The book "Fast Software Encryption" documents the fully peer-reviewed contributions of the 21st International Workshop on Fast Software Encryption, which took place from March 3 to 5, 2014, in London, United Kingdom. It includes 31 revised full texts selected from 99 original submissions. The contributions are divided into thematic sections that address various aspects of cryptography, including designs, cryptanalysis, authenticated encryption, foundations and theory, stream ciphers, as well as hash functions and advanced constructions. This academic book is aimed at professionals and students in the field of computer science and cryptography, providing valuable insights into current research findings and developments in this area.