Unlike any other point in history, hackers, whistleblowers, and archivists now routinely make off with terabytes of data from governments, corporations, and extremist groups. These datasets often contain gold mines of revelations in the public interest and in many cases are freely available for anyone to download. Revelations based on leaked datasets can change the course of history. In 1971, Daniel Ellsberg’s leak of military documents known as the Pentagon Papers led to the end of the Vietnam War. The same year, an underground activist group called the Citizens’ Commission to Investigate the FBI broke into a Federal Bureau of...