zero-order-federated-learning-he
Privacy-enhanced zero-order federated learning using multi-key homomorphic encryption (xMK-CKKS) over wireless channels. Methodology for secure FL aggregation without channel estimation, supporting N-1 client compromise tolerance. Use when designing privacy-preserving federated learning, multi-key HE protocols, or wireless FL systems.
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- June 4, 2026 at 13:32
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- zero-order-federated-learning-he
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- Privacy-enhanced zero-order federated learning using multi-key homomorphic encryption (xMK-CKKS) over wireless channels. Methodology for secure FL aggregation without channel estimation, supporting N-1 client compromise tolerance. Use when designing privacy-preserving federated learning, multi-key HE protocols, or wireless FL systems.
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- {"arxiv_id":"2605.30123","published":"2026-05-28","authors":"Anthony Ayli, Khalil Harris, Jihad Fahs, Mohamad Assaad","tags":["federated-learning","homomorphic-encryption","xmk-ckks","privacy","wireless-ml","zero-order"]}
# Zero-Order Federated Learning with Multi-Key HE
## Overview
Privacy-preserving federated learning protocol combining zero-order FL (one encrypted scalar per device per round) with multi-key homomorphic encryption (xMK-CKKS) over wireless channels — **without channel estimation or pre-equalization**.
## Core Architecture
### Protocol Design (Four-Phase)
1. **Partial Public Key Retransmission**: Devices transmit partial public keys through the same channel realization
2. **Ciphertext Retransmission**: Partial ciphertexts retransmitted through same channel
3. **Algebraic Cancellation**: Dominant large-modulus encryption terms cancel during decryption (same channel → same fading)
4. **Aggregation**: Server aggregates encrypted scalars, decryption noise preserves O(1/√K) convergence rate
### Key Innovation
- **No channel estimation required**: Same-channel retransmission enables algebraic cancellation of fading terms
- **Client-level security**: Each device has its own secret key (multi-key HE vs single-key)
- **Compromise tolerance**: Secure against honest-but-curious server colluding with up to N-1 clients
- **Communication efficiency**: Overhead independent of model dimension (one scalar per round)
### Convergence Analysis
Decoded encryption noise preserves O(1/√K) convergence rate up to negligible noise floor, validated on MNIST with slowly varying LoS-dominant channels.
## When to Use
- Privacy-preserving FL with untrusted clients
- Wireless FL with channel fading (LoS-dominant, slowly varying)
- Scenarios where single-key HE is insufficient (multi-party trust)
- Bandwidth-constrained FL (zero-order scalar transmission)
## Pitfalls
- Requires slowly varying channel (retransmission assumes same channel realization)
- LoS-dominant channels work best; rich scattering may break cancellation
- Multi-key HE has higher computation overhead than single-key
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*Reference: Ayli et al. (2026) "Privacy-Enhanced Zero-Order Federated Learning via xMK-CKKS over Wireless Channels" arXiv:2605.30123*
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