This chapter presents an overview of a set of cryptographic service calls being considered for standardization which allows application programs to request cryptographic functions through a standard interface to a cryptographic facility. The set of calls was recently proposed for consideration as an IEEE POSIX security interface standard. The set of calls include basic cryptographic functions, cryptographic key management functions, and user account management functions. Two cryptosystems providing cryptographic functions are addressed: the secret key cryptosystem, and the public-key cryptosystem. A complete description of each cryptographic service call is given in Appendix B. Those readers who are familiar with cryptography and the secret-key and public-key cryptosystems may skip Sections 5.1 and 5.2 and go directly to Appendix B.