Lattice-based privacy-preserving multimodal retrieval for healthcare

Multimodal data plays a vital role in advancing personalized diagnosis and precision medicine. However, during cross-institutional sharing and collaborative analysis, the protection of patient privacy becomes increasingly critical, particularly in terms of the secure storage and fine-grained retrieval of sensitive medical data. Existing privacy-preserving technologies fail to meet the demands of secure and efficient retrieval over multimodal medical data. To address this challenge, we propose a generic multi-user multimodal searchable encryption framework for healthcare applications, which supports cross-modal retrieval based on trapdoors generated from ciphertexts corresponding to arbitrary modalities. We further design a distributed-decryption searchable encryption scheme, which is the first to combine AudioCLIP and multi-key fully homomorphic encryption for efficient retrieval of encrypted multimodal data. Additionally, we construct an attribute-based multimodal searchable encryption scheme as a complementary solution for implementing fine-grained access control. This enables flexible and controllable management of retrieval permissions over multimodal ciphertexts. Experimental results on MedMNIST and AudioSet demonstrate that our schemes achieve high retrieval efficiency and quantum-resistant security, meeting the requirements of real-world medical applications.