Polynomial Adaptation of Large-Scale CNNs for Homomorphic Encryption-Based Secure Inference

Enabling secure inference of large-scale CNNs using Homomorphic Encryption (HE) requires a preliminary step for adapting unencrypted pre-trained models to only use polynomial operations. Prior art advocates for high-degree polynomials for accurate approximations, which comes at the price of extensive computations. We demonstrate that low-degree polynomials can be sufficient for accurate approximation even for large-scale DNNs. For that, we introduce a dedicated fine-tuning process on unencrypted data that reduces the input range to the activation functions. The resulting models have competitive accuracy of up to 3.5% degradation from the original non-polynomial model, which outperforms prior art on tasks such as ImageNet classification over ResNet and ConvNeXt. Upon adaptation, these models can process HE-encrypted samples and are ready for secure inference. Based on these, we provide optimization insights for activation functions and skip connections, enhancing HE evaluation efficiency. We evaluated ResNet50-152 on encrypted ImageNet samples, an accomplishment not previously reached by polynomial networks, in just 3:13–7:12 min, using commodity hardware under the CKKS scheme with 128-bit security. In comparison to prior high-degree polynomial solutions, our low-degree polynomials boost evaluation latency, for example, by 3× for ResNet-50 and CIFAR-10. We further show our approach versatility, by adapting the CLIP model for secure zero-shot predictions, highlighting new potential in HE and transfer learning.