Physical unclonable function with improved start-up behavior
An electric physical unclonable function (PUF) ( 100 ) is provided comprising a semiconductor memory element ( 110 ) connectable to a PUF control means for reading content from the memory element and for deriving at least in part from said content a digital identifier, such as a secret key. Upon powering the memory element it settles into one of at least two different stable states. The particular stable state into which the memory element settles is dependent at least in part upon random physical characteristics of the memory element introduced during manufacture of the memory element. Settling of the memory element is further dependent upon a control input ( 112 ) of the memory element. The electric physical unclonable function comprises shielding means ( 142, 144 ) for shielding, during a time period including the power-up of the memory element and lasting at least until the settling of the memory element, the control input from receiving control signals upon which the particular stable state into which the memory element settles is dependent. In this way, the dependency of the memory element on its physical characteristics is improved, and dependency on possibly irreproducible control signals is reduced.
1. An electric physical unclonable function (PUF) ( 100 , 200 ) comprising a semiconductor memory element ( 110 ) connectable to a PUF control means ( 120 ) for reading content from the memory element and for deriving at least in part from said content a digital identifier, the memory element being of a type that is configurable into at least two different stable states, the stable states representing the content of the memory element,
the memory element comprising
a power input for electrically powering the memory element and bringing the memory element into a powered-up state, and
a control input ( 112 ) for receiving one or more control signals, the control signals configuring the memory element into a selected one of the at least two different stable states,
the memory element being configured to settle into one of the at least two different stable states upon power-up, the particular stable state into which the memory element settles being dependent at least in part upon the at least partially random physical characteristics of the memory element,
wherein
the particular stable state into which the memory element settles upon power-up is also dependent upon the control input receiving a control signal,
the electric physical unclonable function further comprises shielding means ( 142 , 144 ) for shielding, during a time period including the power-up of the memory element and lasting at least until the settling of the memory element, the control input from receiving control signals upon which the particular stable state into which the memory element settles is dependent, by connecting the control input with a reference voltage line.
2. An electric physical unclonable function (PUF) as in claim 1
wherein the memory element comprises a plurality of interconnected semiconductor gates allowing at least two different stable states when the memory element is in the powered-up state, the physical, at least partially random, characteristics of the memory element comprising the at least partially random physical characteristics of the plurality of interconnected semiconductor gates.
3. An electric physical unclonable function as in claim 2 , wherein the interconnected semiconductor gates comprises at least two gates connected in a cross-coupled loop.
4. An electric physical unclonable function as in claim 3 , wherein at least one of the two gates connected in the cross-coupled loop is a multiple input gate.
5. An electric physical unclonable function as in claim 4 , wherein the memory element is a latch, a flip-flop or a register.
6. An electric physical unclonable function (PUF) as in claim 1 , wherein the time period lasts at least until the reading of the content of the memory element by the PUF control means.
7. An electric physical unclonable function as in claim 1 , wherein the deriving of the digital identifier depends on a first and a second type of data, the first type of data being previously deterministically stored in non-volatile memory ( 122 ), the second type of data depending at least in part upon the at least partially random physical characteristics of a memory element, the data of the second type being read exclusively from one or more memory elements as defined in claim 1 having shielding means.
8. An electric physical unclonable function as in claim 7 , wherein the shielding means comprises a permanent connecting between the control input and the reference voltage line.
9. An electric physical unclonable function as in claim 1 , wherein the memory element comprises a latch, having a set control input and a reset control input, the set control input and the reset control input being each connected to a reference voltage line such that the latch is in a keep state for preventing changes to the stable state into which the latch settled upon power-up.
10. An electric physical unclonable function as in claim 1 , wherein the shielding means comprises a timer, for shielding for a time period starting with the power-up of the memory element and lasting for a predetermined period of time.
11. An electric physical unclonable function as in claim 1 , wherein the control input for receiving one or more control signals comprises a clock input, at least part of the memory element being activated upon receiving a clock tick over the clock input, and wherein the shielding means is configured for shielding the clock input from receiving the clock tick.
12. An electric physical unclonable function as in claim 11 , wherein the memory element comprises a register having a gated clock, the shielding means being configured to control the gating of the gated clock.
13. An electric physical unclonable function ( 500 ) as in claim 11 , wherein the memory element comprises two cross-coupled loops, the shielding means being configured for shielding the clock input by selectably connecting the clock input to a high reference voltage or to a low reference voltage, the selected voltage further selecting one of the two cross-coupled loops, the particular stable state into which the memory element settles upon power-up being dependent upon the at least partially random physical characteristics of the selected one of the two cross-coupled loops but not upon the unselected one of the two cross-coupled loops.
14. An electric physical unclonable function as in claim 1 , wherein the shielding means comprises hardware shielding means for shielding, at least during a time period lasting until completion of the power-up of the memory element, the control input from receiving control signals upon which the particular stable state into which the memory element settles is dependent.
15. An electric physical unclonable function as in claim 14 , wherein the shielding means comprises software shielding means for shielding, at least during a time period lasting from the settling of the memory element until the reading of the content of the memory element by the PUF control means.
16. An electric physical unclonable function as in claim 1 wherein the shielding means is configured to stop the shielding of the memory element after the reading of the content of the memory element by the PUF control means to allow writing of content to the memory element by enabling the memory element to change from one stable state to another upon receiving at least one control signal while the memory element is in the powered-up state.
17. An electric physical unclonable function as in claim 16 comprising anti-aging means for writing anti-aging data into the memory element after the reading content of the memory element.
18. A method of operating an electric physical unclonable function (PUF) comprising a semiconductor memory element, the memory element being of a type that is configurable into at least two different stable states, the stable states representing the content of the memory element, the memory element comprising a control input for receiving one or more control signals, the control signals configuring the memory element into a selected one of the at least two different stable states,
the method comprising
powering the memory element and bringing the memory element into a powered-up state, the memory element settling into one of the at least two different stable states upon the powering,
the particular stable state into which the memory element settles being dependent at least in part upon the at least partially random physical characteristics of the memory element,
reading content of the memory element, and
deriving at least in part from said content a digital identifier,
wherein
the particular stable state into which the memory element settles upon power-up is also dependent upon the control input receiving a control signal, and wherein the method further comprises
shielding, during a time period including the power-up of the memory element and lasting at least until the settling of the memory element, the control input from receiving control signals upon which the particular stable state into which the memory element settles is dependent, by connecting the control input with a reference voltage line.