Secure access to software modules

Abstract

Apparatus and method for obtaining a security value (50) that enables a calling module (7) to achieve secure access to a called module (5) within a digital computer (1). A distributive invertible function f is selected. f may be the cyclic redundancy check function modulo p, where p is a prime number. A desired residual value r associated with the application of f and a desired size of the security value (50) are also selected. Space is allocated within the called module (5) for storing the security value (50). A portion of the called module (5) is designated as a test block (51). f is applied to a numerical representation of test block (51) to generate a first challenge value CV1. f is applied to a numerical value of the security value (50) and a number representative of the location of the security value (50) within the called module (5) to calculate a second challenge value CV2. n is the number of bits from the beginning of the security value (50) to the end of the called module (5). CV1 and CV2 are typically calculated by called module (5). Calling module (7) then calculates the security value (50) based upon CV1 and CV2, and using f. Called module (5) uses this calculated value of the security value (50) in function f. If this application of f provides the desired residual value r, then it is known that calling module (7) had proper access to called module (5). At this point, one or more functions accessible by called module (5) may be activated. Access codes may be coded within residual value r itself.