This book constitutes the thoroughly refereed postproseedings of the 10th International Workshop on Fast Software Encryption, FSE 2003, held in Lund, Sweden in February 2003. The 27 revised full papers presented were carefully reviewed, improved, and selected from 71 submissions. The papers are organized in topical sections on block cipher cryptanalysis, Boolean functions and S-boxes, stream cipher cryptanalysis, MACs, block cipher theory, side channel attacks, new designs, and modes of operation.