This book constitutes the refereed proceedings of the 4th International Workshop on Security, IWSEC 2009, held in Toyama, Japan, in October 2009.

Invited Talk.- The Future of Cryptographic Algorithms.- Block Cipher.- Bit-Free Collision: Application to APOP Attack.- Impossible Boomerang Attack for Block Cipher Structures.- Improved Distinguishing Attacks on HC-256.- Cryptographic Protocols.- A Generic Construction of Timed-Release Encryption with Pre-open Capability.- An Efficient Identity-Based Signcryption Scheme for Multiple Receivers.- Universal Designated Verifier Signatures with Threshold-Signers.- Reducing Complexity Assumptions for Oblivious Transfer.- Protection and Intrusion Detection.- Tamper-Tolerant Software: Modeling and Implementation.- An Error-Tolerant Variant of a Short 2-Secure Fingerprint Code and Its Security Evaluation.- Efficient Intrusion Detection Based on Static Analysis and Stack Walks.- Authentication.- Strongly Secure Authenticated Key Exchange without NAXOS' Approach.- ID-Based Group Password-Authenticated Key Exchange.- A Proposal of Efficient Remote Biometric Authentication Protocol.