A Critical Look into Threshold Homomorphic Encryption for Private Average Aggregation
Abstract
Threshold Homomorphic Encryption (Threshold HE) is a good fit for implementing private federated average aggregation, a key operation in Federated Learning (FL). Despite its potential, recent studies have shown that threshold schemes available in mainstream HE libraries can introduce unexpected security vulnerabilities if an adversary has access to a restricted decryption oracle. This oracle reflects the FL clients' capacity to collaboratively decrypt the aggregated result without knowing the secret key. This work surveys the use of threshold RLWE-based HE for federated average aggregation and examines the performance impact of using smudging noise with a large variance as a countermeasure. We provide a detailed comparison of threshold variants of BFV and CKKS, finding that CKKS-based aggregations perform comparably to BFV-based solutions.
Keywords
Cite
@article{arxiv.2602.22037,
title = {A Critical Look into Threshold Homomorphic Encryption for Private Average Aggregation},
author = {Miguel Morona-Mínguez and Alberto Pedrouzo-Ulloa and Fernando Pérez-González},
journal= {arXiv preprint arXiv:2602.22037},
year = {2026}
}
Comments
This is the author-submitted version (preprint) of a paper published in the Proceedings of the 2nd IEEE International Conference on Federated Learning Technologies and Applications (FLTA 2024). The final version is available in IEEE Xplore: https://doi.org/10.1109/FLTA63145.2024.10840167