To address critical challenges in user privacy and transmission data security in cross-domain environments, as well as practical requirements for user portability and lightweight nodes in cross-domain communication system, this study proposes a blockchain-based secure cross-domain data transmission scheme under highly adversarial conditions. The scheme achieves: 1) decentralized public key management and identity authentication; 2) portable system access for users; 3) trusted key negotiation among communication nodes; 4) key-leakage resistant cross-domain data transmission. The security of the proposed scheme is formally analyzed and proven. Furthermore, a system prototype is implemented to evaluate performance. Experimental results indicate that computation, storage, and communication overheads are all efficiently managed, demonstrating the scheme’s practicality and scalability.