  Investigative journalists collect large numbers of digital documents during
their investigations. These documents can greatly benefit other journalists'
work. However, many of these documents contain sensitive information. Hence,
possessing such documents can endanger reporters, their stories, and their
sources. Consequently, many documents are used only for single, local,
investigations.
  We present DatashareNetwork, a decentralized and privacy-preserving search
system that enables journalists worldwide to find documents via a dedicated
network of peers. DatashareNetwork combines well-known anonymous authentication
mechanisms and anonymous communication primitives, a novel asynchronous
messaging system, and a novel multi-set private set intersection protocol
(MS-PSI) into a *decentralized peer-to-peer private document search engine*. We
prove that DatashareNetwork is secure; and show using a prototype
implementation that it scales to thousands of users and millions of documents.
