## 3 Related Work
* **TrustORAN: Decentralized Zero-Trust with Blockchain** (Milestone): Deployment and testing of a Blockchain-centric, decentralized zero-trust framework for xApp verification and access control across both private and public Blockchains in O-RAN. Reference: [37]. * _Significance_: Empirically demonstrates scalable, decentralized access/authentication, but integration with FL is not realized. * **HSADR: Secure Aggregation + DP for FL in RAN** (Milestone): An FL scheme (HSADR) featuring IND-CCA2 secure aggregation, dropout resilience, and differential privacy using consortium Blockchain. Includes experimental validation under RAN conditions. Reference: [44]. * _Significance:_ Pushes the privacy/security boundary for FL in radio access environments but with limited focus on O-RAN RIC/xApp architectural interfaces or zero-trust orchestration. _2025_
* **Blockchain-based FL Reputation System for O-RAN** (Milestone): Conceptualization and demonstration (on Polygon testnet) of a DApp for on-chain, automated reputation tracking/verification in O-RAN FL, managed by smart contracts and oracles. Reference: [43]. * _Significance:_ Advances reputation and accountability in FL for O-RAN, utilizing Blockchain for transparency/permissioning but lacks architectural zero-trust integration or testbed-based empirical performance. _Trends and Patterns_  
1. **Early Siloing of ZT and FL (2022-2023):** Initial works focused on either O-RAN zero-trust (ZT) security architectures or FL for network optimization and intrusion detection, rarely combining both. 2. **Emergence of Empirical Realism and Testbeds (2023-2024)**: Increasing emphasis on empirical testbeds (OAIC, Colosseum, Polygon testnet, Blockchain deployments) for validation, especially for either ZT [36; 38] or FL [39; 40; 41; 42; 44]. Most testbed work remained within a single domain (ZT or FL), not offering a joint, end-to-end solution. 3. **Shift to Integrated, Decentralized Trust Solutions (Late 2023-2024)**: Rapid adoption of Blockchain and decentralized trust fabrics for access control and reputation mechanisms [37; 43; 44; 45]. Simultaneous evolution toward more advanced privacy protections in FL--differential privacy, secure aggregation [44]. 2024 marks the first proposal of a truly integrated ZT + FL architecture (ZTORAN, [45]) in the O-RAN RIC/xApp context, albeit with simulation-only validation. 4. **Trend Toward Architectures Supporting Accountability and Privacy**: Blockchain/smart contract DApps for reputation [43] and cross-domain authentication [37; 45] become more common. Security proofs (e.g., IND-CCA2) for FL aggregation [44] reflect increasing rigor in privacy guarantees, though not always fully O-RAN-specific. _Significance_  
* **Clear Evolution:** The field migrates from isolated domain solutions (ZT or FL) toward integration, with empirical validation and advanced cryptographic privacy controls being increasingly valued. * **Bottleneck:** No paper achieves physical or fully O-RAN testbed-backed validation of an integrated ZT + FL architecture for RIC/xApps under 6G conditions; most validations remain in simulations or partial proofs-of-concept. * **Current Focus and Gaps:**[45] provides the conceptual milestone for ZT + FL integration for O-RAN, but physical instantiation and measurement against 6G latency/security targets remain an open challenge. Blockchain is now maturely embedded for distributed trust, but micro-segmentation, attestation, and dynamic trust enforcement consistent with strict zero-trust principles are still limited. Table 3 summarizes the key developments in Zero Trust (ZT) and FL for the O-RAN RIC/xApp ecosystems over the past few years. The timeline spans from 2022 to 2025, highlighting important milestones and the methodologies used to validate each approach. In 2022, the Federated DRL for O-RAN Slicing introduced Federated Deep Reinforcement Learning (FL + DRL) for RAN optimization, as detailed in [40].