Paper 2025/1933 Original: eprint.iacr.org/2025/1933 ↗

Revisiting Multi-Key Blind Rotation: Optimized NTRU-based Bootstrapping for MKFHE

Abstract

Multi-key fully homomorphic encryption (MKFHE) extends the capability of fully homomorphic encryption by enabling homomorphic computations on ciphertexts encrypted under different keys. Multi-key blind rotation is the core and most computationally intensive component of MKFHE. The NTRU-based multi-key blind rotation proposed by Xiang et al. (ASIACRYPT 2024) has the potential to achieve smaller key sizes, faster blind rotation, and lower noise compared to its RLWE-based counterpart. However, while the multi-key blind rotation in the RLWE-based scheme proposed by Kwak et al. (PKC 2024) remains compatible with their single-key counterpart, the NTRU-based versions do not. This motivates our work to advance NTRU-based schemes in both efficiency and compatibility. We propose a novel workflow for NTRU-based multi-key blind rotation that achieves compatibility with its single-key counterpart. Our approach significantly reduces both computational and storage complexity compared to the state-of-the-art NTRU-based design, while maintaining a comparable noise magnitude. Building upon this workflow, we further construct two MKFHE schemes for bootstrapping multi-key LWE ciphertexts and multi-key matrix NTRU ciphertexts, both supporting a super-constant number of parties with respect to the ring dimension $N$. Experimental results demonstrate that our method outperforms existing NTRU-based bootstrapping for TFHE-like MKFHE in both computational efficiency and bootstrapping key size. Specifically, our 2-key gate bootstrapping takes only 26ms and requires a bootstrapping key of size 7.34MB, achieving a 3.1× speedup and a 1.9× key size reduction compared to prior NTRU-based works.