A Customer Replaceable Unit Monitoring (CRUM) chip, an image forming apparatus, and a method of authenticating are provided. The apparatus includes a main body that includes a main controller controlling operations of the apparatus, a consumable unit mounted on the main body to communicate with the main controller, and a CRUM chip that is provided in the consumable unit and stores information regarding the consumable unit. The main controller and the CRUM chip transmit and receive signals that include data and integrity detection data between each other. The integrity detection data is generated by using previous integrity detection data.