a
map. Even lovers' X's meant as kisses, were heartlessly deleted if censors thought
they might be a code.
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This display of imagination from both spies and censors points to the
enormously varied ways in which the materiality of media can be leveraged
to transmit information. Indeed, as detailed in the next chapter, cryptog-
raphers themselves would eventually resort to such methods to evade and
ridicule efforts by the intelligence establishment to police the spread of
cryptographic knowledge.
