  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.
