For protecting data stored in an electronic device, an inner security key encrypting and decrypting data stored in the electronic device is encrypted with an outer security key. The outer security key is stored on an external memory to be coupled to the electronic device. The outer security key is generated from at least a unique identifier of the electronic device, a unique identifier of the external memory device, and a personal identification number. Additional constituents of the outer security key may be provided, e.g., an identifier of an auxiliary device to be coupled to the electronic device.