Randomness Analysis of 128 bits Blowfish Block Cipher on ECB and CBC Modes

Randomness of output is a significant factor in measuring the security of any cryptographic algorithm. A non-random block cipher is vulnerable to any type of attack. This paper presents the National Institute of Standard and Technology (NIST) statistical tests of the 128-bit BA to investigate its randomness. The structure of this algorithm resembles that of the 64-bit Blowfish algorithm, except for the block size is 128 bits in the extended version. Moreover, all operations in the 64-bit Blowfish are based on 64 bits instead of 32 bits. The 128-bit BA is a symmetric block cipher with variable key lengths from 64 bits up to a maximum of 896 bits, which enhances the security of the algorithm. The 128-bit BA with the electronic codebook (ECB) and cipher block chaining (CBC) modes is employed for these tests. In addition, comparisons are conducted. The analysis shows that the 128-bit BA with ECB mode is inappropriate for text and image files that contain large strings of identical bytes. This inconsistency in the results of the text and image files is attributed to the fact that the majority of the 188 NIST statistical tests failed in all rounds. Keywords— Block Cipher, Blowfish 128 bits , ECB mode, CBC mode, Randomness Test.