Covert channels in blockchain and blockchain based covert communication: Overview, state-of-the-art, and future directions

Traditional network covert channels have become insecure due to the continuous improvement of traffic analy-sis techniques. As an emerging technology combined with cryptographic techniques, consensus algorithms, P2P network, blockchain has features like decentralization, traceability, immutability, anonymity, transparency, and security, which makes blockchain an ideal platform for covert channel and covert communication. Benefits of blockchain for covert communication include wide access, high capacity covert channels, identity anonymity and information concealment, and robust communication channel. In the paper, we conduct a systematic analysis on covert channels in blockchain from the layer architecture of blockchain. Covert channels are present in data layer, network layer, incentive layer and contract layer, as block structure, transaction structure, cryptographic schemes, P2P network, transaction fee, and smart contract. There are also various covert channels in different layers of blockchain applications. We make a literature review on covert channels in blockchain applications and blockchain based covert communication schemes. Current researches on blockchain based covert communication mainly focus on blockchain based cryptocurrencies, including Bitcoin, Ethereum, Zcash and Monero. There are also some explorations which combine blockchain with images to achieve higher channel capacity for covert communication. Finally, open challenges and future directions on blockchain based covert communication are discussed.