Is it ensure, insure, or assure?
The verification of the identity of a person or process. In a communication system, authentication verifies that messages really come from their stated source, like the signature on a (paper) letter. The most common form of authentication is typing a user name (which may be widely known or easily guessable) and a corresponding password that is presumed to be known only to the individual being authenticated. Another form of authentication is biometrics.