Exploring a Resource-Efficient NTT FPGA Accelerator for Fully Homomorphic Encryption

CKKS encryption scheme stands as one of the most valuable solutions for Fully Homomorphic Encryption (FHE), enabling privacy-preserving computation on encrypted data, at the cost of high computational bottlenecks. In such a scheme, the Number Theoretic Transform (NTT) consumes most of the computational resources due to irregular memory access patterns. Thus, literature accelerates this step on specific hardware devices, such as FPGAs, often exhausting device resources while gaining performance and energy efficiency improvements. However, this prevents further utilization of the FPGA to accelerate other compute-intensive stages. As an alternative, we perform the HW/SW co-design of resource-efficient solutions by integrating them into well-known software libraries implementing CKKS encryption scheme. In particular, we deploy on a Kria KV260 SoC a resource-efficient NTT accelerator with state-of-the-art security parameters (logN ∈ 12,...,16 and logQ ∈ [32,64]), and integrate it into the full-RNS HEANN library - the reference implementation for CKKS scheme. By doing so, we obtain up to 4.47× and 3.63× speedup in the encoding and encryption steps, respectively, while minimizing hardware consumption. These results show the end-to-end improvements achievable without fully utilizing the FPGA resources, leaving headroom for accelerating additional stages of the encryption pipeline.