GAPDiS: Gradient-Assisted Perturbation Design via Sequence Editing for Website Fingerprinting Defense

As deep learning-based website fingerprinting (WF) attacks become increasingly accurate, user privacy faces mounting risks. Existing defenses struggle with the discrete nature of packet direction sequences, rendering gradient-based optimization infeasible and leading to inefficient, heuristic-based perturbation solutions. We propose a novel defense framework that bridges this gap by introducing gradient---aligned offset vectors and a cosine similarity---based reward to evaluate and select perturbation candidates aligned with the gradient direction. We further design a parallel reward computation algorithm to improve efficiency and integrate it into GAPDiS, a universal perturbation generation method that combines gradient guidance with improved tabu search for global optimization. For practical deployment, GAPDiS supports both PT bridge and P4 switch implementations. Experiments on the AWF dataset show that GAPDiS reduces the classification accuracy of WF models from over 98% to below 7% with only 2.56% bandwidth overhead---achieving a 68.1% improvement over state-of-the-art methods.