Fog computing enables low-latency IoT applications but introduces critical privacy risks when fog nodes are untrusted or compromised. Current privacy-preserving frameworks address either cloud security or basic fog-layer encryption, yet none provide comprehensive user-centric privacy enforcement with fine-grained preference composition for distributed IoT-fog-cloud architectures. However, the integration of fog computing–essential for reducing latency in time-critical IoT applications–introduces significant privacy risks when fog nodes are untrusted or compromised. Existing privacy-preserving frameworks primarily address either cloud security or basic fog-layer encryption, but fail to provide comprehensive, user-centric privacy enforcement that accommodates fine-grained preferences, multi-source data fusion, and regulatory compliance in distributed IoT-fog-cloud architectures. This paper presents PrivacyGuard, a novel four-tier privacy-preserving framework specifically designed for personal IoT data protection where fog infrastructure may be untrusted. PrivacyGuard introduces several key innovations: a dedicated edge layer enabling users to specify hierarchical privacy preferences with exceptions and prohibitions through intuitive interfaces; hierarchical data category and purpose taxonomies supporting fine-grained privacy control while maintaining GDPR compliance; privacy preference composition mechanisms automatically deriving least-privilege policies when fusing multi-source data; Trusted Execution Environment (TEE)-based privacy validation at fog nodes enabling secure computation on encrypted data without exposing sensitive information to potentially malicious operators; and hash-based validation result caching optimized for high-latency rural networks. We demonstrate through emulation that PrivacyGuard achieves sub-100ms single-request P99 latency (97.03ms), with graceful degradation to 2,059ms P99 under 100 concurrent users, 91.7% MITM resistance, and 6.37 × cache speedup.
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