  Messaging systems built on mesh networks consisting of smartphones
communicating over Bluetooth have been used by protesters around the world
after governments have disrupted Internet connectivity. Unfortunately, existing
systems have been shown to be insecure; most concerningly by not adequately
hiding metadata. This is further complicated by the fact that wireless
communication such as Bluetooth is inherently a broadcasting medium. In this
paper, we present a new threat model that captures the security requirements of
protesters in this setting. We then provide a solution that satisfies the
required security properties, hides all relevant metadata, scales to moderately
sized protests, and supports group messaging. This is achieved by broadcasting
all messages in a way that limits the overhead of duplicate messages, ensuring
that ciphertexts do not leak metadata, and limiting what can be learned by
observing user behavior. We also build a model of our system and numerically
evaluate it to support our claims and analyze how many users it supports.
Finally, we discuss further extensions that remove potential bottlenecks in
scaling and support substantially more users.
