The authenticity of a program executed by a processor is determined by a security element that computes a hash code over re-ordered segments of a known-to-be-authentic copy of a program executed by the processor. The authenticity of the same segments are re-ordered by and provided by the processor to the security element, which computes a second hash code for the re-ordered segments received from the processor. If the hash values for the segments are identical, the program segments are identical. If the hash values for any segment are different, the two programs are different. When a processor's program is determined to be different from a known good copy, the processor can be stopped or an alarm signal generated.