Fast constant time implementations of ZUC-256 on x86 CPUs

ZUC-256 is a Pseudo Random Number Generator (PRNG) that is proposed as a successor of ZUC-128. Similarly to ZUC-128 that is incorporated in the 128-EEA3 and 128-EIA3 encryption and integrity algorithms, ZUC-256 is designed to offer 256-bit security and to be incorporated in the upcoming encryption and authentication algorithm in 5G technologies. In this context software optimizations of ZUC-256 are desired. This paper proposes several ZUC-256 optimizations for x86 processors, especially, modern processors that have efficient AVX vectorization. Surprisingly, we also show that AES-NI can also be used for ZUC-256 and help creating constant-time implementations. Our results show speedup of up to 4.5 x(per key stream) when computational tasks are parallelized efficiently.