Public Diffusion Models, Private Images: Key-Controlled Inversion for Conditional Reconstruction
Diffusion models are often deployed in settings where model parameters are publicly accessible (e.g., open-source libraries or released checkpoints). This white-box scenario creates a serious security risk: any user who obtains an intermediate latent representation can invert the process to recover the original input image. Most prior work on access control for generative models assumes a black-box model (i.e., parameters are kept secret), typically under an honest-but-curious adversary. By contrast, we address the more challenging and realistic white-box setting where all parameters are public. We present a key-controlled inversion framework that turns the inherent error propagation of diffusion models, which exponentially amplifies small perturbations, into a security asset. By injecting key-dependent noise into the inversion formula, we ensure that only a user with the correct key can reconstruct the original image; any other key yields unrecognizable output. Theoretically, by leveraging existing error-propagation theory for diffusion models, we prove that the resulting ciphertext distribution is IND-CPA secure and derive that the adversary's advantage is exponentially small in a tunable security parameter, hence negligible for any probabilistic polynomial-time (PPT) adversary. Experimentally, we validate these security guarantees across several models and datasets and further demonstrate cross-model robustness, that the injected key noise does not amplify the performance drop caused by model discrepancies.