Every government employee that handles classified information must sign the non-disclosure agreement form for classified information before they do so.
Here is an excerpt from the booklet outlining what you are signing:
As an employee of the Federal Government or one of its contractors, licensees, or grantees who occupies a position which requires access to classified information, you have been the subject of a personnel security investigation. The purpose of this investigation was to determine your trustworthiness for access to classified information. When the investigation was completed, your employing or sponsoring department or agency granted you a security clearance based upon a favorable determination of the investigation results. By being granted a security clearance, you have met the first of three requirements necessary to have access to classified information.
Classified Information Nondisclosure Agreement (Standard Form 312) Briefing Booklet [84 Pages, 0.6MB]
Classified Information Nondisclosure Agreement [3 Pages, 0.9MB]