International Association
for Cryptologic Research

Fully Homomorphic encryption (FHE) allows computation without decryption, but often suffers from a ciphertext expansion ratio and overhead. On the other hand, AES is a widely adopted symmetric block cipher known for its efficiency and compact ciphertext size. However, its symmetric nature prevents direct computation on encrypted data. Homomorphic transciphering bridges these two approaches by enabling computation on AES-encrypted data using FHE-encrypted AES keys, thereby combining the compactness of AES with the flexibility of FHE.

In this work, we present a high-throughput homomorphic AES transciphering based on the CKKS scheme. Our design takes advantage of the ring conversion technique to the conjugate-invariant ring \cite{real_heaan} during the transciphering circuit, including bootstrapping, along with a suite of optimizations that reduce computational overhead. As a result, we achieved a throughput (per-block evaluation time) of 0.994ms—less than 1ms— outperforming the state-of-the-art \cite{xboot} by $1.58\times$ when processing $2^{15}$ AES-128 blocks under the same implementation environment, and support processing $2^{9}$ blocks within $3s$ on a single GPU.