2International Standard. The Directory:Public-key and attribute certificate frameworks[S].Geneva:ISO,2000.
3UnitedStates General Accounting Office. Advances andRemaining Challenges to Adoption of Public Key Infras-tructure Technology[M].Washington,D.C.:GAO,2001.74.
4UnitedStates General Accounting Office. Status of Fed-eral Public Key Infrastructure Activities at Major FederalDepartments and Agencies[M].Washington,D.C.:GAO,2003.7.