Summary

In this paper, we have developed a large block cipher by introducing the basic concepts of permutations and substitutions. The permutations and substitutions are key based. We have taken the key and the plaintext in the form of numbers and characters respectively. Where, each one is converted into its 8 bit binary equivalent based on its ASCII values. In the process of encryption, we have represented the plaintext as a block of 256 bits and developed a block cipher of 256 bits by using the classical feistel network. In each round, we have performed different key based permutations with the help of the equation that we have derived. Similarly, in each round, we have also permuted the key based S-Boxes. The cryptanalysis carried out at end shows that the cipher cannot be broken by any cryptanalytic attack. Key words