Introduction: What's Happening on the Other Channel? (Transcript of Discussion) -- Towards New Security Primitives Based on Hard AI Problems -- Towards New Security Primitives Based on Hard AI Problems (Transcript of Discussion) -- Towards a Theory of Application Compartmentalisation -- Towards a Theory of Application Compartmentalisation (Transcript of Discussion) -- Authentication for Resilience: The Case of SDN -- Authentication for Resilience: The Case of SDN (Transcript of Discussion) -- Verifiable Postal Voting -- End-to-End Verifiable Postal Voting (Transcript of Discussion) -- Independent Computations for Safe Remote Electronic Voting -- Independent Computations for Safe Remote Electronic Voting (Transcript of Discussion) -- ReDABLS: Revisiting Device Attestation with Bounded Leakage of Secrets -- ReDABLS: Revisiting Device Attestation with Bounded Leakage of Secrets (Transcript of Discussion) -- Enhancements to Prepare-and-Measure Based QKD Protocols -- Enhancements to Prepare-and-Measure-Based QKD Protocols (Transcript of Discussion) -- Simple Defences against Vibration-Based Keystroke Fingerprinting Attacks -- Simple Defences against Vibration-Based-Keystroke Fingerprinting Attacks (Transcript of Discussion) -- Communication Setup in Anonymous Messaging -- Communication Setup in Anonymous Messaging (Transcript of Discussion) -- Towards a Stronger Location Integrity -- Towards a Stronger Location Integrity (Transcript of Discussion.-Back Channels Can Be Useful! - Layering Authentication Channels to Provide Covert Communication -- Back Channels Can Be Useful! - Layering Authentication Channels to Provide Covert Communication (Transcript of Discussion) -- Spraying Diffie-Hellman for Secure Key Exchange in MANETs -- Spraying Diffie-Hellman for Secure Key Exchange in MANETs (Transcript of Discussion) -- On the Origin of Yet another Channel -- On the Origin of Yet another Channel (Transcript of Discussion) -- Verifiable Classroom Voting: Where Cryptography Meets Pedagogy.-Verifiable Classroom Voting: Where Cryptography Meets Pedagogy (Transcript of Discussion) -- Creative Uses for the Other Channel.

This book constitutes the thoroughly refereed post-workshop proceedings of the 21st International Workshop on Security Protocols, held in Cambridge, UK, in March 2013. The volume contains 14 revised papers with transcripts of the presentation and workshop discussion and an introduction, i.e. 15 contributions in total. The theme of the workshop was "What's Happening on the Other Channel?".