Cryptography Reference
In-Depth Information
to another ground station. These operations are then
undone, in reverse order, by the intended receiver to
recover the original information.
In the simplest possible example of a true cipher, A
wishes to send one of two equally likely messages to B , say,
to buy or sell a particular stock. The communication must
take place over a wireless telephone on which eavesdrop-
pers may listen in. It is vital to A 's and B is interests that
others not be privy to the content of their communica-
tion. In order to foil any eavesdroppers, A and B agree in
advance as to whether A will actually say what he wishes B
to do, or the opposite. Because this decision on their part
must be unpredictable, they decide by flipping a coin. If
heads comes up, A will say Buy when he wants B to buy
and Sell when he wants B to sell. If tails comes up, how-
ever, he will say Buy when he wants B to sell, and so forth.
(The messages communicate only one bit of information
and could therefore be 1 and 0, but the example is clearer
using Buy and Sell .)
With this encryption/decryption protocol being used,
an eavesdropper gains no knowledge about the actual (con-
cealed) instruction A has sent to B as a result of listening
to their telephone communication. Such a cryptosystem
is defined as “perfect.” The key in this simple example is
the knowledge (shared by A and B ) of whether A is say-
ing what he wishes B to do or the opposite. Encryption is
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