Code protection using cryptography

Abstract

A cryptographic method for discouraging the copying and sharing of purchased software programs allows an encrypted program to be run on only a designated computer or, alternatively, to be run on any computer but only by the user possessing a designated smart card. Each program offering sold by the software vendor is encrypted with a unique file key and then written on a diskette. A user who purchases a diskette having written thereon an encrypted program must first obtain a secret password from the software vendor. This password will allow the encrypted program to be recovered at a prescribed, designated computer having a properly implemented and initialized encryption feature. The encryption feature decrypts the file key of the program from the password, and when the encrypted program is loaded at the proper computer, the program or a portion of it is automatically decrypted and written into a protected memory from which it can only be executed and not accessed for non-execution purposes. In alternative embodiments, the user is not confined to a prescribed, designated computer buy may use the program on other, different computers with a smart card provided the computers have a properly implemented and initialized encryption feature that accepts the smart card. As a further modification, the cryptographic facility may support operations that enable the user to encrypt and decrypt user generated files and/or user generated programs.