id 104 is tested to verify that it represents a valid local username or servicename if the service id 104 is not valid then the request for service is rejected at step 656 if the service id 104 is valid then processing continues to step 616 note that the unique digital signature 132 can also be rejected based upon the system key number 140 for example a particular system key can expire due to the lapse of time e g all keys with system key numbers less than u201c 2 u201d can be rejected automatically because of their creation date or because those system keys have been flagged as u201c used u201d or both according to one embodiment the above described test is performed before step 620 however according to another embodiment the above described test is performed after step 608 in step 616 the system key 108 is retrieved from the authentication log file 432 according to one embodiment the system key number 140 was identified in the service id 136 which was extracted at step 608 once the system key 108 is located the index number 112 is set to one in step 560 in step 624 the index number 112 u201c i u201d is tested to determine whether it is less than or equal to the maximum number of bits available in bit vector 240 u201c max i u201d if the index number 112 is less than or equal to max i then processing continues to step 628 otherwise the process continues to step 656 next in step 628 the index number 112 is tested to determine whether it has already been used the test at.