1. A billing system for billing users on a usage basis for an enciphered application software program for a computer comprising: a hardware security module attached to the computer, the security module carrying therein a fixed computer-readable numeric internal code and also carrying a billing module interface; a removable, portable billing module electrically connected to the security module and electrically connecting to the billing module interface therein, the billing module having fixed and alterable memory therein with a fixed computer-readable numeric external code in the fixed memory and billing information stored in the alterable memory, and a storage medium for the computer carrying an enciphered application program thereon and also carrying thereon security program means for (1) causing the computer to read the internal code and the external code, (2) using the internal and external codes to generate a decipher key which is used in a predetermined algorithm to decipher the application program for use by the user, and (3) writing usage billing information in the alterable memory in the billing module.