A key law authorizing intelligence agencies to acquire more than 250 million online exchanges annually is set to expire at the end of 2017, setting the stage for another battle between privacy and security advocates in Congress.

Section 702 of the Foreign Intelligence Surveillance Act was passed by Congress in 2008, though its full application went unknown to the public until it was exposed through documents leaked by Edward Snowden in 2013. The former intelligence contractor revealed that the CIA, FBI and NSA use the law to see exchanges between users on Facebook, Google, Yahoo and other platforms.