A Simple Algebraic Model based Polyalphabetic Substitution Cipher

Abstract

ABSTRACT Cryptography is considered to be a disciple of science of achieving security by converting sensitive information to an un-interpretable form such that it cannot be interpreted by anyone except the transmitter and intended recipient. An innumerable set of cryptographic schemes persist in which each of it has its own affirmative and feeble characteristics. In this paper we have we have developed a traditional or character oriented Polyalphabetic cipher by using a simple algebraic equation. In this we made use of iteration process and introduced a key K 0 obtained by permuting the elements of a given key seed value. This key strengthens the cipher and it does not allow the cipher to be broken by the known plain text attack. The cryptanalysis performed clearly indicates that the cipher is a strong one. . Keywords Polyalphabetic substitution, variable length key stream, bit-ratio test, frequency test. 1. INTRODUCTION Cryptography is considered to be a collection of tools and techniques related to components of information security such as confidentiality, authenticity, integrity, non-repudiation [1]. Cryptography refers to the science of securing data by changing the data into non-interpretable form, cryptanalysis is the science of analyzing and breaking secure communication. Cryptanalyst or attacker is a person who performs cryptanalysis [2]. Cryptology is a combination of both cryptography and cryptanalysis. A no of cryptanalytic attacks can be found wherein cipher text only, known plaintext, chosen plaintext, chosen cipher text, adaptive chosen plaintext, brute force attack, key guessing attack etc are to name a few [3]. In this paper we propose a simple algebraic model based Polyalphabetic substitution cipher wherein the plain text is converted to cipher text by the use of a key-seed value from which variable length key stream is generated producing different cipher text with every run time for similar entered plain text. In section 2 we have presented the basic terminologies. Section 3 depicts the proposed algorithm both for encryption and decryption. In section 4 we have presented the experimental results. Testing and analysis is given in section 5 comprising of frequency distribution test, bit-ratio test, encryption and decryption time comparison and comparison of plain text size with cipher file size. On the basis of testing and analysis performed conclusions are drawn in section 6.