(1) Verifying the integrity of a transmitted message. See message integrity, e-mail authentication and MAC.

(2) Verifying the identity of a user logging into a network. Passwords, digital certificates, smart cards and biometrics can be used to prove the identity of the client to the network. Passwords and digital certificates can also be used to identify the network to the client. The latter is important in wireless networks to ensure that the desired network is being accessed. See identity management, identity metasystem, OpenID, human authentication, challenge/response, two-factor authentication, password, digital signature, IP spoofing and biometrics.

Four Levels of Proof

There are four levels of proof that people are indeed who they say they are. None of them are entirely foolproof, but in order of least to most secure, they are:

1 - What You Know
Passwords are widely used to identify a user, but only verify that somebody knows the password.

2 - What You Have
Digital certificates in the user's computer add more security than a password, and smart cards verify that users have a physical token in their possession, but both laptops and smart cards can be stolen.

3 - What You Are
Biometrics such as fingerprints and iris recognition are more difficult to forge, but you have seen such systems fooled in the movies all the time!

4 - What You Do
Dynamic biometrics such as hand writing a signature and voice recognition are the most secure; however, replay attacks can fool the system.