Identification and Authentication
No matter what kind of computer security system you're using, the first step is often identification and authentication: Who are you, and can you prove it? Once a computer knows that, it can figure out what you are and are not allowed to do. In other words, access control can't start until identification and authentication is finished.
Let's talk about the problem. Alice has some ability on a computer, and we want to make sure that only she has that ability. Sometimes the ability is access to some information: files, account balances, and so forth. Sometimes the ability is access to the entire computer; no one else can turn the computer on and use her data or programs. Sometimes the ability is more explicit: withdraw money from an ATM, use a cell phone, stop a burglar alarm from ringing. Sometimes the ability is on a Web site: access to her calendar or her brokerage account, for example. Sometimes the ability is access to a cryptographic key that is just too large for her to remember. (PGP uses access control measures to protect private keys.) It doesn't matter what the ability is; what's important is that some access control measure is required to identify Alice.
Actually, the access control measure has to do two things. One, it has to allow Alice in. And two, it has to keep others out. Doing only one is easy—an open door will let Alice, and everyone else, in; a bricked-over door will keep others, as well as Alice, out—but doing both is harder. ...