Study and analysis of eSTREAM cipher Salsa and ChaCha

The eSTREAM project [5] was established to choose new stream ciphers with comparison to existing ciphers (e.g. AES) as to provide a better alternative. The stream cipher Salsa20 [3] as a candidate of the eSTREAM project was accepted for the final phase and again successfully reviewed with 12 round. ChaCha is a variant of Salsa20 aiming at bringing better diffusion for similar performance. Significant effort has been made to analyze & explained Salsa and ChaCha with reduced round in [1] and [2], with some improvements. In this article, first we go through the work done in [1] with complexity 2²⁴⁸ and [2] with complexity 2²⁴³ to provide a view of the existing attack for ChaCha7, i.e., 7 rounds. For Salsa20/8 i.e., 8 rounds, complexity is 2^247.2 [1] and 2^245.5 [2]. Then we go through the method for Differential Cryptanalysis of Salsa20 and ChaCha, improved by correlation attacks and related to the concept of neutral bits to analyze Salsa20/9 & ChaCha with 8 rounds [1]. And to analyze the possibility of reducing the complexity with the existing attack. We determine how a single difference effects after one round to another round and how the reduction can be possible with some particular choices of keys.