An image forming apparatus, comprising: a consumable unit incorporating a custom replaceable unit monitor, CRUM, chip; and a main body configured to perform a first authentication and a second authentication of the consumable unit, when the consumable unit is mounted thereto, wherein the main body comprises: a main controller configured to perform the first authentication according to firmware stored in the image forming apparatus; characterised in that the main body further comprises an authentication controller configured to use at least one ASIC, Application Specific Integrated Circuit, to perform the second authentication of the consumable unit, wherein the authentication controller is operable to perform the second authentication separately from the first authentication performed by the main controller, and the main controller is operable to perform communication with the CRUM chip, when both the first authentication and the second authentication succeed, and wherein the authentication controller comprises: an interface to receive authentication information including a first message authentication code, MAC, from the CRUM chip; an authentication processor comprising an electronic signature verifier to verify an electronic signature included in the authentication information, a section key generator to generate a section key, a MAC generator configured to use the section key to generate a second MAC, and a MAC verifier configured to compare the second MAC with the first MAC; wherein the at least one ASIC includes the electronic signature verifier, section key generator, MAC generator and MAC verifier. a consumable unit incorporating a custom replaceable unit monitor, CRUM, chip; and a main body configured to perform a first authentication and a second authentication of the consumable unit, when the consumable unit is mounted thereto, wherein the main body comprises: a main controller configured to perform the first authentication according to firmware stored in the image forming apparatus; characterised in that the main body further comprises an authentication controller configured to use at least one ASIC, Application Specific Integrated Circuit, to perform the second authentication of the consumable unit, wherein the authentication controller is operable to perform the second authentication separately from the first authentication performed by the main controller, and the main controller is operable to perform communication with the CRUM chip, when both the first authentication and the second authentication succeed, and wherein the authentication controller comprises: an interface to receive authentication information including a first message authentication code, MAC, from the CRUM chip; an authentication processor comprising an electronic signature verifier to verify an electronic signature included in the authentication information, a section key generator to generate a section key, a MAC generator configured to use the section key to generate a second MAC, and a MAC verifier configured to compare the second MAC with the first MAC; wherein the at least one ASIC includes the electronic signature verifier, section key generator, MAC generator and MAC verifier. an interface to receive authentication information including a first message authentication code, MAC, from the CRUM chip; an authentication processor comprising an electronic signature verifier to verify an electronic signature included in the authentication information, a section key generator to generate a section key, a MAC generator configured to use the section key to generate a second MAC, and a MAC verifier configured to compare the second MAC with the first MAC; wherein the at least one ASIC includes the electronic signature verifier, section key generator, MAC generator and MAC verifier.