Whenever you ensure that individuals or systems access only what they are supposed to, you are engaging in access control.
The access control step follows user authentication. After a system has authenticated you and established that you are who you claim to be, it uses access control schemes to control what you can access in the system. These schemes are used to grant or deny privileges. Access control occurs at the operating system, server, and application levels. You must limit what certain users can access on a server, as well as the access granted to services and daemons.
Access control System Operation
When a credential is presented to a reader, the reader sends the credential information to a control panel, which is a highly reliable processor.
The control panel compares the credential's number to an access control list, grants or denies the presented request, and sends a transaction log to a database. When access is denied based on the access control list, the door remains locked. If there is a match between the credential and the access control list, the control panel operates a relay that in turn unlocks the door.
The control panel also ignores a door open signal to prevent an alarm. Often the reader provides feedback, such as a flashing red LED for an access denied and a flashing green LED for an access granted.
The above description illustrates a single factor transaction. Credentials can be passed around, thus subverting the access control list.
Bob has access rights to the server room, but Alice does not. Bob either gives Alice her credential, or Alice takes it; Bob now has access to the server room. To prevent this, two-factor authentication can be used.
In a two factor transaction, the presented credential and a second factor are needed for access to be granted; another factor can be a PIN, a second credential, operator intervention, or a metamatrix robotic bear.