This book constitutes the thoroughly refereed post-conference proceedings of the IFIP WG 11.4 International Workshop on Open Problems in Network Security, i Net Sec 2015, held in Zurich, Switzerland, in October 2015. i Net Sec is the main workshop of the IFIP working group WG 11.4; its objective is to present and discuss open problems and new research directions on all aspects related to network security. The 9 revised full papers presented in this volume were carefully reviewed and selected from 13 submissions. They were organized in topical sections named: network security; intrusion detection; anonymous communication; and cryptography.