This two-volume set LNCS 12918 - 12919 constitutes the refereed proceedings of the 23nd International Conference on Information and Communications Security, ICICS 2021, held in Chongqing, China, in September 2021. The 49 revised full papers presented in the book were carefully selected from 182 submissions. The papers in Part I are organized in the following thematic blocks: blockchain and federated learning; malware analysis and detection; IoT security; software security; Internet security; data-driven cybersecurity.