Abstract:
The importance of data has been increased manifolds in the recent past. Therefore, its protection from the unauthorized persons also demands utmost attention especially when this data is in transit because during this phase it is more vulnerable to attacks from the adversary. The only way to protect this data is with the help of cryptographic techniques. Many different schemes, algorithms and ciphers help users according to their needs. Block ciphers are a kind of symmetric ciphers where a block of text is encrypted with the help of a cryptographic key rather than encrypting one bit at a time as in the case of stream ciphers. The only non-linear and the most vital component in these ciphers is the s-box which provides enhanced security to these ciphers. Therefore, the designing of the s-box of any block cipher holds utmost importance. Block ciphers have many benefits and applications across many different fields in the world. With its growing demand and its usage, these ciphers are also vulnerable to different kinds of attacks. Some of the important known attacks are Linear Cryptanalysis and Differential Cryptanalysis. However, a more powerful attack against these ciphers is known as interpolation attack. Such attacks are effective against ciphers whose s-boxes are represented with simple algebraic functions. Researchers have been applying such attacks on various prototype ciphers like PURE and variants of block ciphers like SHARK and were successful. This definitely raises the concerns in the researcher community that the real world ciphers like DES and AES are also under threat from such attacks and one day when there won’t be any issues of computational and memory resources, these ciphers can also be broken in polynomial time. Therefore, in this thesis we have analyzed s-boxes critically and tried to find what role these s-boxes can play in making ciphers resistant against these attacks. Finally, a design criteria is given for a stronger s-box against interpolation attacks.
