VeCT: Secure and Efficient Constant-Time Code Rewriting with Vector Extensions

Timing channels allow attackers to extract secrets by analyzing the execution time of a victim program. Constant-time (CT) disciplines enforce security against timing attacks via data-flow/control-flow linearization (DFL/CFL). However, the rewritten constant-time code typically considerably increases the memory footprint of the original code, causing significant overhead. We present VeCT, a compiler-based code rewriter that leverages vector extensions to retain constant-time guarantees while improving performance. We first apply rigorous statistical tests to derive practical "safe-use" rules for AVX-512 instructions whose implementation details are proprietary; this analysis also reveals a previously unknown vulnerability in a state-of-the-art constant-time rewriter. Guided by these rules, VeCT introduces a novel strategy that eliminates unnecessary data loads in rewritten code, and enables vectorization to further improve efficiency. We implement VeCT based on LLVM to automatically transform code into AVX-512-based constant-time equivalents. On real-world applications like AES and Blowfish, VeCT reduces the overhead of transformed code by up to 98.9% compared to the state-of-the-art, while preserving constant-time behavior.