1. To establish, usually by challenge and response, that a transmission attempt is authorized and valid. 2. Verify the identity of a user, user device, or other entity, or the integrity of data stored, transmitted, or otherwise exposed to unauthorized modification in an information system (IS,) or establish the validity of a transmission. 3. A challenge given by voice or electrical means to attest to the authenticity of a message or transmission. 4. To establish the validity of a claimed identity. 5. To establish the validity of a claimed identity.