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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 automated information system, or establish the validity of a transmitted message. [NIS] 3. A challenge given by voice or electrical means to attest to the authenticity of a message or transmission. [JP1]