  Despite the pervasiveness of Internet censorship, we have scant data on its
extent, mechanisms, and evolution. Measuring censorship is challenging: it
requires continual measurement of reachability to many target sites from
diverse vantage points. Amassing suitable vantage points for longitudinal
measurement is difficult; existing systems have achieved only small,
short-lived deployments. We observe, however, that most Internet users access
content via Web browsers, and the very nature of Web site design allows
browsers to make requests to domains with different origins than the main Web
page. We present Encore, a system that harnesses cross-origin requests to
measure Web filtering from a diverse set of vantage points without requiring
users to install custom software, enabling longitudinal measurements from many
vantage points. We explain how Encore induces Web clients to perform
cross-origin requests that measure Web filtering, design a distributed platform
for scheduling and collecting these measurements, show the feasibility of a
global-scale deployment with a pilot study and an analysis of potentially
censored Web content, identify several cases of filtering in six months of
measurements, and discuss ethical concerns that would arise with widespread
deployment.
