The present invention relates to a device for creating challenge-response pairs.
The use of “physically uncloneable functions” (PUFs) for security purposes is known, e.g. from WO 2005/048179. Incorporating a PUF into a product such as a smartcard, chip, or storage medium makes it extremely difficult to produce a “clone” of the product. In this document “clone” means either a physical copy of the product or a model that is capable of predicting the input-output behavior of the product with reliability. The difficulty of physical copying arises because the PUF manufacturing is an uncontrolled process and the PUF is a highly complex object. Accurate modeling is extremely difficult because of the PUF's complexity; slightly varying the input results in widely diverging outputs. The uniqueness and complexity of PUFs makes them well suited for identification, authentication or key generating purposes.
Typically, a proving party should prove access to a secret by providing a PUF with a challenge from which a unique and unpredictable response is created. This response is supplied to a verifying party, for verification that the proving party actually has access to the secret. Of course, this proving/verifying procedure should be undertaken without revealing the secret, which typically involves encryption/decryption. A PUF can only be accessed via an algorithm that is inseparable from the PUF, and any attempt to bypass or manipulate the algorithm will destroy the PUF. PUFs are e.g. implemented in tokens employed by users to authorize themselves and thus get access to certain services or devices. The token may for example comprise a smart card communicating by means of radio frequency signals or via a wired interface (such as USB) with the device to be accessed.
To this end, an optical PUF may be employed, which comprises a physical structure containing light scattering material arranged in such a manner that directions in which light is scattered are randomly distributed. The light scattering material can e.g. consist of a piece of epoxy, which contains glass spheres, air bubbles or any kind of scattering particles and/or one or more semi-reflective layers with a predetermined roughness. The epoxy can also be replaced by some other transparent means. Shining a laser through such an optical PUF produces a speckle pattern which strongly depends on properties of the incoming wave front and on the internal structure of the PUF. The input (wave front) can be varied by shifting or tilting the laser or by changing the focus of the laser beam.
Typically, the PUF is illuminated from an input side with a light source (e.g. a laser) and the light scattering material produces speckle patterns on an output side of the PUF which may be detected by means of a camera sensor. The randomness and uniqueness of the light scattering in this material is exploited to create challenge-response pairs and cryptographic key material to be used in authentication and identification schemes. The input (i.e. the challenge) to the optical PUF can e.g. be angle of incidence of the laser, focal distance or wavelength of the laser, a mask pattern blocking part of the laser beam, or any other change in laser beam wave front. The output (i.e. the response) of the optical PUF is the speckle pattern. The input-output pair is usually referred to as a challenge-response pair (CRP). Replicating an optical PUF is very difficult, since even if the exact location of the scattering elements are known, precise positioning of scattering elements in a replica is virtually impossible and very expensive to attain.
An object of the present invention is to provide a device for producing challenge-response pairs, which device is cost-effective to manufacture. This object is accomplished by a device in accordance with the independent claim attached hereto. Preferred embodiments of the invention are defined by dependent claims.
According to a first aspect thereof, the present invention provides a device for creating challenge-response pairs, which comprises a radiation source, a challenge-modifying element, a radiation scattering element, and a radiation-detecting element. The radiation source is arranged to create a challenge by irradiating said challenge-modifying element. The laser beam is either incident directly on the challenge-modifying element, or is guided from the laser to the challenge-modifying element by means of for example a reflective element, such as a mirror or a prism etc.
The challenge-modifying element is arranged to alter radiation received from said radiation source and direct said modified radiation towards said radiation scattering element.
The radiation scattering element is arranged to scatter light, which is received from said light source via said challenge-modifying element, and direct said light towards said radiation-detecting element.
The radiation-detecting element is arranged to create a response to said modified and scattered light, which is received from said radiation source via said radiation scattering element. Further, said radiation scattering element is preferably arranged such that the scattered radiation, which reaches said radiation detection element, passes an imaginary plane between said radiation scattering element and said radiation detection element, and said challenge-modifying element and said radiation-detecting element are both arranged on the same side of said imaginary plane.
One advantage of providing the radiation-detecting element and the challenge-modifying element on the same side with respect to said radiation scattering element, is that the device becomes easier to assemble, as the arrangement of the electric wiring of the components is facilitated.
Said challenge-modifying element is preferably arranged to modify said challenge by altering the point of incidence of said radiation at said radiation scattering element, the angle at which said radiation is incident at said radiation scattering element and/or the phase of said radiation incident on said radiation scattering element. In other words, by altering or modifying the challenge, one will also modify the response that corresponds to the modified challenge.
Advantageously, said challenge-modifying element and said radiation-detecting element are both arranged on the same substrate. This facilitates the manufacturing of the device, and it also facilitates the alignment of the components within the device.
Preferably, the radiation source is also arranged on the same side of said imagery plane as is the radiation detection element. Even more preferably, said radiation source is arranged on the same substrate as said challenge-modifying element and said radiation detection element. Hence, one compact integrated element is obtained, comprising all electrically controllable components of the device, which facilitates the assembling of the device. Further, such a compact integrated device facilitates the generation of speckles with an optimum size for being detected by the radiation-detecting elements, based on e.g. CMOS-technology.
According to one embodiment of the invention the challenge-modifying element comprises a translatable and/or pivotable lens. Hence, different challenges can be created by changing the position of said lends and/or by changing the inclination of said lens with respect to its main axis. In this document terms like “transparent” and “reflective” are used for objects which are transparent and reflective, respectively, to a radiation portion emitted from said radiation source, which radiation portion said radiation detection element is sensitive to, possibly the radiation has been frequency converted before it reaches said radiation detection element. One advantage of using a translatable lens, instead of e.g. a static SLM or SPM, is that less components are required for controlling the lens compared to controlling or addressing an SLM comprising a large number of mirrors.
Advantageously, said lens is provided with a reflective surface, and said reflective surface is arranged optically after said lens. In this document, when a first surface is arranged optically after a component, this means that the radiation first reaches said component before it reaches said first surface. The advantage of providing a reflective surface optically after said lens, is that the radiation can easily be directed towards different positions on said scattering element by means of reflection.
Advantageously, said challenge-modifying element comprises a pivotable mirror, which provides an accurate way of changing the angle of incidence of radiation at said light scattering element. According to one embodiment of the invention, said pivotable mirror is also translatable such that different portions of the incident radiation can be reflected by adjusting the position of the mirror.
Advantageously, said challenge-modifying element comprises several separately controllable areas arranged such that each area is able to modify a portion of said incident radiation independently of the other controllable areas. One example, said challenge-modifying element comprises an array of mirrors, wherein each mirror is pivotable independently of the other mirrors. Moreover, each mirror can be set in a number of different inclination states, each state corresponding to a different inclination of the mirror. Hence, by arranging different mirrors in different inclination states a large number of different challenges can be provided.
Said areas can also be liquid crystal (LC) elements or picture elements, which are able to alter the phase of incident radiation individually of each other. In other word, by activating the picture elements, the light which is incident on them will be reflected towards the light scattering element, and a plurality of different challenge-response pairs may be created, as will be described in the following. When liquid crystal elements are exposed to light (either directly from the light source or via the scattering element), light beams will be reflected at the LC elements and undergo a phase change (or a change in polarization state). By arranging the LC elements such that they can be set in a great umber of optical states, the phase of the light appears to change in a continuous manner as compared to a situation where the LC elements are switched between an off-state and an on-state. The reflected light will incide on the light scattering element. Hence, the light which is incident on the scattering element from the light source—the challenge—is modified by the light reflected at the LC elements and a new, modified challenge is created and input to the scattering element. The light scattering element scatters incident light such that a random speckle pattern is created and spread over the light detecting elements. This random pattern is detected by the light detecting elements, and a response to the modified challenge is thus created. Thus, the LC elements will act as a phase or polarization modulator for incident light, which has as an effect that the light which is supplied to the scattering element is modified. Typically, the degree of modification of the challenge is dependent on the number of activated picture elements, as well as actual combination(s) of activated picture elements. A great number of activated picture elements will result in a high degree of challenge modification as well an increase of challenge space. Each new challenge provided to the light scattering element will result in a different speckle pattern for the light which illuminates the light detecting elements. Consequently, each new combination of activated picture elements will render a new, modified challenge and a corresponding new response. A new challenge-response pair is thus created.
Preferably, said challenge-modifying element is a Micro Electro Mechanical System device (MEMS), e.g. a Spatial Light Modulator (SLM) or a Spatial Phase Modulator (SPM), comprising a two dimensional array of movable mirrors.
Said radiation scattering element is preferably arranged optically after said radiation source and optically before said challenge-modifying element. Further, said radiation scattering element is arranged to direct light from said radiation source towards said challenge-modifying element. Moreover, said radiation scattering element is preferably arranged to shape the radiation beam such that its cross section is adapted to the area of the challenge-modifying element. When an SLM, SPM or other relatively large challenge-modifying element is used, said radiation scattering element preferably comprises an elliptically or spherically shaped portion, which collimates the radiation beam before it is incident on said SLM. When a small translatable mirror is used as challenge-modifying element said radiation scattering element preferably comprises a focusing portion, e.g. being elliptically or spherically shaped.
Advantageously, said radiation scattering element is provided with a retro-reflection element, arranged to prevent light from being specularly reflected onto said radiation detection elements, said retro-reflection element preferably being a reflective surface.
Said radiation source is preferably a laser. Said radiation detection element is preferably a CMOS detector.
A basic idea of the present invention is to arrange a radiation detection element, a challenge-modifying element and preferably also a light source on the same side of an imaginary plane, which separates said radiation-detecting element from a radiation scattering element in a device for providing challenge response pairs. Hence, generation of a speckle pattern having a desired minimum speckle size is facilitated and a more easily assembled device is provided. In more detail, a challenge in the form of light emitted onto a light scattering element, which light will be scattered in the light scattering element and detected as a response to the challenge by light detecting elements. A light source in the form of e.g. a laser diode is typically used to produce the light that is emitted onto the scattering element. The light which is incident on the scattering element is referred to as a challenge. The emitted light is scattered and spread across the light detecting elements, wherein a response to the challenge is sensed by the light detecting elements. The light scattering element comprises a transmissive material which contains randomly distributed light scattering particles or simply physical irregularities, which scatter incident light such that a random speckle pattern is created and spread over the light detecting elements. This random pattern is detected by the light detecting elements, and is known as the response to the challenge (i.e. the light) that was supplied to the light scattering element. Hence, a challenge-response pair is created.
Further, by integrating a display comprising a plurality of picture elements (preferably arranged in a matrix), the possible number of challenge-response pairs that can be produced will increase greatly, as has been described in the above.
A detailed description of preferred embodiments of the present invention will be given in the following with reference made to the accompanying drawings, in which:
Like reference numbers and like designations in these figures refer to like embodiments.
Further, said device 100 comprises a challenge-modifying element 102 in order to vary the challenge, i.e. modify the radiation incident on said radiation scattering element 103 such that a different radiation pattern is sensed by said radiation-detecting element 103. Advantageously, said device 100 comprises an optical element 106 which substantially collimates the laser beam, in order to distribute said laser light evenly over the active area of said challenge-modifying element 102. According to this embodiment the challenge-modifying element 102 comprises an SLM, which in turn comprises pivotable reflective elements, such that the direction of a selected portion of the incident light beam can be altered. By altering the direction of said laser beam, the point of incidence of said laser radiation at said light scattering element 103 is also altered. Hence, the speckle pattern imaged on said radiation detectors 105 is altered, as the laser beam is scattered differently by said radiation scattering element 103. Consequently, the detectors return a different response when the point of incidence is altered. Preferably, the SLM is arranged such that the reflective elements can be rotated independently in two orthogonal directions, such that as many challenges as possible can be obtained.
Alternatively, an SPM can be used instead of said SLM. This SPM can for example be a MEMS (Micro Electro Mechanical System) device consisting of a two-dimensional array of movable mirrors. The activated mirrors cause the light reflected against these mirrors to have a different path length compared to the light reflected by the non-activated pixels or mirrors, and herewith spatially change the phase distribution of the reflected light. For each challenge a different distribution of the mirror-array can be set.
Generally, the radiation scattering element 103 is arranged on a first side of an imaginary plane 107, and said challenge-modifying element 102 and said radiation-detecting element 105 are arranged on a second side of said imaginary plane 107. Consequently, the laser light passes through said imaginary plane at least twice. Once after it has been reflected by said challenge-modifying element 102 but before it is scattered by said radiation scattering element 103, and once after it has been scattered by said radiation scattering element 103 but before it incides said radiation-detecting element 105. When said radiation-detecting element comprises a flat radiation sensitive surface, said imaginary plane is preferably parallel with said radiation sensitive surface. According to this embodiment of the invention the imaginary plane 107 is not parallel to the sensitive surface of said detecting elements. Optionally, additional scattering means 113 can be arranged at the outgoing surface of said scattering element 103.
Generally, a portion of the scattered light 408 will reach the sensitive area of the light detection element on the silicone substrate. The wavelength of the laser radiation, the diameter of the scattered beam emerging from the light scattering element and the distance between the light scattering element and the light detecting element will substantially determine the minimum speckle size on the sensor. The larger the distance between the light scattering particles 104 and the light detecting element 105, the larger the minimum speckle size will be. For a wavelength of 0.8 μm, a beam diameter of 0.4 mm and a distance of 0.5 mm, the minimal speckle size equals 2 μm. In order to accurately determine the speckle pattern, the pixel size should then be less than 1 μm, which is practically obtainable.
Naturally, the interspersement of the challenge-modifying elements and the radiation detection elements can be used in all the above-described embodiments, provided that the challenge-modifying element comprises several separately controllable areas, or matrix elements.
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All the drawings of the embodiments 1 through 7 are two-dimensional representations of a three-dimensional device. Certain optical elements in the drawings, however, need not to be located in one plane. For example light entering the light scattering device will be partly spatially reflected by the entrance surface. In order to avoid this spatially reflected light from reaching the light detectors, these detectors are preferably placed before or after the drawing plane.
In the detailed description of preferred embodiments of the present invention hereinabove, when employing LC technology, the cover glass should be provided with a transparent conducting layer, which is provided with a (constant) voltage.
Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. The described embodiments are therefore not intended to limit the scope of the invention, as defined by the appended claims.
Number | Date | Country | Kind |
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05109654.3 | Oct 2005 | EP | regional |
05112740.5 | Dec 2005 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB06/53737 | 10/11/2006 | WO | 00 | 4/16/2008 |