This four-volume set LNISCT 687-690 constitutes the proceedings of the 21st EAI International Conference on Security and Privacy in Communication Networks, SecureComm 2025, held in Xiangtan, China, during July 4 - 6, 2025.The 119 full papers included in these volumes were carefully reviewed and selected from 341 submissions. They are organized in the following topical sections:Part I: Distributed and Network Security; ML/AI Security.Part II: ML/AI Security; CyberSecurity.Part III: CyberSecurity; Cryptography and Authentication.Part IV: Cryptography and Authentication; Security and Optimization.