Physical unclonable function

1. A physical unclonable function comprising

a plurality of cross-coupled loops of two invertors, each cross-coupled loop of the plurality of cross-coupled loops being configured to settle into one of at least two different stable states upon power-up, the particular stable state into which a particular cross-coupled loop of the plurality of cross-coupled loops settles being dependent at least in part upon at least partially random physical characteristics of the particular cross-coupled loop,

a reading circuit for reading the plurality of stable states into which the plurality of cross-coupled loops settled after a power-up, the physical unclonable function being configured for the plurality of cross-coupled loops to be read-only, and

a switchable power domain, the plurality of cross-coupled loops being comprised in the power domain and at least a part of the physical unclonable function being outside the power domain, the power domain being configured for selectively connecting and disconnecting the plurality of cross-coupled loops from a power supply while the at least a part of the physical unclonable function is connected to the power supply.

2. A physical unclonable function as in claim 1 , wherein the plurality of cross-coupled loops of two invertors are a plurality of bus-keepers.

3. A physical unclonable function as in claim 1 , wherein at least part of the reading circuit is outside the power domain.

4. A physical unclonable function as claim 1 , wherein the power domain is only powered during read out of the cross-coupled loops.

5. A physical unclonable function as in claim 1 , wherein the plurality of cross-coupled loops, the power domain, and the at least a part of the physical unclonable function are implemented on a single integrated circuit.

6. A physical unclonable function as in claim 1 , comprising an isolation circuit for isolating the plurality of cross-coupled loops from the at least a part of the physical unclonable function while the plurality of cross-coupled loops is disconnected from the power supply.

7. A physical uncloable function as in claim 6 , wherein the isolation circuit comprises a plurality of and-gates.

8. A physical unclonable function as in claim 1 , wherein at least one cross-coupled loop comprises exactly one data connection, and wherein the exactly one data connection is only configured for reading the stable-state into which the at least one cross-coupled loop settled upon power-up.

9. A physical unclonable function as in claim 1 wherein the reading circuit comprises a plurality of multiplexers for selecting cross-coupled loops from the plurality of cross-coupled loops.

10. A physical unclonable function as in claim 9 , wherein each cross-coupled loop of the plurality of cross-coupled loops is connected to a multiplexer of the plurality of multiplexers.

11. A physical unclonable function as in claim 9 , wherein at least two cross-coupled loops of the plurality of cross-coupled loops are connected to the same multiplexer of the plurality of multiplexers.

12. A physical unclonable function as in claim 9 , wherein the plurality of multiplexers is arranged as a multiplexer-tree, the reading circuit being arranged for reading the plurality of stable states through the multiplexer-tree.

13. A physical unclonable function as in claim 9 , wherein each different cross-coupled loop of the plurality of cross-coupled loops is connected to a different multiplexer of the plurality of multiplexers.

14. A physical unclonable function as in claim 1 , comprising PUF control logic for deriving an identifier from the plurality of stable states.

15. A physical unclonable function as in claim 14 , wherein the PUF control logic is configured for applying an error correcting algorithm to the plurality of stable states and to helper data for deriving the identifier.

16. A physical unclonable function as in claim 14 , wherein the PUF control logic is configured for deriving a random number from the plurality of stable states.

17. A physical unclonable function as in claim 14 , wherein the PUF control logic is configured for a read-cycle comprising:

powering down the plurality of cross-coupled loops,

powering up the plurality of cross-coupled loops, and

reading the plurality of cross-coupled loops.

18. A method for obtaining PUF data comprising

powering-up a plurality of cross-coupled loops comprised in a switchable power domain configured for selectively connecting and disconnecting the plurality of cross-coupled loops from a power supply,

allowing each one of the plurality of cross-coupled loops to settle into one of at least two different stable states, the particular stable state into which a particular cross-coupled loop of the plurality of cross-coupled loops settles being dependent at least in part upon at least partially random physical characteristics of the particular cross-coupled loop,

reading the plurality of stable states into which the plurality of cross-coupled loops settled after a power-up through a reading circuit, the plurality of cross-coupled loops being read-only.

19. A non-transitory tangible computer readable storage medium comprising computer program code adapted to perform, when the computer program is run on a computer;

powering-up a plurality of cross-coupled loops comprised in a switchable power domain configured for selectively connecting and disconnecting the plurality of cross-coupled loops from a power supply,

allowing each one of the plurality of cross-coupled loops to settle into one of at least two different stable states, the particular stable state into which a particular cross-coupled loop of the plurality of cross-coupled loops settles being dependent at least in part upon at least partially random physical characteristics of the particular cross-coupled loop,

reading the plurality of stable states into which the plurality of cross-coupled loops settled after a power-up through a reading circuit, the plurality of cross-coupled loops being read-only.