This book constitutes the refereed proceedings of the 4th International Conference on Cryptology and Network Security, CANS 2005, held in Xiamen, China in December 2005. The 28 revised full papers presented together with 2 invited papers were carefully reviewed and selected from 118 submissions. The papers are organized in topical sections on cryptanalysis, intrusion detection and viruses, authentication and signature, signcryption, e-mail security, cryptosystems, privacy and tracing, information hiding, firewalls, denial of service and DNS security, and trust management.