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Shuffling Is Not Enough: Breaking Permutation-Based Model Confidentiality (arxiv.org)
1 point by sbulaev 19 days ago | hide | past | pdf | discuss on HN

In plain words: Systems that hide a model by shuffling and noising layer outputs can be broken: one query more than the layer has inputs reveals it exactly. The attack rebuilt a ResNet-20's layers with zero error using 5,712 queries, and also worked on large image models.

Abstract · Shuffling is Not Enough: Breaking Permutation-Based Model Confidentiality in Hybrid FHE Inference

Hybrid fully homomorphic encryption~(FHE) inference improves the practicality of private inference by letting the server evaluate linear layers homomorphically while the client decrypts and applies nonlinearities. Recent schemes attempt to protect model confidentiality by returning noisy, output-permuted responses and appealing to shuffle-model differential privacy~(DP). We show that this protection fails in the correctness regime required by hybrid FHE systems. For a $d$-input linear layer, $d+1$ admissible queries suffice for exact recovery of a permutation-invariant layer summary, hence for perfect model distinguishability. We further show that input DP is orthogonal to model confidentiality and that the local-DP premise required for shuffle amplification cannot hold under correctness-bounded noise. We recover all linear layers of a \safhire{}-style ResNet-20 end-to-end from TFHE transcripts with zero error, using $d+1$ queries per layer for a total of $5{,}712$ direct queries. Under the same query model, we also confirm exact per-layer recovery on pretrained ImageNet-scale CNNs and ViT-B/16. The leaked spectra enable fingerprinting, lineage attribution, and improved logit-based extraction, while suppressing them destroys inference utility.

Jiseung Kim, Hyung Tae Lee
arXiv:2609.12911 · cs.CR · submitted Sep 11, 2026
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