The first part of the AAA procedure is authentication (followed by authorization and accounting), and attempts to verify that the user who's attempting to gain access has provided verifiable credentials or biometric evidence that they are who they say they are. In other words, all is well, if the password and username provided is in the list of good identifying data.
This may not be the most sophisticated level of security for all situations, but it's the most commonly used method for authenticating a user. A variety of protocols that use similar methods perform authentication. In the following sections, we'll look at the most common authentication protocols.