Deniable Upload and Download via Passive Participation

Downloading or uploading controversial information can put users at risk, making them hesitant to access or share such information. While anonymous communication networks (ACNs) are designed to hide communication meta-data, already connecting to an ACN can raise suspicion. In order to enable plausible deniability while providing or accessing controversial information, we design CoverUp: a system that enables users to asynchronously upload and download data. The key idea is to involve visitors from a collaborating website. This website serves a JavaScript snippet, which, after user's consent produces cover traffic for the controversial site / content. This cover traffic is indistinguishable from the traffic of participants interested in the controversial content; hence, they can deny that they actually up- or downloaded any data.

CoverUp provides a feed-receiver that achieves a downlink rate of 10 to 50 Kbit/s. The indistinguishability guarantee of the feed-receiver holds against strong global network-level attackers who control everything except for the user's machine. We extend CoverUp to a full upload and download system with a rate of 10 up to 50 Kbit/s. In this case, we additionally need the integrity of the JavaScript snippet, for which we introduce a trusted party. The analysis of our prototype shows a very small timing leakage, even after half a year of continual observation. Finally, as passive participation raises ethical and legal concerns for the collaborating websites and the visitors of the collaborating website, we discuss these concerns and describe how they can be addressed.