L4 Introduction to Classical Cipher systems

2-1 Introduction.
Classical ciphers have been used since ancient Egypt to exchange classified
messages between authorized persons. Since then, different methods and techniques are used in order to increase security level of such information. Most of these methods and techniques are based on the idea that each natural language has its own distribution characteristics.
The encryption process aims to uniformly flatten the statistical characteristics of the language and obscure any correlation and dependencies between plaintext and ciphertext by diffusion and confusion. In the other hand, cryptanalysis tries to make use of remaining information (such as structured properties, redundancy, correlation, and dependencies) to recover the plaintext or the used key during encryption process.
Cryptographic systems have been classified into different types in many
cryptography literatures. However, most of these literatures classify them into two main categories; symmetric (one-key) cryptosystems, and asymmetric (two-key) systems . In symmetric or one-key systems, the enciphering and deciphering key are the same. In asymmetric cryptosystems enciphering and deciphering keys are different in such a way the at least one key is computationally infeasible to determine from the other.
From the ancient ages till the last few decades, all cryptosystems were one-key systems, thus one-key systems referred to as conventional or (classical) systems.