Key schedule algorithms play an important role in modern encryption algorithms, and their security is as crucial as the security of the encryption algorithms themselves. Many studies have been performed on the cryptographic strength evaluation of the encryption algorithms; however, strength evaluation of the key schedule algorithms often obtains less attention that can lead towards the possible loophole in the overall encryption process. In this paper, a criterion is proposed to evaluate the cryptographic strength of the key schedule algorithms. This criterion includes different methods of data generation from subkeys and a suitable set of statistical tests. The statistical tests are used to explore the cryptographic properties such as diffusion, confusion, independence, and randomness in the subkeys generated by the key schedule algorithm. The proposed criterion has been applied to some of the key schedule algorithms of different block ciphers. The results confirm that the proposed criterion can effectively differentiate between strong- and weak-key schedule algorithms.

AES type S-boxes are constructed by replacing the affine matrix of AES S-box equation with 8x8 invertible affine matrices. The 8x8 S-boxes of AES produced in GF(2 8 ) are a nonlinear transformation that have significant effect on the strength of entire system. In this paper, 46 AES like S-boxes are generated by replacing the affine matrix and only 10 randomly selected Sboxes are included. The cryptographic properties of 10 AES like S-boxes are analyzed. The S-box must satisfy these properties in order to yield a secure and reliable cipher. These include: Avalanche, Strict Avalanche, Bit Independence criteria and Nonlinearity measurement. The experimental results indicate that most of the generated S-boxes mimic the AES original S-box with respect to these properties and some of them are stronger than AES S-box.

No abstract