Cryptography Reference
In-Depth Information
,
!
!
G
Figure 10.15
Shannon's model of a symmetric encryption system.
an encryption and decryption algorithm 40 and are both fed with the same secret key
k generated by a key source. It is assumed that there exists a secure channel between
the key source and the encryption and decryption devices. The encryption device
turns the plaintext message m into a ciphertext c , and the decryption device does
the opposite. It is assumed that the adversary has only access to the ciphertext c and
that he or she has no information about the secret key other than that obtained by
observing c . In this situation, the adversary tries to obtain useful information about
the plaintext message m or the secret key k .
A cryptographic technique not covered by Shannon's model is probabilistic
or randomized encryption. Figure 10.16 shows a model of a randomized symmetric
encryption system. In addition to the components of the original Shannon model, this
model includes a random source that generates a random input s for the encryption
process. The random input may either be used as an additional nonsecret “key” that
is transmitted to the destination, and multiplexed with the ciphertext, or it may be
used to randomize the plaintext, in which case the adversary does not obtain the
randomizer in the clear. In either case, it is important to note that the decryption
process cannot be randomized and hence that the decryption process need not be fed
with s .
An encryption (process) that takes place in a symmetric encryption system
(
) can also be viewed as a discrete random experiment. In this case,
M and K represent real-valued random variables that are distributed according
to P M :
M
,
C
,
K
,
E
,
D
+ (see Definition 4.2 for the notion
of a random variable). Note that P M typically depends on the plaintext language
in use, whereas P K is often uniformly distributed over all possible keys (i.e., all
+
M→ R
and P K :
K→ R
40
More specifically, they implement the families E and D of encryption and decryption functions.
Search WWH ::




Custom Search