Cryptography Reference
In-Depth Information
“QENMO” from above. Any five-letter word could be the underlying
message because any key is possible. The name, “BRUNO”, for in-
stance, would have generated “QENMO” if the key numbers were 15,
13, 19, 25, and 0. If all possibilities are available, then the attacker
can't use any of the information about English or the message itself
to rule out solutions. The entropy of the message itself should be
greater than or equal to the entropy in the key. This is certainly the
case here because each byte of the message could be any value be-
tween 0 and 255 and so could the key. In practice, the entropy of the
key would be even greater because the distribution of the values in
the message would depend on the vagaries of language while the key
can be chosen at random.
A real one-time pad would not be restricted to uppercase char-
acters. You could use a slightly different encryption process that em-
ployed all 256 possible values of a byte. One popular method is to use
the operation known as exclusive-or (XOR), which is just addition in
the world of bits. ( 0+0=0 , 0+1=1 ,and 1+1=0 because it wraps
around.) If the one-time pad consists of bytes with values between 0
and 255 and these values are evenly distributed in all possible ways,
then the result will be secure. It is important that the pad is not used
again because statistical analysis of the underlying message can re-
veal the key. The United States was able to read some crucial cor-
respondence between Russia and its spies in the United States dur-
ing the early Cold War because the same one-time pad was reused.
[Age95] The number of bits in the key was now less than the number
of bits of information in the message, and Shannon's proof that the
one-time pad is a perfect encryption no longer holds.
The one-time pad is an excellent encryption system, but it's also
very impractical. Two people who want to communicate in secret
must arrange to securely exchange one-time pads long before they
need to start sending messages. It would not be possible, for in-
stance, for someone to use their WWW browser to encrypt the credit
card numbers being sent to a merchant without exchanging a one-
time pad in person. Often, the sheer bulk of the pad makes it too
large to be practical.
Many people have tried to make this process more efficient by
using the same part of the pad over and over again. If they were en-
crypting a long message, they might use the key 90210 over and over
again. This makes the key small enough to be easily remembered,
but it introduces dangerous repetition. If the attackers are able to
guess the length of the key, they can exploit this pattern. They would
know in this case that every fifth letter would be shifted by the same
amount. Finding the right amount is often trivial and it can be as
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