Preface. A rich stream of papers and many good books have been written on cryptography and computer security, but most of them assume a scholared reader who has the time to start at the beginning and work his way through the entire text. The goal of Encyclopedia of Cryptography and Security, Second Edition is tomake important notions of cryptography and computer security accessible to readers who have an interest in a particular keyword related to cryptology or computer security, but who lack the time to study one of the many books in these areas. The second edition is intended as a replacement of Encyclopedia of Cryptography and Security that was edited by one of us (Henk van Tilborg) and published by Springer in 2005.The current edition provides a comprehensive reference to 750 topics in the broad field of cryptology and computer security; compared to the first edition the reader can find 300 new entries. This Encyclopedia improves on the earlier edition in several important and interesting ways. First, entries in the first edition have been updated when needed. Second, it provides a broader coverage to the field, and the new entries are mostly in the area of Information and System Security. Third, computer technology has evolved drastically in the short timespan of just five years. We have tried to make an effort to be comprehensive and include coverage of several newer topics. Fourth, the Encyclopedia is available not only as a printed volume but as an online reference work as well. The title includes a list of entries and of authors.The online edition is an XML e-Reference searchable version that allows cross-references.The online edition will also allow us to make regular updates as new trends and terms arise.