BCDH: Blockchain-based covert Diffie-Hellman key exchange scheme

The purpose of the covert communication scheme is to conceal the communication behavior entirely. In such schemes, the sender and receiver rely on secret keys to establish a covert channel. However, conventional key exchange protocols would expose the key exchange process between them. An adversary who observes the key exchange would be aware of the existence of communication behavior. The keys used in covert communication are not suitable to be generated through conventional key exchange schemes. To address this, we propose a blockchain-based covert elliptic-curve Diffie-Hellman key exchange scheme (BCDH) to conceal the process of the key exchange in blockchain transactions. Following a straightforward setup, BCDH allows the sender and receiver to covertly exchange a secret key on a blockchain without direct communication. Furthermore, we expand the BCDH approach to operate across multiple blockchains, further enhancing its covertness and stability. We analyze BCDH from several perspectives, including covertness, security, randomness, etc. Additionally, we implement a prototype of BCDH on the Ethereum platform to assess its feasibility and performance. Our evaluation demonstrates that BCDH is efficient and well-suited for real-world applications.