Book excerpt: This book constitutes the refereed proceedings of the 21th International Conference on Information and Communications Security, ICICS 2019, held in Beijing, China, in December 2019. The 47 revised full papers were carefully selected from 199 submissions. The papers are organized in topics on malware analysis and detection, IoT and CPS security enterprise network security, software security, system security, authentication, applied cryptograph internet security, machine learning security, machine learning privacy, Web security, steganography and steganalysis.