  Transport layer data leaks metadata unintentionally$\unicode{x2013}$such as
who communicates with whom. While tools for strong transport layer privacy
exist, they have adoption obstacles, including performance overheads
incompatible with mobile devices. We posit that by changing the objective of
metadata privacy for $\textit{all traffic}$, we can open up a new design space
for pragmatic approaches to transport layer privacy. As a first step in this
direction, we propose the $\textit{hybrid model}$, a system model that allows
one to practically combine, and formally reason about network traffic with
different privacy guarantees ($\textit{regular}$ and $\textit{deniable}$) in
one joint system. Using techniques from information flow control we present a
principled approach to construct a formal model and prove that deniable traffic
achieves transport layer privacy against strong
adversaries$\unicode{x2013}$this constitutes the first bridging of information
flow control and anonymous communication to our knowledge. Additionally, we
show that existing state-of-the-art protocols can be extended to support
transport layer privacy, by designing a novel protocol for $\textit{deniable
instant messaging}$ (DenIM), which is a variant of the Signal protocol. As an
instantiation of the hybrid model, we implement and evaluate a proof-of-concept
instant messaging system running both DenIM and regular Signal. We empirically
show that the hybrid model can maintain low-latency for regular Signal traffic
without breaking existing features, while at the same time supporting deniable
Signal traffic.

Cryptography and Security