A Hash-Driven Evolving Permutation Stream Cipher with Encrypted Keystream Generation

Abstract

Stream ciphers are widely used in low-latency and high-throughput settings, but practical designs must balance internal-state complexity, statistical quality of the keystream, and implementation cost. This paper presents a hash-driven stream cipher construction that maintains a 256-byte Dynamic Permutation State (DPS). In each round, SHA-512 is applied to the concatenation of the key, IV, and the prior DPS to derive a nonlinear control array that performs permutation updates via swap operations; the updated DPS is then masked with AES-128 to output an encrypted keystream block. Randomness was evaluated using the NIST SP 800-22 Statistical Test Suite, and the generated keystream achieved passing proportions consistent with the STS acceptance criteria across major tests. Sensitiv-ity experiments show near-ideal diffusion under single-bit key changes (average Hamming ratio ≈ 0.5), and an overlapping-round analysis indicates negligible correlation between successive round outputs. A timing study highlights a trade-off between keystream quality and speed: the prototype implementation is slower than AES-CTR and ChaCha20 but remains practical for software-based generation of robust keystreams. Finally, brute-force analysis indicates that while the nominal key size matches AES-128, the per-key evaluation time is increased due to repeated hashing and permutation evolution, raising the computational effort of exhaustive testing in practice.