Devices with modifiable physically unclonable functions

1. A method for modifying a physically unclonable function (PUF) source of a device by changing a resistance of a fuse of the PUF source in response to a tamper attempt, the method comprising:

transmitting a first signal through the fuse of the PUF source;

measuring a current or voltage of the first signal with a sensor of the device;

generating a pre-tamper PUF value using the measured current or voltage of the first signal;

generating a cryptographic key using the pre-tamper PUF value;

detecting an attempt to tamper with the device;

modifying the PUF source in response to detection of the attempt, the modifying comprising transmitting through the fuse a second signal of sufficiently high current or voltage to change a resistance of the fuse;

transmitting a third signal through the fuse after the modifying;

measuring a current or voltage of the third signal with the sensor of the device; and

generating a post-tamper PUF value using the measured current or voltage of the third signal.

2. The method of claim 1 , wherein the fuse is included in a plurality of fuses, and wherein the modifying randomly changes responses of the plurality of fuses to the third signal.

3. The method of claim 1 , wherein the device is a payment reader, and wherein the method further comprises using the cryptographic key to perform a payment transaction.

4. A method for modifying a physically unclonable function (PUF) source of a device, the PUF source having at least one fuse embedded in the device, the method comprising:

transmitting a first signal through the at least one fuse of the PUF source;

measuring a current or voltage of the first signal with a sensor;

providing a PUF value from the PUF source based on the measured current or voltage of the first signal;

processing the PUF value within the device to perform an operation;

detecting an event at the device subsequent to the processing;

determining, with processing circuitry of the device, to modify the PUF source based on the detected event; and

modifying the PUF source in response to the determining, the modifying comprising transmitting through the at least one fuse a second signal of sufficiently high current or voltage to change a resistance of the at least one fuse.

5. The method of claim 4 , further comprising applying an input to the PUF source, wherein the transmitting is performed in response to the input, and wherein the modifying changes a response of the PUF source to the input.

6. The method of claim 4 , wherein the at least one fuse comprise a plurality of fuses, wherein the method further comprises applying an input to the PUF source, wherein the transmitting is performed in response to the input, and wherein the modifying randomly changes responses of the plurality of fuses to the input.

7. The method of claim 4 , wherein the event is a reception of a user input by the device.

8. The method of claim 4 , wherein the event is an attempt to tamper with the device.

9. The method of claim 4 , wherein the device is a payment reader, and wherein the operation is performance of a payment transaction.

10. The method of claim 4 , further comprising authenticating the device based on the PUF value.

11. The method of claim 4 , wherein the processing comprises generating a cryptographic key based on the PUF value.

12. The method of claim 4 , wherein the processing comprises generating an authentication token based on the PUF value.

13. The method of claim 4 , wherein the at least one fuse has a dielectric layer, and wherein the voltage of the second signal exceeds a breakdown voltage of the dielectric layer.

14. The method of claim 4 , wherein the at least one fuse has a layer of amorphous silicon, and wherein the voltage of the second signal is sufficiently high to convert the amorphous silicon to a polycrystalline silicon-metal alloy.

15. The method of claim 4 , wherein the at least one fuse has a layer through which the first signal passes, and wherein the PUF value is based on a thickness of the layer.

16. The method of claim 15 , wherein the layer comprises a dielectric material, and wherein the voltage of the second signal exceeds a breakdown voltage of the dielectric material.

17. The method of claim 15 , wherein the layer comprises amorphous silicon, and wherein the voltage of the second signal is sufficiently high to convert the amorphous silicon to polycrystalline silicon-metal alloy.

18. A device, comprising:

a physically unclonable function (PUF) source having at least one fuse, the PUF source responsive to an input for providing a PUF value based on measurement of a current or voltage of a first signal transmitted through the at least one fuse; and

circuitry configured to process the PUF value to perform an operation within the device, the circuitry further configured to select the at least one fuse for modification based on an event detected by the circuitry and to modify the at least one fuse by transmitting a second signal of sufficiently high current or voltage to change a resistance of the at least one fuse, thereby changing a response of the PUF source to the input.

19. The device of claim 18 , wherein the event is a reception of a user input by the device.

20. The device of claim 18 , wherein the event is an attempt to tamper with the device.

21. The device of claim 18 , wherein the device is a payment reader, and wherein the operation is performance of a payment transaction.

22. The device of claim 18 , wherein the circuitry is configured to generate a cryptographic key based on the PUF value.

23. The device of claim 18 , wherein the circuitry is configured to generate an authentication token based on the PUF value.

24. The device of claim 18 , wherein the at least one fuse has a dielectric layer, and wherein the voltage of the second signal exceeds a breakdown voltage of the dielectric layer.

25. The device of claim 18 , wherein the at least one fuse has a layer of amorphous silicon, and wherein the voltage of the second signal is sufficiently high to convert the amorphous silicon to a polycrystalline silicon-metal alloy.