Establishing efficient and robust covert channels is crucial for secure communication within insecure network environments. With its inherent benefits of decentralization and anonymization, blockchain has gained considerable attention in developing covert channels. To guarantee a highly secure covert channel, channel negotiation should be contactless before the communication, carrier transaction features must be indistinguishable from normal transactions during the communication, and communication identities must be untraceable after the communication. Such a full-lifecycle covert channel is indispensable to defend against a versatile adversary who intercepts two communicating parties comprehensively (e.g., on-chain and off-chain). Unfortunately, it has not been thoroughly investigated in the literature. We make the first effort to achieve a full-lifecycle covert channel, a novel blockchain-based covert channel named ABC-Channel. We tackle a series of challenges, such as off-chain contact dependency, increased masquerading difficulties as growing transaction volume, and time-evolving, communicable yet untraceable identities, to achieve contactless channel negotiation, indistinguishable transaction features, and untraceable communication identities, respectively. We develop a working prototype to validate ABC-Channel and conduct extensive tests on the Bitcoin testnet. The experimental results demonstrate that ABC-Channel achieves substantially secure covert capabilities. In comparison to existing methods, it also exhibits state-of-the-art transmission efficiency.

翻译：在不安全的网络环境中，建立高效且稳健的隐蔽信道对于安全通信至关重要。凭借其去中心化和匿名化的固有优势，区块链在开发隐蔽信道方面获得了广泛关注。为了保证高度安全的隐蔽信道，通信前的信道协商应实现无接触，通信中的载体交易特征必须与正常交易不可区分，通信后的身份必须不可追踪。这种全生命周期的隐蔽信道对于防御能够全面拦截通信双方（例如链上和链下）的全能型对手而言不可或缺。然而，文献中尚未对此进行深入探究。我们首次尝试实现全生命周期隐蔽信道，即一种名为ABC-Channel的新型区块链隐蔽信道。我们解决了一系列挑战，例如链下接触依赖性、随交易量增长而增加的伪装难度，以及随时间演化、可通信但不可追踪的身份，从而分别实现无接触信道协商、不可区分的交易特征和不可追踪的通信身份。我们开发了一个工作原型来验证ABC-Channel，并在比特币测试网上进行了广泛测试。实验结果表明，ABC-Channel实现了高度安全的隐蔽能力。与现有方法相比，它还展现出最先进的传输效率。