- TU-T Recommendation X.509 | ISO/IEC 9594-8 defines a framework for public-key certificates and attribute certificates. These frameworks may be used by other standards bodies to profile their application to Public Key Infrastructures (PKI) and Privilege Management Infrastructures (PMI). Also, this Recommendation | International Standard defines a framework for the provision of authentication services by Directory to its users. It describes two levels of authentication simple authentication, using a password as a verification of claimed identity; and strong authentication, involving credentials formed using cryptographic techniques. While simple authentication offers some limited protection against unauthorized access, only strong authentication should be used as the basis for providing secure services.