A study of some block ciphers for secure transmission of information

Abstract

In this thesis, we have investigated several problems in the area of cryptography. In these problems, we have developed several block ciphers, wherein, iteration plays a fundamental role. The first chapter is introductory in nature. It deals with the basic concepts and a survey of the relevant literature in the area of cryptography. In the second chapter, we have developed a block cipher by taking a large key matrix (of size 384 binary bits) and a plaintext matrix containing two columns (112 bits). In this, the plaintext column vectors, operated by the key matrix are thoroughly interlaced at each stage of the iteration. The cryptanalysis carried out in this chapter clearly indicates that the cipher is difficult to break by any conventional cryptanalytic attack. In chapter 3, we have developed a large block cipher by modifying the Hill cipher. This is an extension of the analysis carried out in chapter 2, to a plaintext block of unlimited size. Chapter 4 is devoted to the study of a block cipher basing upon the modification of the Hill cipher. In this, we have considered the effect of key dependent permutation together with interlacing. The analysis clearly reveals the strength of the cipher. In chapter 5, we have dealt with another block cipher, wherein the size of the key matrix is 384 bits and the size of the plaintext is as large as we please. Here also, we have studied the effects of the key dependent permutation, interlacing and the iteration. Here, we have noticed that all these agents are strengthening the cipher in a remarkable manner. Chapter 6 deals with another modification of the Hill cipher. In this, we have introduced interweaving in each step of the iteration. Here, we have found that the interweaving of the resulting plaintext, at each stage of the iteration, and the multiplication with the key matrix strengthen the cipher quite significantly. In the investigation presented in chapter 7, we have extended the analysis of the modified Hill cipher involving interweaving.