Lower-Layer Security in Mobility Yih-Chun Hu

Wireless Physical Layer Security The problem: prevent the jamming of broadcast to a group of nodes, some of which are malicious – Without malicious nodes, use spread spectrum – Unicast doesn’t have malicious nodes The underlying mechanism: – Spread spectrum takes a data and a key – Spreads the data across the spectrum using the key – Anyone who doesn’t have the key is at a substantial disadvantage for jamming

Elements to a Solution Key distribution: – Similar to multicast key distribution – Use a binary tree scheme Detection: – Use multiple keys simultaneously – If one key is received and another is not, suspect jamming – Detection code set must be random [Chiang and Hu, “Extended Abstract: Cross-Layer Jamming Mitigation,” MobiCom 2007]

Tree Coding Strategy Cover Probe Codes Jamming Detected

Theoretical Results Finite number of successful jamming attempts until the attacker can no longer jam Size of the cover: – Naïve Approach: n codes – Basic Tree Scheme: j log n codes – With “Remerging”: 2 j codes – Best Possible: j +1 codes

Simulation Results 50 nodes, equal-power base station and jammers

Mobility: The Next Step Jamming is not only a physical-layer problem: – Even with perfect strategy, jammers can still cause substantial loss – Near-far problem in real environments increases jammer’s ability to cause loss Jamming-induced loss at the network layer: – Route to bypass jammed areas – Increased mobility: more frequent neighbor change (Near-far problem is only a problem when the attacker is near)