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