Abstract: A data processing apparatus for processing secure data is disclosed. The data processing apparatus comprising: processing circuitry comprising a plurality of state retention cells in the form of scan chains for holding a current state of said processing circuitry, at least some of the state retention cells being arranged in series; encryption circuitry; and a hibernate signal input; said data processing apparatus being responsive to receipt of a hibernate signal at said hibernate signal input to switch from an operational mode in which said data processing apparatus is powered up, to a low power mode in which at least said processing circuitry is powered down, said data processing apparatus being operable prior to powering down said processing circuitry, to output a state of said processing circuitry from said plurality of state retention cells and to encrypt said output state using said encryption circuitry and to save said encrypted state to said storage device.