LiLAC: Lightweight Low-Latency Anonymous Chat
John Podolanko, Revanth Pobala, Hussain Mucklai, George Danezis, Matthew Wright · 2017
Low latency anonymity systems, like Tor and I2P, support private online communications, but offer limited protection against powerful adversaries with widespread eavesdropping capabilities. It is known that general-purpose communications, such as web and file transfer, are difficult to protect in that setting. However, online instant messaging only requires a low bandwidth and we show it to be amenable to strong anonymity protections. In this paper, we describe the design and engineering of LiLAC, a Lightweight Low-latency Anonymous Chat service, that offers both strong anonymity and a lightweight client-side presence. LiLAC implements a set of anonymizing relays, and offers stronger anonymity protections by applying dependent link padding on top of constant-rate traffic flows. This leads to a key trade-off between the system's bandwidth overhead and end-to-end delay along the circuit, which we study. Additionally, we examine the impact of allowing zero-installation overhead on the client side, by instead running LiLAC on web browsers. This introduces potential security risks, by relying on third-party software and requiring user awareness; yet it also reduces the footprint left on the client, enhancing deniability and countering forensics. Those design decisions and trade-offs make LiLAC an interesting case to study for privacy and security engineers.