International Association
for Cryptologic Research

The magic of Fully Homomorphic Encryption (FHE) is that it allows operations on encrypted data without decryption. Unfortunately, the slow computation time limits their adoption. The slow computation time results from the vast memory requirements (64Kbits per ciphertext), a bootstrapping key of 1.3 GB, and sizeable computational overhead (10240 NTTs, each NTT requiring 5120 32-bit multiplications). We accelerate the FHEW bootstrapping in hardware on a high-end U280 FPGA.

To reduce the computational complexity, we propose a fast hardware NTT architecture modified from with support for negatively wrapped convolution. The IP module includes large I/O ports to the NTT accelerator and an index bit-reversal block. The total architecture requires less than 225000 LUTs and 1280 DSPs.

Assuming that a fast interface to the FHEW bootstrapping key is available, the execution speed of FHEW bootstrapping can increase by at least 7.5 times.