Multi-channel Protocols

We examine several ad-hoc pairing protocols that strengthen their radio exchanges with additional transmissions over another channel, for example a screen showing graphically encoded information to a camera. Additional channels may have limited capacity and may still be subject to eavesdropping, but they may offer specific advantages over radio such as data origin authenticity. A single protocol may profitably use more than one channel, each with its own specific security properties, for different messages in its trace. Making this option explicit allows for further advances in protocol design. We also present an intriguing asymmetric protocol that achieves results comparable to mutual authentication even though the verification happens only in one direction.