GP-STCKS: A GeoPrefix-based Spatio-Temporal Controllable Keyword Search Scheme for Privacy-Preserving Electronic Health Systems

The proliferation of cloud-based e-health systems necessitates outsourcing sensitive Electronic Health Records (EHRs), raising critical privacy and access control concerns. While Attribute-Based Keyword Search (ABKS) enables secure encrypted retrieval, existing schemes inherently lack fine-grained spatio-temporal constraints. To address this, we propose GP-STCKS, a novel privacy-preserving spatio-temporal controllable keyword search scheme. By seamlessly integrating Geohash hierarchical indexing into the Ciphertext-Policy ABKS (CP-ABKS) framework, our scheme enforces strict dual access constraints over specific time intervals and spatial hierarchies. Rigorous formal analysis proves its resilience against chosen-keyword and multi-dimensional key collusion attacks while guaranteeing keyword confidentiality. Extensive experiments demonstrate that GP-STCKS maintains lightweight overhead and optimal search efficiency, perfectly balancing stringent privacy protection with real-world e-health deployability.