TEMPERATURE INDEPENDENT PHYSICALLY UNCLONABLE FUNCTION DEVICE

Abstract

A physically unclonable function (PUF) device comprises a plurality of conductors, at least some of which are arranged so that they interact electrically and/or magnetically with one another. A media surrounds at least a portion of each of the conductors and a plurality of temperature compensation particles are arranged throughout the media, where the temperature compensation particles have a temperature coefficient selected such that they compensate for temperature-related effects in the PUF device by making the permittivity and/or permeability of the media substantially temperature independent. Circuitry applies an electrical challenge signal to at least one or the conductors and receives an electrical output from at least one of the other conductors to generate an identifying response to the challenge signal that is unique to the device.

Claims

1. A physically unclonable function (PUF) device comprising: a plurality of conductors, at least some of which are arranged so that they interact electrically and/or magnetically with one another; a media surrounding at least a portion of each of the conductors; circuitry for applying an electrical challenge signal to at least one of the conductors and for receiving an electrical output from at least one of the other conductors to generate an identifying response to the challenge signal that is unique to the device; and a plurality of temperature compensation particles arranged throughout the media, the temperature compensation particles having a temperature coefficient selected such that they compensate for temperature-related effects in the PUF device by making the permittivity and/or the permeability of the media substantially temperature independent.

2. The PUF device of claim 1, wherein the temperature compensation particles comprise at least two different materials.

3. The PUF device of claim 2, wherein a temperature coefficient of permittivity and/or temperature coefficient of permeability of a first material of the at least two different materials has an opposite sign to a temperature coefficient of permittivity and/or temperature coefficient of permeability of a second material of the at least two different materials.

4. The PUF device of claim 2, wherein the at least two different materials are sintered together.

5. The PUF device of claim 1, wherein the temperature compensation particles are micro-particles and/or nano-particles.

6. The PUF device of claim 1, wherein the conductors comprise electrically insulated wires overlapping one another; are embedded within a substrate material with vias to allow for overlapping routing; or are formed from a complex media of mixed permittivity, permeability and conductivity.

7. The PUF device of claim 1, wherein the circuitry for applying the challenge signal is arranged to vary the conductors to which the challenge signal is applied and/or the conductors from which the response is received after each challenge is applied to the device.

8. The PUF device of claim 1, wherein the circuitry for applying the challenge signal is arranged to vary the number of conductors to which the challenge signal is applied and/or the number of conductors from which the response is received after each challenge is applied to the device.

9. The PUF device of claim 1, wherein the circuitry is arranged to apply at least a second electrical challenge signal to at least one of the conductors and to receive at least a second electrical output from at least one of the other conductors to generate an identifying response to the challenge signal that is unique to the device.

10. The PUF device of claim 9, wherein the electrical challenge signal is a first electrical challenge signal, and wherein the second electrical challenge signal is applied to a different set of conductors than the first electrical challenge signal.

11. The PUF device of claim 9, wherein the second electrical output is received from a different set of conductors than the first electrical output.

12. A method of fabricating temperature compensation particles for a PUF device, the temperature compensation particles arranged to be placed in a surrounding media of the PUF such that they compensate for temperature-related effects in the PUF device by making the permittivity and/or the permeability of the media substantially temperature independent, the method comprising: combining at least two different materials to form a composite material, the composite material having a temperature coefficient selected such that the permittivity and/or the permeability of the composite material is substantially temperature independent; grinding the composite material into temperature compensation particles.

13. The method of claim 12, wherein a temperature coefficient of permittivity and/or temperature coefficient of permeability of a first material of the at least two different materials has an opposite sign to a temperature coefficient of permittivity and/or temperature coefficient of permeability of a second material of the at least two different materials.

14. The method of claim 12, wherein combining the first material and the second material comprises at least one of the following: bonding the first material to the second material; welding the first material to the second material; fusing the first material to the second material; growing the second material on the first material; or depositing the second material on to the first material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Examples of the present invention will now be described with reference to the accompanying drawings:

[0036] FIG. 1 shows a schematic diagram of an example PUF device according to the invention;

[0037] FIG. 2 shows a schematic diagram of an example stand-alone PUF device according to the invention;

[0038] FIG. 3 shows a schematic diagram of an example PUF according to the invention which encases other components;

[0039] FIG. 4 shows an example of a temperature compensation particle according to the invention; and

[0040] FIG. 5 shows an example of a composite material to be ground into temperature compensation particles.

DETAILED DESCRIPTION

[0041] FIG. 1 shows a simplified conceptual diagram of the PUF network, with a reduced number (eg. five) of conducting paths 2 spread across two layers (solid black and dashed grey). The surrounding media 3 is, in this example, heterogeneous and is different for each PUF. Temperature compensation particles 10 (not shown) are arranged throughout the surrounding media 3 in order to introduce further heterogeneity whilst also compensating for temperature-related effects in the PUF. The PUF interface circuitry 1 applies electrical stimuli to a subset of the conducting paths. The interface circuitry 1 or an external challenge input 4 determines the selection of paths; and amplitude, phase and frequency of the stimuli applied to these paths.

[0042] The currents induced, in response to the challenge, within a different subset of conducting paths are received by the circuitry 1 and provide an identifying response 5 which is output from the PUF. The output 5 will be application specific, derived from the behaviour of the PUF 2, 3 and inference by circuitry 1.

Construction of the PUF Element

[0043] The conducting paths may be arranged such that all paths have a good probability of interacting in the absence of the heterogeneous media. This arrangement of the conducting paths ensures that the response of the PUF instantiation is unpredictable. The arrangement of the paths may be calculated by an optimisation algorithm whereby the cost function is related to the deviation of the integrated path couplings. The path routing may also be changed between different instantiations of the PUF, provided the integrated coupling along the lengths of each path to all others is sufficient to provoke a complex, non-predictable, tamper-proof response.

[0044] The conducting paths may comprise electrically insulated wires overlapping one another; be embedded within a substrate material with vias to allow for overlapping routing; or be formed from a complex media of mixed permittivity, permeability and conductivity. The heterogeneous media surrounding the conductors may then be applied in the form of some setting material such as epoxy, or by ‘doping’ existing substrate material such as FR4.

PUF Interfacing

[0045] The interface circuitry 1 between the PUF device and any application may be application specific. In a digital circuit, the challenge and response may be digital signals, which might be converted into analogue stimuli by the PUF interface. Alternatively, the challenge and response could themselves be analogue, in which case they may not need conversion before being transmitted to the conducting paths.

[0046] The circuity 1 for encoding challenges into the appropriate waveforms may be implemented as an Application Specific Integrated Circuit (ASIC), or by a combination of commercial off-the shelf components enclosed within the effective tamperproof region. This may also provide control access to the PUF, reducing the effectiveness of ‘brute force’ attacks by limiting the number of challenge-response pair requests within a given period.

[0047] In one embodiment, the PUF device may be a self-contained, standalone element. This is shown in FIG. 2. In this arrangement, the PUF is protecting the relationship that maps challenges to responses, i.e. the control electronics 1 for the PUF. The PUF interface 1 is contained within the PUF network (the conducting paths 2 in heterogenous media 3). It is difficult to reproduce the precise arrangement and nature of the conducting paths within the heterogeneous media, making it difficult to clone the PUF. Containment 6 may be provided to surround the PUF, and provide physical protection/robustness. This can also include a metallic component/ground plane to inhibit electrical measurement of the PUF. This containment element is not essential to operation of the PUF device however. The PUF device is shown mounted on a supporting structure 7, such as a printed circuit board) for clarity, although this is not key to operation of the PUF, and is not essential. Interconnects 8, pass through the heterogeneous media and containment to the circuit board to allow for communication between the PUF and the application circuit.

[0048] In another example, the PUF network may be used to fully/partially enclose other elements to provide protection to those elements, this is shown in FIG. 3. The PUF interface 1 and other protected components 9 are contained within the PUF network 2,3. For example, the protected components could be a microprocessor and encrypted storage module. In this case, the PUF can be used to generate the encryption key for the storage module. The PUF network has been formed around the interface electronics and protected components, and is connected to the interface internally. Attempts to disassemble or probe inside the PUF will cause a change in the electrical properties of the PUF network, leading to a change in the response generated for a given challenge, and preventing decryption of the storage module. Communication with the rest of the circuit can then be performed via the interconnects to the PCB.

[0049] If the control electronics are embedded within the PUF, they should also include means to correct any errors to ensure a repeatable key is produced within the required response time. This may be fuzzy logic, such as a fuzzy extractor, that ensures that small changes in the physical response (e.g. noise) do not lead to changes in the response.

[0050] In both FIGS. 2 and 3, the PUF element is shown as a dashed line to illustrate how it encloses the components to be protected. However, in a practical implementation the conducting paths 2 would extend to the edges of the heterogeneous media 3, with secondary conduction paths (due to the media and any additives) extending throughout the entire volume of the media.

[0051] The entire device may be fixed to a solid structure, or made flexible and shaped to the desired form factor for the initial registration process. The material and substrate selection may also be altered to adjust the entropy of the system, and to meet other constraints of the system such as thermal control. The device may then be also enclosed within a ground plane, forming a Faraday cage, to shield the unit from external electromagnetic interference and prevent side-channel attacks on the unit.

[0052] In use the PUF is passed a challenge from an external circuit via the PUF Interface 1. The interface 1 converts the request, which could be received as a serial command, to a challenge which can be fed into the PUF device. For example, in an instantiation with ten conducting paths, four may be stimulated with a signal (of varying waveform shape, frequency, amplitude and phase offset), with the response measured on any number of the other six paths. The presence of non-linear materials, such as ferro-magnetics, within the heterogeneous media will introduce a dependency of the response to the amplitude of the challenge. Furthermore, eddy currents within the media will alter the response detected by an individual conductor and introduce frequency dependence. The amplitude at the given stimulus frequency on each of the response wires is then converted into a response vector, which may then be converted back to a serial stream via the PUF interface 1 and fed back to the circuit. A serial example is provided here, but may also be implemented by a parallel bus or any other electrical interface circuitry.

[0053] Between challenges, the number of paths involved in the challenge may change, as may the number of paths used to detect the response. The specific paths used within each challenge may be changed, or kept the same between different challenges. Similarly, the conductors 2 used to detect the response may also change between each challenge.

[0054] The frequencies of conductor excitation may be in the audio range, utilising low cost and readily available transmit/receive electronics, or may operate at higher frequencies to develop a more complex electromagnetic interaction in which time delays become significant, and at which physical effects such as the skin effect play a large role in the interaction between the conductors.

[0055] If an adversary attempts to probe the PUF device to measure the electrical characteristics of the instantiation, the presence of the probe should cause a sufficient deviation of the response for a given challenge to invalidate the PUF, and render the device temporarily unreadable.

[0056] The initial registration process will be dependent on the use of the proposed invention, but is a necessary step to use the PUF device in a practical implementation. For remote authentication, the challenge-response pairs may be queried and stored securely during a registration process. For secure key storage, this is a one time and irreversible procedure.

[0057] As previously noted, environmental factors such as temperature may influence the response of the PUF. This can be mitigated through the introduction of temperature compensation particles into the PUF. Factors such as humidity can be mitigated by instantiating the PUF within a hermetically sealed enclosure. Measurements of other parameters, for example by use of a thermocouple in the control ASIC, may then be used in the generation of a challenge to the PUF. Furthermore, the temperature of the PUF can be locally controlled as part of the challenge process, further increasing the difficulty of modelling the PUF device response.

Temperature Compensation Particles

[0058] A temperature compensation particle, as employed in the present invention, is a particle which has a property (or properties) which are substantially temperature independent and when introduced into a PUF device they improve its resilience to the effects of temperature and thereby increase the device's operating temperature range.

[0059] Examples of the temperature independent properties are permittivity and permeability. In the case of the property being permittivity, the particle will have a temperature coefficient of permittivity close to zero while in the case of the property being permeability the particle will have a temperature coefficient of permeability close to zero.

[0060] As shown in FIG. 4, the low magnitude temperature coefficient may be the result of the particle 10 comprising materials with temperature coefficients which counteract each other, such as a first material 11 with a temperature coefficient of opposite sign to the temperature coefficient of a second material 12. For example, ilmenite has a dielectric constant of 16 at 25° C., +120 ppm/° C. and barium polytitanate has a dielectric constant of 37 at 25° C., −25 ppm/° C. If these materials are mixed in the ratio of 1:3.191 then the effective permittivity of the mixture is 31.9899 over the temperature range 25-150° C. The temperature coefficient of the dielectric constant of the resulting mixture is reduced to 0.5 ppm, thereby reducing the temperature effect on the dielectric constant by a factor of 98%. Though this example mixture is made up of only two different materials, temperature compensation particles may comprise more than two different materials.

[0061] Another combination of materials resulting in a substantially temperature independent permittivity is the mixing of neodymium, samarium and other rare-earth oxides to form a compound with negligible dependence of dielectric constant over the temperature range −55 to +125° C.

[0062] A particle 10 with temperature independent magnetic permeability could also be formed from the mixing of a ferromagnetic or ferrimagnetic material with an antiferromagnetic material, provided that the curie temperature of the ferromagnetic or ferrimagnetic material exceeds the Neel temperature of the antiferromagnet. Examples of materials which could be mixed according to this scenario include iron (a ferromagnet) or iron oxide (a ferrimagnet) and chromium or manganese oxide.

[0063] The temperature compensation particles 10 may be selected for inherently having a desired temperature coefficient or may be designed and the product of a particle fabrication process.

[0064] There are a number of methods for fabricating temperature compensation particles 10 and methods are not limited to the following example.

[0065] In this example, two feeder materials with complementary properties, such as a first material 11 with positive temperature coefficient of permittivity and a second material 12 with negative temperature coefficient of permittivity, are selected. The ratio of the amount of first material 11 used in the fabrication process, relative to the second material 12, is such that the resultant particles will have substantially temperature independent permittivity.

[0066] A sheet, or multiple sheets, of the first material 11 is combined with a sheet, or multiple sheets, of the second material 12 to form a composite material 13. The materials may be combined through processes including, but not limited to, bonding, welding, fusing, growing and deposition. An example of a composite material 13 formed by the combination of a sheet of a first material 11 with a sheet of a second material 12 is shown in FIG. 5.

[0067] Once the composite material 13 has been formed, it may be fed into a grinder and ground into a powder comprising the temperature compensation particles 10. On average, the composition of each particle 10 of the powder will be the same as the ratio of the feeder materials and so the particles 10 will exhibit the net temperature dependence of the target property (in this case permittivity) as the mixture of materials used in their fabrication.

[0068] Optionally, the powder may be sintered to further control the shape and size of the temperature compensation particles 10.

[0069] The temperature compensation particles 10 may be arranged throughout the media 3 before the media 3 is applied to the PUF or after the media 3 is applied but before it has set.