This book constitutes the thoroughly refereed post-workshop proceedings of the 20th International Workshop on Security Protocols, held in Cambridge, UK, in April 2012.Following the tradition of this workshop series, each paper war revised by the authors to incorporate ideas from the workshop, and is followed in these proceedings by an edited transcription of the presentation and ensuing discussion. The volume contains 14 papers with their transcriptions as well as an introduction, i.e. 29 contributions in total. The theme of the workshop was "Bringing protocols to life".