This book constitutes the refereed proceedings of the 4th International Conference on Applied Cryptography and Network Security, ACNS 2006, held in Singapore in June 2006. Book presents 33 revised full papers, organized in topical sections on intrusion detection and avoidance, cryptographic applications, DoS attacks and countermeasures, key management, cryptanalysis, security of limited devices, cryptography, authentication and Web security, ad-hoc and sensor network security, cryptographic constructions, and security and privacy.