As Internet and its users grow dramatically multi-media data are getting common, many types of electronic mail applications are showing up. Internet s/w developers and users would like to electronic mail system for commercial purposes. They also would like to protect their electronic mail somehow. For these purpose, the security feature using cryptography is one of the essential functions. In this paper, we describe the library function development for handing MOSS(MIME Object Security Service), the security version of MINE(Multi -Purpose Internet Mail Extension). For these purpose, we explain the security features and structures of MOSS and defined the necessary function and we also discussed design issues for the MOSS implementation.