Abstract
The rapid and extensive usage of Internet in the present decade has put forth information security as an utmost concern. Most of the commercial transactions taking place over the Internet involves a wide variety of data including text, images, audio and video. With the increasing use of digital techniques for transmitting and storing Multimedia data, the fundamental issue of protecting the confidentiality, integrity and authenticity of the information poses a major challenge for security professionals and hassled to the major developments in Cryptography . In cryptography, an S-Box (Substitution-box) is a basic component of symmetric key algorithms, which performs substitution and is typically used to make the relationship between the key and the cipher text non linear and most of the symmetric key algorithms like DES, Blowfish makes use of S boxes. This paper proposes a new method for design of S boxes based on chaos theory. Chaotic equations are popularly known for its randomness, extreme sensitivity to initial conditions and ergodicity. The modified design has been tested with blowfish algorithm which has no effective crypt analysis reported against its design till date because of its salient design features including the key dependant s boxes and complex key generation process. However every new key requires pre-processing equivalent to encrypting about 4 kilobytes of text, which is very slow compared to other block ciphers and it prevents its usage in memory limited applications and embedded systems. The modified design of S boxes maintains the non linearity [3] [5] and key dependency factors of S boxes with a major reduction in time complexity of generation of S boxes and P arrays. The algorithm has been implemented and the proposed design has been analyzed for size of key space, key sensitivity and Avalanche effect. Experimental results on text and Image Encryption show that the modified design of key generation continues to offer the same level of security as the original Blowfish cipher with a less computational overhead in key generation.
Highlights
An S box can be thought of as a miniature substitution cipher
The chaotic value generated will be converted to a suitable index within the range by the formula zj=* ((indexmax-indexmin)/(fmax-fmin))+ indexmin where, zj is the index to be identified for generating the element in P array or S box, ci is the chaotic value generated during ith iteration, fmax and fmin are the maximum and minimum values generated by the chaotic equation, index max and index min are the maximum and minimum values of the indices in key index table. 3.4
The values of P arrays and S boxes depend on the initial seed and its related parameters associated with the chaotic equation
Summary
An S box can be thought of as a miniature substitution cipher. The input to an s box could be a n bit word, but the output can be an m bit word where m and n are not necessarily the same. An S box can be keyed or keyless and linear or non-linear. The idea of diffusion is to hide the relation ship between the plain text and the cipher text, which will frustrate the adversary who uses cipher text statistics to find the plain text. The idea of confusion is to hide the relation ship between the cipher text and the key, which will frustrate the adversary who uses cipher text to find the key. If a single bit in the key is changed, most or all bits in the cipher text will be changed. Every iteration makes use of S boxes, P Boxes, and other non linear operations in order to provide diffusion and confusion
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