A data processing device comprises a protection key unit, a dummy key unit, and a control unit. The protection key unit provides a protection key. The dummy key unit provides a dummy key. The dummy key unit has a set of two or more allowed dummy key values associated with it and is configurable by a user or a host device to set the dummy key to any value selected from said set of allowed dummy key values. The control unit is connected to the dummy key unit and to the protection key unit and arranged to set the protection key to the value of the dummy key in response to a tamper detection signal (fatal_sec_vio) indicating a tamper event. The value of the dummy key may notably be different from zero.A method of protecting a data processing device against tampering is also described.