Prehistory of Cryptography - A Classical Introduction to Cryptography: Applications for Communications Security - page 16

Enemy cryptanalyst

Message
source

X Encipherer

C

Decipherer
C −1

Message X Cryptogram Y

Key K

Key
source

Figure 1.5. The Shannon encryption model.

Following the Shannon Theory, we do not encrypt fixed messages, but messages
coming from a plaintext source. The plaintext source generates random texts according
to some given probability distribution. For instance, with the distribution of plain
English texts, the probability that the plaintext is hello world is much greater
(particularly when the message is a textbook about a programming language) than the
probability that it is gbwiub oafp (except maybe in some Vogon poetry). 16

Following the Shannon Theory, a cipher is given by

1. a plaintext source (with the corresponding distribution),
2. a secret key distribution,
3. a ciphertext space,
4. a rule which transforms any plaintext X andakey K by a ciphertext Y

=

C K ( X ),
5. a rule which enables recovering X from K and Y

C − 1
K

= C K ( X )as X =

( Y ).

(see Fig. 1.5.) A more intuitive definition of a cipher includes

1. a plaintext space, a ciphertext space, a key space,
2. a key generation algorithm,
3. an encryption algorithm,
4. a decryption algorithm.

These definitions relate to conventional cryptography .

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