Cryptography Reference
In-Depth Information
the act by
A
of either saying what he wants done or not as
determined by the key, while decryption is the interpre-
tation by
B
of what
A
actually meant, not necessarily of
what he said.
This example can be extended to illustrate the second
basic function of cryptography, providing a means for
B
to assure himself that an instruction has actually come
from
A
and that it is unaltered—i.e., a means of authen-
ticating the message. In the example, if the eavesdropper
intercepted
A
's message to
B
, he could—even without
knowing the prearranged key—cause
B
to act contrary
to
A
's intent by passing along to
B
the opposite of what
A
sent. Similarly, he could simply impersonate
A
and tell
B
to buy or sell without waiting for
A
to send a message,
although he would not know in advance which action
B
would take as a result. In either event, the eavesdropper
would be certain of deceiving
B
into doing something that
A
had not requested.
To protect against this sort of deception by outsiders,
A
and
B
could use the following encryption/decryption
protocol.
.
They secretly flip a coin twice to choose one of four
equally likely keys, labeled HH, HT, TH, and TT, with
both of them knowing which key has been chosen. The
outcome of the first coin flip determines the encryption
rule just as in the previous example. The two coin flips
