This book constitutes the thoroughly refereed post-proceedings of the 17th International Workshop on Security Protocols, SP 2009, held in Cambridge, UK, in April 2009. The 17 revised full papers presented together with edited transcriptions of some of the discussions following the presentations have gone through multiple rounds of reviewing, revision, and selection. The theme of this workshop was "Brief Encounters". In the old days, security protocols were typically run first as preliminaries to, and later to maintain, relatively stable continuing relationships between relatively unchanging individual entities. Pervasive computing, e-bay and second life have shifted the ground: we now frequently desire a secure commitment to a particular community of entities, but relatively transient relationships with individual members of it, and we are often more interested in validating attributes than identity. The papers and discussions in this volume examine the theme from the standpoint of various different applications and adversaries.