Secret message sharing using online social media

Recently, there have been proposals to evade censors by using steganography to embed secret messages in images shared on public photo-sharing sites. However, establishing a covert channel in this manner is not straightforward. First, photo-sharing sites often process uploaded images, thus destroying any embedded message. Second, prior work assumes the existence of an out-of-band channel, using which the communicating users can exchange metadata or secret keys a priori; establishing such out-of-band channels, not monitored by censors, is difficult. In this paper, we address these issues to facilitate private communications on photo-sharing sites. In doing so, first, we conduct an in-depth measurement study of the feasibility of hiding data on four popular photo-sharing sites. Second, based on the understanding derived, we propose a novel approach for embedding secret messages in uploaded photos while preserving the integrity of such messages. We demonstrate that, despite the processing on photo-sharing sites, our approach ensures reliable covert communication, without increasing the likelihood of being detected via steganalysis. Lastly, we design and implement a scheme for bootstrapping private communications without an out-of-band channel, i.e., by exchanging keys via uploaded images.