Breaking $\mathsf{TraintoDefend}$ via Adaptive Neuron Clustering in Cryptanalytic Model Extraction
Cryptanalytic model extraction poses a fundamental threat to deep learning model confidentiality by enabling precise parameter recovery with theoretical guarantees of functional equivalence. To counter this growing threat, $\mathsf{TraintoDefend}$ (NeurIPS 2025) was proposed as the first dedicated defense against cryptanalytic extraction and remains the only defense believed to be effective to date. It encourages neurons to learn similar weights so that they switch activation states together, making their individual contributions harder to distinguish. However, we notice that neurons with similar weights need not switch activation states together under small perturbations. Instead, weight similarity can cause the original clustering rule to merge signatures from distinct neurons when only a subset of their normalized weight coordinates agree. Motivated by this observation, we introduce $\mathsf{SimBreak}$, an adaptive clustering rule that breaks the weight-similarity defense of $\mathsf{TraintoDefend}$. $\mathsf{SimBreak}$ estimates a relative scale from all jointly observed weight coordinates and uses the same scale to test both weight direction and bias consistency. Across six protected fully connected networks trained on MNIST, $\mathsf{SimBreak}$ raises target layer signature recovery from $0\%$ to $100\%$, with a maximum relative recovery error of $2^{-22.27}$. Our work challenges the use of neuron weight similarity as a sufficient basis for protection and establishes adaptive evaluation as essential to assessing cryptanalytic extraction defenses.