This volume comprises the proceedings of the 4th Conference on Advanced - cryption Standard, ‘AES — State of the Crypto Analysis,’ which was held in Bonn, Germany, during 10–12 May 2004. The conference followed a series of events organized by the US National - stitute of Standards and Technology (NIST) in order to hold an international competition to decide on an algorithm to serve as the Advanced Encryption Standard (AES). In 1998, at the ?rst AES conference (AES 1), 15 di?erent al- rithmswerepresented,discussed,reviewedandveri?ed.Asecondconferencewas organizedinApril1999,andbyAugust1999only?vecandidateswerestillinthe running: MARS, RC6, Rijndael, Serpent and Two?sh. After a further conference devoted to veri?cation, testing and examination of the candidate algorithms in order to prove their performance and security, one winning algorithm remained. The encryption scheme Rijndael, designed by the Belgian cryptographers Joan Daemen and Vincent Rijmen, was selected in 2000 to become the successor to the famous DES (Data Encryption Standard) and it is now the Advanced - cryption Standard.