Quantum-resistant homomorphic encryption for privacy-preserving demand flexibility and grid modernization: A systematized review

Demand flexibility programs could unlock 180 GW of building capacity in the United States by 2030, which is enough to avoid building nearly 200 large power plants. However, a fundamental conflict stalls this progress. Grid operators need granular consumer data to coordinate these resources, but privacy risks deter the participation required to unlock this capacity. Without privacy-preserving solutions, utilities must build expensive infrastructure instead of leveraging existing flexible loads. Homomorphic encryption (HE) offers a cryptographic solution by enabling computation directly on encrypted data, yet a comprehensive review evaluating whether quantum-secure HE schemes can meet the operational requirements of grid modernization and demand flexibility has been lacking. Quantum-resistant schemes, built on lattice-based cryptography resistant to attacks from both classical and quantum computers, are essential as adversaries can harvest encrypted data today and decrypt it once quantum computers mature. This is a critical concern given power infrastructure's multi-decade operational lifetime. This systematized review assesses 34 studies (with database searches through May 2025) against demand flexibility requirements, revealing critical gaps between current capabilities and deployment needs. We introduce a six-level taxonomy comprising Application Context, Operational Requirements, Threat & Security Model, HE Scheme, System Architecture, and Performance & Deployment to connect application needs with cryptographic design and deployment outcomes. Our analysis reveals that research is concentrated in distribution-level applications (79%), with energy theft monitoring/detection (29%) and smart meter data aggregation (26%) as the dominant workloads. However, the field's maturity is nascent, with 97% of studies remaining in proof-of-concept or lab prototype stages, and performance limitations create orders-of-magnitude gaps for real-time control. While the field has adopted advanced schemes like Cheon-Kim-Kim-Song (CKKS) (used in 53% of total studies), only 18% of applications target sub-minute operational timescales. We identify five critical research opportunities to bridge this theory-practice gap, including developing low-latency protocols and standardized benchmarks. Our findings provide a roadmap for future work, concluding that while quantum-resistant HE holds long-term promise, significant advances in algorithmic efficiency and hardware acceleration are required for widespread deployment in next-generation energy systems.