  Peer-to-Peer (P2P) content sharing systems are susceptible to the content
pollution attack, in which attackers aggressively inject polluted contents into
the systems to reduce the availability of authentic contents, thus decreasing
the confidence of participating users.
  In this paper, we design a pollution-free P2P content sharing system, Green,
by exploiting the inherent content-based information and the social-based
reputation. In Green, a content provider (i.e., creator or sharer) publishes
the information of his shared contents to a group of content maintainers
self-organized in a security overlay for providing the mechanisms of redundancy
and reliability, so that a content requestor can obtain and filter the
information of his requested content from the associated maintainers. We employ
a reputation model to help the requestor better identify the polluted contents,
and then utilize the social (friend-related) information to enhance the
effectiveness and efficiency of our reputation model. Now, the requestor could
easily select an authentic content version for downloading. While downloading,
each requestor performs a realtime integrity verification and takes prompt
protection to handle the content pollution. To further improve the system
performance, we devise a scalable probabilistic verification scheme.
  Green is broadly applicable for both structured and unstructured overlay
applications, and moreover, it is able to defeat various kinds of content
pollution attacks without incurring significant overhead on the participating
users. The evaluation in massive-scale networks validates the success of Green
against the content pollution.
