Low complexity and hardware-friendly spectral modular multiplication

The Schönhage-Strassen Algorithm (SSA) is an asymptotically fast multiplication algorithm with the complexity of O(l log l log log l) where l is the operand size. It outperforms other multiplication algorithms when l is large enough. One possible usage of such long integer multiplication is for cryptography. Innovated from SSA, the Interleaved Spectral Montgomery Modular Multiplication (ISM³) algorithm is proposed to accelerate the modular multiplication. ISM3 algorithm primarily interleaves the Montgomery modular multiplication algorithm between time and spectral (frequency) domain. We show that the tasks in each step of the proposed algorithm have little data dependency, and hence, extremely suitable for hardware implementation. We present the parallel ISM³ architecture and implement it on Xilinx Virtex-II and Virtex-6 FPGAs. Experimental results show that our 3838-bit ISM3 is faster than the previous Montgomery multiplier. Moreover, our design can complete a 7678-bit modular multiplication in 3398 cycles in 17.98 μs on a Virtex-6 device.