ReFHE-NTT: Resource-Driven NTT FPGA Architecture for Fully Homomorphic Encryption

Fully Homomorphic Encryption (FHE) enables privacy-preserving computation on encrypted data at a high computational cost. Among the existing schemes, CKKS is gaining traction thanks to its support for approximate arithmetic over real numbers. In such a scheme, the Number Theoretic Transform (NTT) is dominant, involving intensive modular arithmetic, twiddle-factor handling, and nontrivial memory access patterns. As a result, NTT acceleration has gained significant interest, with many FPGA designs achieving high throughput by aggressively exploiting device resources. While effective for NTT-centric workloads, this approach is ill-suited to full FHE pipelines, where the NTT must coexist with other compute-intensive kernels. Thus, we propose ReFHE-NTT, a resource-efficient NTT accelerator tailored to such settings. Our design combines on-the-fly twiddle-factor fusion with specialized modular arithmetic for pseudo-Mersenne primes, reducing memory footprint and arithmetic cost without storing precomputed tables. We co-design and validate the accelerator on the KV260 MPSoC, supporting polynomial degrees log N Element; 12...16 and CKKS parameter sets with moduli up to 64 bits per prime. To the best of our knowledge, ReFHE-NTT is the first solution targeting embedded platform to support full-scale CKKS parameter sets. Compared to prior FPGA designs, it achieves up to a 20.2× improvement in slice-equivalent efficiency over the fastest open-source accelerator and a 1.98× improvement over the most resource-efficient one. Integrated into HEAAN CKKS library, ReFHE-NTT delivers top end-to-end speedup of 15× for encoding and 7.9× for encryption, demonstrating how resource-driven NTT design can substantially improve FHE performance on embedded platforms.