Blockchain uses a peer-to-peer (P2P) network, rather than a central authority (e.g., a central bank or financial institution), to conduct transactions (e.g., financial transactions). Due to the decentralized nature, blockchain allows one to undertake and verify transactions in real-time. In recent times, a number of blockchain-based applications have been presented in the literature. One such popular real-world example application is Bitcoin. However, there are underlying security and privacy issues. For example, once a public key has been linked with an individual’s identity, one could browse prior transactions on the blockchain and view all transactions associated with the particular public key. One key research challenge is how to balance security and privacy of an individual with accountability. For example, how can we allow the tracing of illegal activities or users (e.g., money laundering and cybercrime) without breaking the decentralization property of the system?