Papers by Emiliano De Cristofaro

Censorship in the Wild: Analyzing Internet Filtering in Syria

Last week we looked at a case study of Internet filtering in Pakistan; this week we have a case study of Syria. (I think this will be the last such case study I review, unless I come across a really compelling one; there’s not much new I have to say about them.)

This study is chiefly interesting for its data source: a set of log files from the Blue Coat brand DPI routers that are allegedly used [1] [2] to implement Syria’s censorship policy, covering a 9-day period in July and August of 2011. leaked by the Telecomix hacktivist group. Assuming that these log files are genuine, this gives the researchers what we call ground truth: they can be certain that sites appearing in the logs are, or are not, censored. (This doesn’t mean they know the complete policy, though. The routers’ blacklists could include sites or keywords that nobody tried to visit during the time period covered by the logs.)

With ground truth it is possible to make more precise deductions from the phenomena. For instance, when the researchers see URLs of the form http://a1b2.cdn.example/adproxy/cyber/widget blocked by the filter, they know (because the logs say so) that the block is due to a keyword match on the string proxy, rather than the domain name, the IP address, or any other string in the HTTP request. This, in turn, enables them to describe the censorship policy quite pithily: Syrian dissident political organizations, anything and everything to do with Israel, instant messaging tools, and circumvention tools are all blocked. This was not possible in the Pakistani case—for instance, they had to guess at the exact scope of the porn filter.

Because the leaked logs cover only a very short time window, it’s not possible to say anything about the time evolution of Syrian censorship, which is unfortunate, considering the tumultuous past few years that the country has had.

The leak is from several years ago. There is heavy reliance on keyword filtering; it would be interesting to know if this has changed since, what with the increasing use of HTTPS making keyword filtering less useful. For instance, since 2013 Facebook has defaulted to HTTPS for all users. This would have made it much harder for Syria to block access to specific Facebook pages, as they were doing in this study.