It's generally accepted among security experts that a single method of authentication, such as a password, is not adequate for protecting systems.
Strong authentication is achieved by combining at least two of the above attributes.
LDAP's increasing use as a standardized method for exchanging directory-based information also makes it useful for authentication via smart cards and digital certificates, for instance. This can be particularly useful if a network manager has to provide both traditional dial-in remote access and VPN access. It also makes transitions from one method to the other easier, since the authentication databases don't have to change. Upping the ante A similar situation exists for digital certificates, which are being used for an increasing number of applications, including electronic commerce and secure messaging, as well as user authentication. Since many of the products that have been developed for managing digital certificates use LDAP, it's possible to link LDAP-aware devices, such as firewalls and VPN gateways, to these certificates servers for authenticating users.