International Association
for Cryptologic Research

Number theoretic transform (NTT) is widely utilized to speed up polynomial multiplication, which is the critical computation bottleneck in a lot of cryptographic algorithms like lattice-based post-quantum cryptography (PQC) and homomorphic encryption (HE). One of the tendency for NTT hardware architecture is to support diverse security parameters and meet resource constraints on different computing platforms. Thus flexibility and Area-Time Product (ATP) become two crucial metrics in NTT hardware design. The flexibility of NTT in terms of different vector sizes and moduli can be obtained directly. Whereas the varying strides in memory access of in-place NTT render the design for different radix and number of parallel butterfly units a tough problem. This paper proposes an efficient conflict-free memory mapping scheme that supports the configuration for both multiple parallel butterfly units and arbitrary radix of NTT. Compared to other approaches, this scheme owns broader applicability and facilitates the parallelization of non-radix-2 NTT hardware design. Based on this scheme, we propose a scalable radix-2 and radix-4 NTT multiplication architecture by algorithm-hardware co-design. A dedicated schedule method is leveraged to reduce the number of modular additions/subtractions and modular multiplications in radix-4 butterfly unit by 20% and 33%, respectively. To avoid the bit-reversed cost and save memory footprint in arbitrary radix NTT/INTT, we put forward a general method by rearranging the loop structure and reusing the twiddle factors. The hardware-level optimization is achieved by excavating the symmetric operators in radix-4 butterfly unit, which saves almost 50% hardware resources compared to a straightforward implementation. Through experimental results and theoretical analysis, we point out that the radix-4 NTT with the same number of parallel butterfly units outperforms the radix-2 NTT in terms of area-time performance in the interleaved memory system. This advantage is enlarged when increasing the number of parallel butterfly units. For example, when processing 1024 14-bit points NTT with 8 parallel butterfly units, the ATP of LUT/FF/DSP/BRAM n radix-4 NTT core is approximately 2.2 × /1.2 × /1.1 × /1.9 × less than that of the radix-2 NTT core on a similar FPGA platform.

The lattice-based CRYSTALS-Dilithium scheme is one of the three thirdround digital signature finalists in the National Institute of Standards and Technology Post-Quantum Cryptography Standardization Process. Due to the complex calculations and highly individualized functions in Dilithium, its hardware implementations face the problems of large area requirements and low efficiency. This paper proposes several optimization methods to achieve a compact and high-performance hardware architecture for round 3 Dilithium. Specifically, a segmented pipelined processing method is proposed to reduce both the storage requirements and the processing time. Moreover, several optimized modules are designed to improve the efficiency of the proposed architecture, including a pipelined number theoretic transform module, a SampleInBall module, a Decompose module, and three modular reduction modules. Compared with state-of-the-art designs for Dilithium on similar platforms, our implementation requires 1.4×/1.4×/3.0×/4.5× fewer LUTs/FFs/BRAMs/DSPs, respectively, and 4.4×/1.7×/1.4× less time for key generation, signature generation, and signature verification, respectively, for NIST security level 5.

NewHope-NIST is a promising ring learning with errors (RLWE)-based postquantum cryptography (PQC) for key encapsulation mechanisms. The performance on the field-programmable gate array (FPGA) affects the applicability of NewHope-NIST. In RLWE-based PQC algorithms, the number theoretic transform (NTT) is one of the most time-consuming operations. In this paper, low-complexity NTT and inverse NTT (INTT) are used to implement highly efficient NewHope-NIST on FPGA. First, both the pre-processing of NTT and the post-processing of INTT are merged into the fast Fourier transform (FFT) algorithm, which reduces N and 2N modular multiplications for N-point NTT and INTT, respectively. Second, a compact butterfly unit and an efficient modular reduction on the modulus 12289 are proposed for the low-complexity NTT/INTT architecture, which achieves an improvement of approximately 3× in the area time product (ATP) compared with the results of the state-of-the-art designs. Finally, a highly efficient architecture with doubled bandwidth and timing hiding for NewHope-NIST is presented. The implementation results on an FPGA show that our design is at least 2.5× faster and has 4.9× smaller ATP compared with the results of the state-of-the-art designs of NewHope-NIST on similar platforms.