Strong and scalable metadata security for voice calls

Abstract

This dissertation presents a scalable approach to protecting metadata (who is communicating with whom) in a communication system. The emphasis in this dissertation is on hiding metadata for voice calls, but the approach is applicable to any two-way communication between users. Our approach is embodied in a new system named Yodel, the first system for voice calls that hides metadata from a powerful adversary that controls the network and compromises servers. Voice calls require sub-second message latency, but low latency has been difficult to achieve in prior work where processing each message requires an expensive public key operation at each hop in the network. Yodel avoids this expense with the idea of self-healing circuits, reusable paths through a mix network that use only fast symmetric cryptography. Once created, these circuits are resilient to passive and active attacks from global adversaries. Creating and connecting to these circuits without leaking metadata is another challenge that Yodel addresses with the idea of guarded circuit exchange, where each user creates a backup circuit in case an attacker tampers with their traffic. We evaluate Yodel across the internet and it achieves acceptable voice quality with 990 ms of latency for 5 million simulated users.