The cornerstone for the effective functioning of modern covert communication and proxy tools is a persistent and blocking-resistant control channel. These channels are needed to frequently update client-side configurations to stay ahead of countermeasures against such tools. However, all existing control channels require clients to explicitly query for updates, which is prohibitively difficult in adversarial networks. Further, this client-initiated communication relies mostly on ad-hoc and out-of-band channels, which are hard to maintain and scale.

In this talk, I will present CenPush, the first server-side control channel using push notification services. CenPush allows tool maintainers to update client configurations automatically without the need for clients to initiate a query themselves. We develop a general-purpose design for integrating push notifications as a control channel into circumvention tools, while testing and evaluating its effectiveness across various parameters, including blocking resistance, reliability, data-carrying capacity, ease of integration, and cost. We utilize the design to integrate and implement push notifications for use in the popular circumvention tools such as Tor and demonstrate the practical utility of CenPush.