Efficient Federated Learning with Low-Rank Updates under Homomorphic Encryption

Federated Learning has been widely adopted for its ability to collaboratively train models without exposing raw data. However, the server-side aggregation process may still leak sensitive information about client data. Homomorphic Encryption enables privacy-preserving aggregation, but it introduces substantial communication overhead for clients and high computational costs for the server. To address these challenges, we propose HEAL-FL, a federated learning framework that is based on low-rank shared basis vectors across clients. Instead of transmitting full encrypted model updates, clients send only encrypted low-rank coefficients, thereby reducing both communication costs and server-side aggregation overhead. Furthermore, HEAL-FL incorporates a communication-efficient basis update scheme that relies exclusively on homomorphic addition at the server. Our evaluation across various homomorphic encryption schemes shows that HEAL-FL reduces client communication and server aggregation costs, leading to improved efficiency of Federated Learning systems. Notably, these savings translate into up to a significant reduction of 38.6% in total training time compared to conventional homomorphic FedAvg with full model parameter transmission, demonstrating the practical benefits of our approach.