The Internet e-mail security software and standards, such as PGP (Pretty Good Privacy) and PEM (Privacy Enhanced Mail), have several limitations that should be overcome for their further applications to the Internet and network environments. In order to improve and reengineer those software, details of the As-Is software processing should be analyzed. One of the possible techniques for software analysis is IDEFO function modeling. Although IDEFO has been mainly used for BPR as one of the industrial engineering techniques, it has been rarely applied to the analysis of software processing and reengineering in computer and software engineering fields. Additionally, no sufficient details of PGP and PEM processing are analyzed in the literature. The objective of this paper is to demonstrate the application of the IDEFO to the systems analysis of the Internet e-mail security software as well as to provide software developers with the basis for software improvements.