Access controls define what actions a user can perform or what objects a user can access. These controls assume that the identity of the user has already been verified. It is the job of authentication mechanisms to ensure that only valid users are admitted. Described another way, authentication uses some mechanism to prove that you are who you claim to be. Three general methods are used in authentication. To verify your identity, you can provide the following:
Something you know
Something you have
Something you are (something unique about you)
The most common authentication mechanism is to provide something that only you, the valid user, should know. The most frequently used example of this is the common user ID (or username) and password. In theory, since you are not supposed to share your password with anybody else, only you should know your password, and thus by providing it you are proving to the system that you are who you claim to be. In theory, this should be a fairly decent method to provide authentication. Unfortunately, for a variety of reasons, such as the fact that people have a tendency to choose very poor and easily guessed passwords, this technique is not as reliable as it should be. Other authentication mechanisms are consequently always being developed and deployed.
Another method to provide authentication involves the use of something that only valid users should have in their possession. A physical-world example of this would be a simple lock and key. Only those individuals with the correct key will be able to open the lock and thus provide admittance to a house, car, office, or whatever the lock was protecting. A similar method can be used to authenticate users for a computer system or network (though the key may be electronic and may reside on a smart card or similar device). The problem with this technology is that people will lose their keys (or cards), which means they can't log in to the system and somebody else who finds
the key can then access the system, even though that person is not authorized. To address this problem, a combination of the something-you-know/something-you-have methods is often used so that the individual with the key can also be required to provide a password or passcode. The key is useless unless you know this code. An example of this is the ATM card most of us carry. The card is associated with a personal identification number (PIN), which only you should know. Knowing the PIN without having the card is useless, just as having the card without knowing the PIN will not give you access to your account.