ABSTRACT
This chapter discusses some simple cryptosystems popularly used in the precomputer era. These are called classical cryptosystems. The chapter also discusses some possible attacks on those cryptosystems. In contrast to modern cryptographic algorithms, these cryptosystems can be solved easily. In block cipher, the key and the encryption algorithm is applied to the block of data at once, instead of applying on each single alphabet/bit individually. There are two types of block cipher: Substitution Cipher and Transposition Cipher. The shift cipher and affine cipher are special cases of substitution cipher. Hill cipher is a polygraphic substitution cipher based on linear algebra. The idea of permutation cipher is to keep the plaintext character unchanged but effect a change in their position using some permutation. The rail fence cipher is a form of transposition cipher. It derives its name from the way in which it is encoded.
