TheINDOCRYPTconferenceseriesstartedin2000, andINDOCRYPT2003was the fourth one in this series. This series has been accepted by the international research community as a forum for presenting high-quality crypto research, as is evident from the 101 submissions this year, spread over 21 countries and all ?ve continents. The accepted papers were written by authors from 16 countries, covering four continents. A total of 101 papers were submitted for consideration to the program c- mittee, and after a careful reviewing process 30 were accepted for presentation. One of the conditionally accepted papers was withdrawn by the authors as they found an errorin the paper that could not be repairedin the short time between the noti?cation of the reviewand the ?nalversionsubmission.Thus the ?nal list contains29acceptedpapers.Wewouldliketo thanktheauthorsofallsubmitted papers, including both those that wereaccepted and those which, unfortunately, could not be accommodated. The reviewing process for INDOCRYPT was very stringent and the schedule was extremely tight. The program committee members did an excellent job in reviewing and selecting the papers for presentation. During the review process, theprogramcommitteememberscommunicatedusingareviewsoftwarepackage developed by Bart Preneel, Wim Moreau and Joris Claessens. We acknowledge them for providing this software. These proceedings include the revised versions of the 29 selected papers. Revisions were not checkedby the programcommittee and the authors bear the full responsibility for the contents of the respective papers. Our thanks go to all the program committee members and the external reviewers(alistofthem isincludedintheproceedings)whoputintheirvaluable time and e?ort in providing important feedback to the authors.