https://www.centos.org/docs/5/html/5.1/Deployment_Guide/sso-sc-login-concept.html
This section provides a brief overview of the process of logging in using a smart card.
When the user inserts their smart card into the smart card reader, this event is recognized by the PAM facility, which prompts for the user's PIN.
The system then looks up the user's current certificates and verifies their validity. The certificate is then mapped to the user's UID.
This is validated against the KDC and login granted.
Figure 43.5. How Smart Card Login Works
You cannot log in with a card that has not been enrolled, even if it has been formatted. You need to log in with a formatted, enrolled card, or not using a smart card, before you can enroll a new card.
Refer to Section 43.6, “Kerberos” and Section 43.4, “Pluggable Authentication Modules (PAM)” for more information on Kerberos and PAM.
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