Data processing device and method for protecting a data processing device against tampering

1. A data processing device, comprising:

a protection key unit for providing a protection key;

a dummy key unit for providing a dummy key, wherein 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 in a set of two or more allowed dummy key values; and

a control logic 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 indicating a tamper event, wherein the protection key unit comprises a set of n binary memory elements for representing the protection key and wherein the dummy key unit comprises a set of m binary memory elements for representing the dummy key, wherein m is less than n.

2. The data processing device of claim 1 , wherein the control unit is arranged to set the dummy key to a new value upon setting the protection key to the value of the dummy key.

3. The data processing device of claim 2 , wherein said new value of the dummy key is zero.

4. The data processing device of claim 1 , wherein the protection key unit and the dummy key unit are connectable to a host device so as to enable the host device to set the dummy key to any value in said set of two or more allowed dummy key values.

5. The data processing device of claim 1 , comprising a secure unit arranged to be kept continuously on power, the secure unit-comprising at least the protection key unit, the dummy key unit, and the control unit.

6. The data processing device of claim 5 , wherein the secure unit comprises one or more batteries arranged to keep it continuously on power.

7. The data processing device of claim 1 , wherein said binary memory elements are flip-flops.

8. The data processing device of claim 1 , wherein the dummy key unit comprises:

a seed unit for providing a seed representative of the dummy key, the seed having a bit length of m; and

a wiring mesh circuit having an input connected to the seed unit and an output connected or connectable to the protection key unit, wherein the wiring mesh unit defines a binary matrix and is arranged to multiply the seed by said binary matrix to generate a wiring mesh output value representative of the dummy key, the wiring mesh output value having a bit length of n which is greater than m.

9. The data processing device of claim 1 , comprising one or more functional hardware units connected to the protection key unit and arranged to encrypt or decrypt data on the basis of the protection key.

10. The data processing device of claim 1 wherein the protection key has a first value and the dummy key has a second value different from the first value.

11. The data processing device of claim 10 , wherein the second value is non-zero.

12. The data processing device as set forth in claim 10 , wherein said first value is chosen at random.

13. A method of protecting a data processing device against tampering, comprising:

providing within the data processing device a protection key and a dummy key, the dummy key having a non-zero value different from the protection key;

detecting a tamper event; and

in response to the detection of the tamper event, setting the protection key to the value of the dummy key, wherein the protection key is a bit sequence of length n and the dummy key is a bit sequence of length m, wherein m is less than n.

14. The method of claim 13 , further comprising, after said operation of setting the protection key to the value of the dummy key:

zeroing the dummy key.