With the rapid advancements in information and communications technology (ICT) and ‘expansion’ of cyber space, cyber security is of crucial importance to the stability of our Internet-connected society. For example, how do we ensure secure communications between servers, network nodes, terminals and user applications across public and private networks? Defending our cyber space is both a research challenge and an operational challenge. Designing effective security solutions is complicated by the need to carefully balance between security and usability, as well as the amount of efforts and resources required. For example, how do we achieve better security without compromising on communication speed? This special issue aims to solicit state-of-the-art research advances in cyber-security threat mitigation and defense solutions, including the underlying cryptographic techniques. The issue welcomes both theoretical research, and applied research (e.g. implementations and applications).