TheCodeBreakers
channel of communication. Examples are static on the radio, "snow" on a
television screen, misprints, background
chatter at a cocktail party, fog, a bad connection on the telephone, a
foreign accent, perhaps even mental preconceptions. Shannon is
suggesting that noise is analogous to encipherment. "The chief
differences in the two cases," he wrote, "are: first, that the operation of
the enciphering transformation is generally of a more complex nature
than the perturbing noise in a channel; and, second, the key for a
secrecy system is usually chosen from a finite set of possibilities while
the noise in a channel is more often continually introduced, in effect
chosen from an infinite set."
When Carl W. Helstrom, author of Statistical Theory of Signal
Detection, was asked whether the techniques of isolating signals from
noise had any relevance to crypt-analysis, he replied: "I suspect that the
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