International Association for Cryptologic Research

International Association
for Cryptologic Research


Multi-Parameter Support with NTTs for NTRU and NTRU Prime on Cortex-M4

Erdem Alkim , Dokuz Eylul University, Izmir, Turkey
Vincent Hwang , National Taiwan University, Taipei, Taiwan; Academia Sinica, Taipei, Taiwan
Bo-Yin Yang , Academia Sinica, Taipei, Taiwan
DOI: 10.46586/tches.v2022.i4.349-371
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Abstract: We propose NTT implementations with each supporting at least one parameter of NTRU and one parameter of NTRU Prime. Our implementations are based on size-1440, size-1536, and size-1728 convolutions without algebraic assumptions on the target polynomial rings. We also propose several improvements for the NTT computation. Firstly, we introduce dedicated radix-(2, 3) butterflies combining Good–Thomas FFT and vector-radix FFT. In general, there are six dedicated radix-(2, 3) butterflies and they together support implicit permutations. Secondly, for odd prime radices, we show that the multiplications for one output can be replaced with additions/subtractions. We demonstrate the idea for radix-3 and show how to extend it to any odd prime. Our improvement also applies to radix-(2, 3) butterflies. Thirdly, we implement an incomplete version of Good–Thomas FFT for addressing potential code size issues. For NTRU, our polynomial multiplications outperform the state-of-the-art by 2.8%−10.3%. For NTRU Prime, our polynomial multiplications are slower than the state-of-the-art. However, the SotA exploits the specific structure of coefficient rings or polynomial moduli, while our NTT-based multiplications exploit neither and apply across different schemes. This reduces the engineering effort, including testing and verification.
  title={Multi-Parameter Support with NTTs for NTRU and NTRU Prime on Cortex-M4},
  journal={IACR Transactions on Cryptographic Hardware and Embedded Systems},
  publisher={Ruhr-Universität Bochum},
  volume={2022, Issue 4},
  author={Erdem Alkim and Vincent Hwang and Bo-Yin Yang},