This book constitutes the thoroughly refereed post-conference proceedings of the IFIP WG 11.4 International Workshop on Open Problems in Network Security, iNetSec 2015, held in Zurich, Switzerland, in October 2015. iNetSec 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.

Network Security.- 1 Forwarding Accountability: A Challenging Necessity of the Future Data Plane.- A Metric for Adaptive Routing on Trustworthy Paths.- Intrusion Detection.- Early Warning Systems for Cyber Defence.- Catching Inside Attackers: Balancing Forensic Detectability and Privacy of Employees.- Intrusion Detection in the Smart Grid based on an analogue technique.- Anonymous Communication.- On Building Onion Routing into Future Internet Architectures.- Anonymity Online for Everyone: What is missing for zero-effort privacy on the Internet.- Cryptography.- Reviving the Idea of Incremental Cryptography for the Zettabyte era Use case: Incremental Hash Functions Based on SHA-3.- The Quest for Scalable Blockchain Fabric: Proof-of-Work vs. BFT Replication.