From one-one to one-many: ORCA enables scalable and revocable group covert communication on blockchain

The decentralized and immutable nature of blockchainprovides a resilient foundation for covert communication in adversarial and untrusted environments, specifically in scenarios requiring unobservable multi-recipient messaging. Most existing schemes, however, are limited to one-to-one transmission and lack mechanisms to handle untrusted receivers, which constrains their scalability and security. To address these challenges, we propose ORCA (Orthogonal Covert Architecture), a group covert communication framework based on strictly orthogonal, integer-valued codewords. ORCA selects codewords from a Hadamard matrix and applies secret column permutations to ensure decoding isolation and resistance against inference attacks. Each receiver recovers only its assigned message through projection, without coordination or leakage. This encoding structure supports scalable embedding, seamless receiver revocation, and clean integration with standard transaction fields. In contrast to prior work, we analyze the impact of imperfect orthogonality and provide theoretical bounds on decoding interference. Extensive experiments on real-world Bitcoin blockchain data and comparative evaluation against representative covert communication schemes confirm ORCA’s robustness, high embedding capacity, and statistical indistinguishability from normal blockchain activity. These results establish ORCA as a scalable and secure solution for multi-recipient covert communication in adversarial environments.