Identity card with physical unclonable function

Abstract

An identity card, comprising a card body and a physical unclonable function arranged within the card body, wherein the physical unclonable function comprises a first light influencing layer and a second light influencing layer.

Claims

1. An identity card, comprising: a card body; a physical unclonable function arranged within the card body, wherein the physical unclonable function comprises a first light influencing layer and a second light influencing layer, said first light influencing layer and said second light influencing layer being arranged within the card body; a light source for emitting light towards the first light influencing layer and the second light influencing layer; and an optical sensor for sensing light from the first light influencing layer and the second light influencing layer in response to the emitted light.

2. The identity card of claim 1, wherein the first light influencing layer and the second light influencing layer have light transmittances which are smaller than a light transmittance of the card body.

3. The identity card of claim 1, wherein the respective light transmittance layer is an optically linear layer or an optically nonlinear or optically excited layer.

4. The identity card of claim 3, wherein the optically linear layer is one of the following layers: a metal layer or a printing ink layer or a nanoparticle layer or a layer having a light refraction index which is different than a light refraction index of the card body, or a diffractive layer, wherein the optically nonlinear layer is a nonlinear polymer layer, and wherein the optically excited layer is a fluorescent material layer.

5. The identity card of claim 1, wherein the first light influencing layer or the second light influencing layer comprises spaced surface elements, in particular stripes or rectangles or circular elements or oval elements.

6. The identity card of claim 1, wherein the first light influencing layer comprises a pattern, in particular a periodic pattern or a non-periodic pattern or a grating, of spaced surface elements having smaller transmittance than the card body, and wherein the second light influencing layer comprises a pattern, in particular a periodic pattern or a non-periodic pattern or a grating, of spaced surface elements having smaller transmittance than the card body.

7. The identity card of claim 6, wherein the respective pattern is one-dimensional or two-dimensional.

8. The identity card of claim 1, wherein the first light influencing layer comprises a first pattern of spaced surface elements, wherein the second light influencing layer comprises a second pattern of spaced surface elements, and wherein the first pattern of spaced surface elements and the second pattern of spaced surface elements are arranged above each other.

9. The identity card of claim 8, wherein the spaced surface elements of the first light influencing layer and the spaced surface elements of the second light influencing layer are arranged exactly above each other within a tolerance; or wherein the first pattern and the second pattern are displaced with respect to each other.

10. The identity card of claim 8, the surfaces of the spaced surface elements of the first light influencing layer have equal or different dimensions than surfaces of the spaced surface elements of the second light influencing layer.

11. The identity card of claim 1, wherein the respective light influencing layer comprises spaced surface elements respectively having a wavelength-scale surface dimensions or thicknesses, or wherein the respective light influencing layer comprises spaced surface elements respectively spaced apart by a wavelength-scale distance, or wherein a distance between the first light influencing layer and the second light influencing layer is of wavelength scale.

12. The identity card of claim 1, wherein the first light influencing layer and the second light influencing layer jointly form an optical lens.

13. The identity card of claim 1, wherein the card body comprises laminated transparent layers, wherein the first light influencing layer is arranged between successive two laminated transparent layers of the card body, and wherein the second light influencing layer is arranged between successive two laminated transparent layers of the card body.

14. A method for manufacturing an identity card, the method comprising: providing a plurality of transparent laminate layers; arranging a physical unclonable function within the plurality of transparent laminate layers, wherein the physical unclonable function comprises: a first light influencing layer, a second light influencing layer, a light source for emitting light towards the first light influencing layer and the second light influencing layer, and an optical sensor for sensing light from the first light influencing layer and the second light influencing layer in response to the emitted light; and laminating the plurality of transparent laminate layers and the physical unclonable function to obtain the identity card.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further embodiments will be described with respect to the accompanying figures, in which:

(2) FIG. 1 shows an identity card with PUF;

(3) FIG. 2 shows an identity card;

(4) FIG. 3 shows a stack of light influencing layers;

(5) FIG. 4 shows intensities of light;

(6) FIG. 5 shows a schematic diagram of a stack of three light influencing layers; and

(7) FIGS. 6a, 6b, 6c show the light influencing layers.

DETAILED DESCRIPTION

(8) The identity card 100 comprises a card body 101 with a PUF 101 which is arranged within a card body and forms a probed physical structure. The card body is not explicitly shown in FIG. 1.

(9) The identity card 100 further comprises an integrated circuit 103, e.g. a card chip, for integrating the circuitry for the PUF measurements, pre- and post-processing and the memories and applications into a complex material structure which is stimulated and measured by the enclosed optional integrated security function 105. The integrated security function 105 is configured to implement an application such as encryption or protocols.

(10) The integrated circuit 103 further comprises an optional memory 107 coupled to the integrated security function 105, and an interface 109 for communicating with the integrated security function 105.

(11) The integrated circuit 103 further implements an control function 111 for controlling the PUF 101, wherein the control function 111 e.g. comprises an optional pre-processing 113, an optional challenge processing 115, and a stimulus 117 for stimulating the PUF 101 e.g. with light. The stimulus 117 may comprise a light emitting element such as a LED or a LED array.

(12) The control function 111 further comprises one or more sensors 119 for sensing light from the PUF 101 in response to the stimulus generated by the stimulus 117, an optional response processing 121 and an optional post-processing 121.

(13) As shown in FIG. 1, the integrated circuit 103, e.g. a security chip, is connected with the card body both physically and logically so that the integrated circuit 103 is able to verify the integrity of its surroundings.

(14) According to some embodiments, the integrated security function 105 may output a Call PUF signal towards the pre-processing 113 which in response thereto may generate output start and challenge signals towards the challenge processing 115 to trigger the stimulus 117 to e.g. emit light towards the PUF 101.

(15) The light response from the PUF 101 is sensed by the sensor 119 providing a sensor signal to the response processing 121. The response processing 121 generates in response thereto the response and ready signals which are provided to the post-processing 123. The post processing 123 which provides a get secret signal to the integrated security function 105 for e.g. encryption.

(16) According to some embodiments, a device-unique material structure forming the PUF as e.g. key storage or optical fingerprint can be used which can have multiple advantages. Because the material carries the secrets, it is possible to bind a system to a physical object, which is one of our primary goals. Furthermore, invasive influencings would probably change or destroy the material and thereby render the embedded secret useless.

(17) For the scenario of an identity card as a smartcard, the choice of an optical PUF is favourable. Optical measurements are robust against Electro-Magnetic Interference (EMI) and in addition not only enable the use of the card body as the physical structure to be measured, but also provide the possibility of involving the printing on the card. However, resource constraints in smartcards, e.g. limited power consumption as well as static placement of the optical measurement mechanisms, do not allow the reading of optical PUFs by modulating, moving or tilting the laser beams.

(18) One way to generate several different light stimuli by or within the integrated circuit 103 is the integration of an LED array in the stimulus 117.

(19) In general, the challenge-response behaviour of the PUF 101 can be complex in order to resist model-building attacks and to provide enough entropy for the desired applications.

(20) According to some embodiments, the controlled PUF 101 is integrated in order to protect against modelling attacks and ensure secure usage of the PUF 101, e.g. for key embedding and reconstruction. Following the principles of a controlled PUF, not only does the material enclosure protect the inner circuits, but the control circuits also protect the system from read-out or other methods of unauthorized access to the core of the PUF 101 which could enable modelling attacks. Because of this, a component having access to the challenge-response interface of the PUF 101 can be the control logic circuitry implementing the control function 111. All other application logics on the identity card can communicate with the control logics in order to make use of the functionality of the PUF 101. As a result, mutual protection of control logic circuits and PUF structure can be achieved.

(21) FIG. 2 shows an identity card 200, comprising a card body 201 and a physical unclonable function 203, 207 arranged within the card body 201, wherein the physical unclonable function comprises a first light influencing layer 203 and a second light influencing layer 207.

(22) The first light influencing layer 203 comprises spaced surface elements 205, e.g. metal stripes, being arranged to form a one dimensional periodic pattern (grating). Accordingly, the second light influencing layer 207 comprises spaced surface elements 209, e.g. metal stripes, being arranged to form a one dimensional periodic pattern (grating). The first light influencing layer 203 and the second light influencing layer 207 are arranged above each other within the card body 201, which is more transparent than the spaced surface elements 205, 209. The card body 201 can be formed from a transparent polymer.

(23) The surface elements 205, 209 have dedicated surfaces being directed towards the optional light source 211, i.e. stimulus, arranged e.g. in the card body 201. The light source 211 may comprise an LED or a LED array. Thus the surface elements 205, 209 form light barriers interacting with the light, e.g. reflecting, refracting or diffracting light emitted by the light source 211 towards the surface elements 205, 209.

(24) The identity card further comprises a sensor 213 for sensing light from the surface elements 205, 209 in response to the light emitted by the light source 211.

(25) The light source 211 and the sensor can be integrated within an integrated circuit 215, which may have the architecture as shown in FIG. 1.

(26) The first light influencing layer 203 and the second light influencing layer 207 collectively form the PUF as a probed physical structure arranged in a probed volume V.sub.p.

(27) The maximum amount of information that can be extracted from a PUF increases with the space that can be measured e.g. by the chip, i.e. the integrated circuit 215. The card area that can be accessed by the measurement is the probed volume V.sub.p.

(28) According to some embodiments, the size of the probed area is defined on the one hand by chip specificationssuch as measurement sensitivity and relative positions of light sources and sensorsand on the other hand by the influencing properties of the card materials. The optical properties can be optimized in terms of both the sensitivity of the sensors and the desired penetration depth. A parameter which can be considered is the back-influencing of light, since the sensors on the chip measure only the light that is reflected or scattered back to the integrated circuit 215. This can be demonstrated using the one-dimensional model structure consisting of N equal partially reflective layers, each with reflection R. Intensities of light emitted and reflected by a single layer n are given by formula I.sub.tn=(1R)I.sub.t(n-1) and I.sub.rn=RI.sub.t(n-1) respectively, where I.sub.t(n-1) is intensity of light emitted by the previous layer. The total intensity of light reflected by the stack of k layers is given by formula

(29) $I_{\mathrm{rk}}=\underset{n=1}{\overset{k}{.Math.}}{I}_{\mathrm{rn}}.$

(30) FIG. 3 shows a stack of k (k=1/N) light influencing layers 301 collectively forming a PUF. The light influencing layers 301 may be reflective layers respectively comprising a grating of surface elements such as metal stripes.

(31) FIG. 4 depicts the intensities of light emitted and reflected by the stack of k (k from 1 to N) light influencing layers 301 as well as the intensity of light reflected by a single layer n. In particular, FIG. 4 shows normalized intensities of light transmitted 401 (solid line), reflected 403 by the stack of k layers (circles) and reflected 405 by a single layer n (triangles).

(32) The contribution from a single layer decreases with the layer number n. For example, if a single layer reflects 10% of light (R=10%), the maximum contribution from a single layer to the signal does not exceed 10% and the contribution from the 24.sup.th layer becomes smaller than 1%. The contribution from the remaining N-24 layers does not exceed 8% of the total signal. This model clearly shows the relation between sensitivity of sensor, material structure and the maximum size of the probed physical structure.

(33) The maximum amount of information that can be obtained from a probed area with an optical PUF can be roughly estimated, assuming that the smallest material structure that can be resolved by optical measurement has a size comparable to the wavelength of the probing light. The probed volume V.sub.p can be divided into N.sub.e elementary volumes .sup.3. In the simplest case, each elementary volume represents 1 bit of information and the maximum amount of information is given by the number of elementary volumes N.sub.e=V.sub.p/.sup.3.

(34) For example, from a volume V.sub.p of size 1.01.00.3 mm.sup.3 probed by a wavelength of 700 nm, one can extract a maximum of 8.710.sup.5 bits.

(35) With regard to light propagation and in order to describe the properties of light distribution at the exit surface of the medium, the influence of fabrication tolerances on the resulting light distribution and in order to relate the optimum medium structure and size to the integrated measuring means, the theoretical light propagation in the designed media can be calculated.

(36) The calculation approach can, by way of example, be based on elementary structures. A disordered structure is mathematically resolved into a finite series of elementary periodical structures. Light propagation in such structures can be numerically calculated in a volume compared to the probed volume V.sub.p. Any single parameter of the structure, like the number of layers, period, contrast or spatial shift, can be independently varied and the resulting change in light distribution can be calculated.

(37) FIG. 5 shows a schematic diagram of a stack of three light influencing layers 501, 503, 505, respectively comprising spaced surface elements 507, 509, 511, e.g. metal stripes, being arranged to form a periodic pattern (grating). The grating can be characterized by modulated refraction and absorption n(x), (x), period P.sub.x, layer distance d and a relative layer shift a.

(38) In order to determine the arrangement and/or structure of the light influencing layers 501, 503, 505, the so called stitch method can be employed. According to the stitch method, a volume is divided in a plurality of sub-volumes with e.g. 303030 m3 or elementary structures. The light distribution can then be determined for each sub-volume or elementary structure

(39) The light influencing layers 501, 503, 505, are arranged above each other, with or without a displacement in x direction. Furthermore, the surfaces of the spaced surface elements 507, 509, 511 can be aligned to show in the same direction, i.e. having normal showing in the z-direction and/or can be inclined with normal showing in the z-x-direction.

(40) FIG. 6a shows the light influencing layers 501, 503, 505 shown in FIG. 5, which are arranged within a transparent card body 601 of an identity card 600. The identity card further comprises a plurality of light sources 603 and a plurality of light sensors 605. FIGS. 6b and 6c show corresponding light distributions obtained for reflective layer thicknesses of 10 nm, FIG. 6b, and 20 nm, FIG. 6c.

(41) The light influencing layers 501, 503, 505 can comprise spaced surface elements being formed by thin (compared to the distance between the layers) metal stripes embedded in a transparent polymer material forming the card body 601. The metal stripes can have different, e.g. staggered, widths as shown in FIG. 6a. The cross section of the structure is shown schematically in FIG. 6a. The period P.sub.x between subsequent spaced surface elements can set to 50 m (wavelength-scale), and the distance between the light influencing layers 501, 503, 505 to about 100 nm (wavelength-scale).

(42) The thickness of the light influencing layers 501, 503, 505 can be varied between 10 and 30 nm or even 300 nm. The volume can be illuminated with parallel light of a wavelength of 630 nm under normal incidence.

(43) The graphs of FIGS. 6b and 6c show the resulting spatial distributions of light intensity normalized to the intensity of incident light. There is a difference in the light distribution for the different metal layer thicknesses of 10 and 30 nm. The latter results in a focussed light thus in even a further improved contrast of the light pattern. Furthermore, the light is redistributed by the structure in the entire calculated volume while the distribution changes with distance from the structure. The distribution achieves higher contrast at some distance, while in the area close to the structure the contrast and the characteristic size of the light spots remain smaller. The results show how the light distribution is affected by a 20 nm difference in layer thickness.

(44) In the following, the material selection for the light influencing layers will be addressed.

(45) The light influencing layers can have a different light characteristic, e.g. transmittance, then the card body surrounding the light influencing layers. According to some embodiments, the light influencing layers can be structured using known card production techniques, including printing, spraying, dispersing, embossing or vacuum deposition techniques as well as holographic recording.

(46) Most optical materials do not interact with light, thus providing simultaneous and linear response. The signal from a PUF based on such materials is measured by a sensor at the same moment as the light is emitted by a light source and its value is proportional to the intensity of the probe light.

(47) Under the assumption that sensors integrate light over time t.sub.m, the type of suitable materials can be extended by material that can interact with light within time<t.sub.m. Optically non-linear materials change their optical properties under irradiation. This change is strongly dependent on the light intensity, is induced within a very short time and relaxes after the light is switched off. Fluorescent materials absorb light in one spectral region and emit it in another, usually red-shifted, spectral region. Emission follows absorption within a very short time and depends strongly on the molecular surroundings of the emitting units. Typical emission times are within the range of 0.5 to 20 ns.

(48) Of particular interest are time-resolved or time-delayed measurements, where the response is measured with some delay relative to the probe light flash. An approach of this kind only makes sense if the material response changes reversibly during or after irradiation. Photochromic materials are examples of such materials. Under irradiation with actinic light, the photochromic unit undergoes a transformation to its other form, which has a different absorption spectrum. Relaxation to the initial form occurs thermally or under irradiation. The most effective photochromic materials are stilbenes, spiropyranes, azobenzenes as well as bacteriorhodopsin.

(49) The material types and their related properties are summarized in Table 1.

(50) TABLE-US-00001 TABLE 1 Material overview and properties Material types Related properties Optically linear: Simultaneous response: printing inks, metallic response depends on light intensity; broadband inks, dispersed micro absorption spectra; probing with different and nanoparticles, wavelengths is desirable in order to increase materials with high variation of PUF response. refractive indices Optically nonlinear: Simultaneous response: variety of nonlinear response changes with time, but the response polymers, time is very short (shorter than measurement bacteriorhodopsin time); response depends on light intensity; nonlinear effects require very high light intensities or electrical fields. Optically excited: Simultaneous response (s): fluorescent materials response is spectrally shifted relative to the excitation; typical excitation in UV or blue region of VIS; requires sensor sensitivity in different spectral regions. Optically or thermally Time-resolved response: excited: photochromic change their colour under irradiation or heat; polymers, thermochromic response depends on light intensity (or heat); polymers response time varies widely (from ms to minutes) depending on material structure; relaxation may last even longer; typically require irradiation in UV or blue region of VIS; response depends on the modulation frequency. Optically excited: Time resolved response (seconds to hours): phosphorescent materials very long relaxation time.

(51) Regarding the light generation and measurement, the integration of light sources and sensors into the security chip (integrated circuit) that is embedded in the card can be performed.

(52) According to some embodiments, the structure of the PUF can be used for generating an encryption key, wherein authentication by a challenge-response protocol directly using the unique mapping of challenges to responses of a PUF can be provided.

(53) According to some embodiments, material-based PUFs can be used where the measurement circuitry is integrated into the smartcard controller and the PUF structure is part of the card material surrounding the chip. Such PUF system is considered resistant against laser fault injection attacks and micro probing, since any physical change results in different measurement data and therefore in altered PUF responses.