ULD-Net: Enabling Ultra-Low-Degree Fully Polynomial Networks for Homomorphically Encrypted Inference

Fully polynomial neural networks—models whose computations comprise only additions and multiplications—are attractive for privacy-preserving inference under homomorphic encryption (HE). Yet most prior systems obtain such models by post-hoc replacement of nonlinearities with high-degree or cascaded polynomials, which inflates HE cost and makes training numerically fragile and hard to scale. We introduce ULD-Net, a pretraining methodology that enables ultra-low-degree (multiplicative depth ≤ 3 for each operator) fully polynomial networks to be trained from scratch at ImageNet and transformer scale while maintaining high accuracy. The key is a polynomial-only normalization, PolyNorm, coupled with a principled choice of normalization axis that keeps activations in a well-conditioned range across deep stacks of polynomial layers. Together with a special set of polynomial-aware operator replacements, such as polynomial activation functions and linear attention, ULD-Net delivers stable optimization without resorting to high-degree approximations. Experimental results demonstrate that ULD-Net outperforms several state-of-the-art open-source fully and partially polynomial approaches across both CNNs and ViTs on diverse datasets, in terms of both accuracy and HE inference latency. Specifically, ULD-Net achieves +0.39% accuracy and a 2.76× speedup compared to the best fully polynomial baseline; up to +3.33% accuracy and a 3.17× speedup compared to the best partial polynomial baseline. Applying ULD-Net to ViT-Small and ViT-Base yields 76.7% and 75.2% top-1 accuracy on ImageNet, demonstrating the first fully polynomial models scaled to the ViT/ImageNet level.