Analysis of Quarter Rounds of Salsa and Chacha Core and Proposal of an Alternative Design to Maximize Diffusion

Background/Objectives: Salsa and ChaCha are commonly used encryption primitives. Both Salsa and ChaCha core use Quarter round as its core function. The objective of the paper is to analyze the diffusion property of Quarter round of both these algorithms and propose an alternative design named Modified ChaCha Core (MCC). Methods: The Quarter round functions of all these three algorithms are compared using the diffusion matrices that reflect change in output words with a small change in input words. For each algorithm we generated more than a million diffusion matrices depending on the possible permutations of rotations constants used in Quarter round. Findings: Results of our experiment reflected that for Salsa and ChaCha core, there are high number of alternative rotation constants that generate more diffusion than the rotation constants prescribed by the authors. The comparison of diffusion matrices of all three competing structures also concluded that quarter round of MCC exhibits more diffusion than Quarter round of Salsa and ChaCha and it does so in lesser operations. Applications: MCC core; the design proposed in this paper, may be used to generate stream ciphers or may be used to generate collision resistant compression function for a cryptographic hash algorithm.