This book constitutes the refereed proceedings of the 4th International Workshop on Security, IWSEC 2009, held in Toyama, Japan, in October 2009. The 13 revised full papers presented together with 1 invited talk were carefully reviewed and selected from 46 submissions. The papers are organized in topical sections on block cipher, cryptographic protocols, contents protection and intrusion detection, as well as authentication. The Fourth International Workshop on Security (IWSEC 2009) was held at Toyama International Conference Center, Toyama, Japan, October 28¿30, 2009. The workshop was co-organized by CSEC, a special interest group on computer security of the IPSJ (Information Processing Society of Japan) and ISEC, a technical group on information security of IEICE (The Institute of Electronics, Information and Communication Engineers). The excellent Local Organizing Committee was led by the IWSEC 2009 General Co-chairs, Kazuo Takaragi and Hiroaki Kikuchi. IWSEC2009received46papersubmissionsfromallovertheworld.Wewould like to thank all the authors who submitted papers. Eachpaper was reviewedby at least three reviewers. In addition to the Program Committee members, many externalreviewersjoinedthereviewprocessintheirparticularareasofexpertise. We were fortunate to have this energetic team of experts, and are grateful to all of them for their hard work. The hard work includes very active discussion; the discussion phase was almost as long as the initial individual reviewing. The review and discussion were supported by a very nice Web-based system, iChair. We thank its developers. Following the review phases, 13 papers were accepted for publication in this volume of Advances in Information and Computer Security. Together with the contributed papers, the workshop featured an invited talk and a hash function panel both of which were respectively given and chaired by eminent researcher, Bart Preneel (Katholieke Universiteit Leuven). An abstract of the talk, titled ¿TheFutureofCryptographicAlgorithms,¿isincludedinthisvolume.Wedeeply appreciate his contribution. Many people contributed to the success of IWSEC 2009. We wish to - press our deep appreciation for their contribution to informationand computer security.