Efficient LLM Adversarial Training via Low-Rank Defense and Circuit-Guided Surrogates

Adversarial training is one of the most effective defenses against adversarial attacks, yet the computational cost remains prohibitive at modern scales, especially for large language models (LLMs). While existing mitigation strategies, e.g., latent adversarial training (LAT), have been developed, they still incur a high computational cost. In this work, focusing on LLM-based classifiers, we investigate computation-efficient strategies to speed up LAT from two complementary perspectives: (1) Defense-side optimization: We explore representation fine-tuning (ReFT) within LAT, and reveal a potential issue if there is a mismatch on which tokens to apply ReFT and the attack. (2) Attack-side optimization: When computing adversarial attacks in each LAT iteration, we extract only the relevant circuits from the LLM to construct a lightweight surrogate model, avoiding the computation in the forward-backward passes through the full model during the attack generation. For both perspectives, we provide theoretical justifications and numerical evidence to illustrate the effectiveness of the proposed strategies. Compared to standard LAT with full fine-tuning, our method on average reduces per-step adversarial-training FLOPs by 29.6% while requiring only 0.0118% trainable parameters, at a moderate cost in robustness.