Authentication

Authentication is a term used in computer security for the process by which a computer user (or occasionally a computer program) gains priviledges to perform operations in a protected computer system. Practical examples include:

withdrawing cash from an ATM.
controlling a remote computer over the Internet.
using an Internet banking system.

The methods by which a human can authenticate themselves are classified into three classes:

Something the user is (e.g., fingerprint or retinal scanners)
Something the user has (e.g., ID card)
Something the user knows (e.g., a password, a passphrase or a PIN)

Sometimes a combination of methods is used, e.g., a bank card and a PIN number.

Historically, methods of authentication have used increasingly sophisticated mathematics in order to remain reliable against newly discovered techniques of impersonation.