Device and method for generating a key

A device for generating a key has a multimode interferometer which can be coupled to a light source and has a light path having an electro-optical material, the light path being configured to obtain light at an input side, influence the light under the influence of a locally varying refraction index of the electro-optical material and provide influenced light at an output side. The device has a receiver configured to receive the influenced light at the output side, and has an evaluator configured to perform an evaluation based on the influenced light and to generate the key based on the evaluation.

BACKGROUND OF THE INVENTION

The present invention relates to a device for generating a key, like a bit sequence, using a multimode interferometer. The present invention also relates to a method for providing a key and to a cryptographic multimode interferometer or electro-optically programmable multimode interferometer as a cryptographic key.

There is demand for concepts for deriving keys for the purpose of authentication and/or encryption. Passwords or other shared secrets may, for example, be used, which allow determining whether the respective communication partner is in possession of the knowledge entailed.

Well-known software or hardware-based algorithms for performing an encryption can only be mapped insufficiently, i.e. with insufficient precision and/or with too high computing complexity.

Thus, a concept for generating a key which can generate the key with high precision and low computing complexity would be desirable.

SUMMARY

According to an embodiment, a device for generating a key may have: a multimode interferometer which can be coupled to a light source and has a light path having a material having a controllable refraction index, the light path being configured to obtain light at an input side and influence the light under the influence of a locally varying refraction index of the material in order to provide influenced light at an output side; receiving means configured to receive the influenced light at the output side; and evaluating means configured to perform an evaluation based on the influenced light and to generate the key based on the evaluation; an electrode arrangement configured to generate the locally varying refraction index based on locally varying electrical fields of the electrode arrangement; wherein the receiving means has an array of photodetectors and the evaluating means is configured to perform, for each of the photodetectors, a threshold value decision as to whether the quantity detected in the respective photodetector is to be transferred to a binary 0 or a binary 1 and to obtain a bit sequence as the key by lining up the threshold value decisions.

According to another embodiment, a method for generating a key may have the steps of: guiding light from an input side of a light path to an output side of the light path under the influence of a locally varying refraction index of a material of the light path, the material having a controllable refraction index; generating the locally varying refraction index by an electrode arrangement based on locally varying electrical fields of the electrode arrangement; providing influenced light at the output side; receiving the influenced light at the output side by receiving means which has an array of photodetectors; performing an evaluation based on the influenced light by performing a threshold value decision, for each of the photodetectors, as to whether a quantity detected in the respective photodetector is to be transferred to a binary 0 or a binary 1; and generating the key based on the evaluation by lining up the threshold value decisions.

One finding of the present invention is having recognized that, by using a multimode interferometer, a key can be generated on a hardware basis, i.e. at low computing complexity, which can be obtained by making use of the optical characteristics of a multimode interferometer with high precision.

In accordance with an embodiment, a device for generating a key comprises a multimode interferometer which can be coupled to a light source and comprises a light path having a material comprising a controllable refraction index, the light path being configured to obtain light at an input side under the influence of a locally varying refraction index of the material, to influence the same and to provide influenced light at an output side. In addition, the device comprises receiving means configured to receive the influenced light at the output side, and evaluating means configured to perform an evaluation based on the influenced light and to generate the key based on the evaluation.

In accordance with an embodiment, the device is configured to obtain a locally varying influence of light based on the locally varying refraction index. This allows obtaining the key by the local variation of the refraction index within the material, obtainable at low computing complexity and high precision.

In accordance with an embodiment, the device comprises a light source connected to the light path and configured to emit the light. This allows obtaining a functionally integrated circuit where light source and material are mutually matched.

In accordance with an embodiment, the light source is a narrow-band light source, like laser or a light source having a filter, which may receive broad-band light and output a narrow-band portion.

In accordance with an embodiment, the receiving means comprises a filter configured to filter the influenced light and to provide narrow-band filtered light at a filter output, the evaluating means being configured to perform the evaluation based on the narrow-band filtered light. This allows using comparably broad-band light sources, allowing a simple technical design, and filtering the light used at the receiving means so that keys having an unchanged high precision can be obtained with an unchanged high precision.

In accordance with an embodiment, the evaluating means is configured to determine a local intensity distribution of the influenced light or filtered light and to generate the key based on the local intensity distribution. It is of advantage here that an intensity distribution can be determined at low computing complexity, like using threshold values.

In accordance with an embodiment, the evaluating means is configured to perform the local intensity distributions in mutually different sub-regions of a total region of the light path. The key comprises a plurality of key portions, wherein each key portion is associated to a sub-region. This allows obtaining complex keys at low a complexity of the system.

In accordance with an embodiment, the device comprises an electrode arrangement configured to generate the locally varying refraction index of the material based on locally varying electrical fields of the electrode arrangement. The electrode arrangement may be part of the multimode interferometer. It is of advantage here that electrical fields can be generated with high precision, high reproducibility and low technical complexity.

In accordance with an embodiment, the electrode means comprises a number of spatially separate, i.e. mutually insulated, electrode elements configured to influence the refraction index of the material in a spatially separate manner. The device comprises driving means configured to drive the electrode elements such that a pattern in the influenced light is unambiguously associated to each pattern of driven electrodes of the electrode means. It is of advantage that a drive signal for driving the electrodes can be unambiguously transferred to a pattern of the influenced light.

In accordance with an embodiment, the electrode means comprises a plurality of spatially separate electrode elements arranged in a two-dimensional array. With regard to the influence of light guided through the light path, the electrodes are formed to be asymmetrical relative to at least one direction of the two-dimensional array. This can be done to an extent such that each electrode generates an unambiguous influence in the influenced light, and/or that each combination of electrode elements driven produces an unambiguous pattern. It is of advantage here that high an entropy can be obtained in the key and a high range of values of the key is obtained.

In accordance with an embodiment, the device is configured to generate an asymmetrical influence of the light guided through the light path relative to at least one direction of the two-dimensional array by mutually different electrode geometries and/or by mutually different electrical voltages at the electrode elements. This allows driving the electrodes at low complexity, like by pre-configured electrode geometries and/or applying constant and mutually different or varying voltages, which is simple as far as computing is concerned.

In accordance with an embodiment, the electrode means comprises a plurality of spatially separate electrode elements arranged in rows and columns of a two-dimensional array. Electrodes within a row comprise a mutually different dimension, unambiguous within the row, along a row direction. Alternatively or additionally, electrodes within a column comprise a mutually different dimension, unambiguous within the column, along a column direction. These criteria may apply to one, several or all the rows and/or columns. It is of advantage here that a compact arrangement of electrode geometries can be obtained.

In accordance with an embodiment, a quotient of the dimension of any two adjacent electrodes along the row direction comprises a uniform quotient value and/or a quotient of the dimension of any two electrodes along the column direction comprises the uniform quotient value. This allows easily obtaining individual electrode elements.

In accordance with an embodiment, the quotient value comprises a value within a range of values of at least 1.5 and at most 10, like a value of at least 2, like 2. A neighboring electrode along the column direction or row direction exemplarily comprises half a dimension when compared to the other electrode. Such a quotient value is of particular advantage for designing the individual electrode elements.

In accordance with an embodiment, the multimode interferometer is configured to vary the refraction index of the material in a locally varying manner based on a bit sequence comprising a first number of bits. The evaluating means is configured to provide, for the key, a bit sequence having the first number of bits for the key. This allows obtaining an n-bit key based on an n-bit driving of the multimode interferometer.

In accordance with an embodiment, the device is configured to provide the bit sequence at a signal output and to receive, at a signal input, an input signal which comprises a reference key, the device being configured to compare the reference key to the key and to evaluate an identity of a transmitter of the input signal based on a result of the comparison. This allows checking whether the other device knows the shared secret. Alternatively or additionally, the device can be configured to derive the key based on a bit sequence obtained and to provide the key so that the device receiving the key can check the identity of the device.

In accordance with an embodiment, the key is a first key. The device is configured to guide first light through the light path during a first time interval in order to obtain the first key, and to guide second light through the light path during a second time interval in order to obtain a second key. The evaluating means is configured to combine the first key and the second key to form a total key. This allows synergetically repeatedly using the multimode interferometer which, in interaction with different light, like light of different wavelengths, can excite different modes or propagate in different modes and thus generate different patterns in the influenced light so that the type of light or light source used is another degree of freedom which can be used to increase the bits used or generated in the key while maintaining the high entropy.

In accordance with an embodiment, the multimode interferometer is a first multimode interferometer. The device comprises at least a second multimode interferometer which is coupled to an output of the light path. This allows further influencing the already influenced light in the second multimode interferometer, allowing high robustness of the key generated.

In accordance with an embodiment, the device comprises at least a third multimode interferometer which is coupled to the output of the light path in parallel to the second multimode interferometer and is configured to obtain a local intensity distribution of the light path which differs from the first multimode interferometer. This means that the output of the first multimode interferometer can be divided into at least two further multimode interferometers which obtain information which is at least partly disjunctive from one another and continue to influence the same. This allows a further increased robustness of the key generated based on the outputs of the second and the third multimode interferometer.

In accordance with an embodiment, the multimode interferometer is a first multimode interferometer which is arranged to be interleaved with a second multimode interferometer. This allows high complexity of the key generated and, thus, high robustness of the key generated.

In accordance with an embodiment, the light path is a first light path and comprises at least two spatially spaced apart outputs of the light path which are configured to output different spatial intensity distributions of the light path. The spatially spaced apart outputs are coupled to an input of a second light path of the second multimode interferometer at different lateral positions. This means that the light path of the second multimode interferometer can obtain light which is influenced differently at laterally different positions so that light, which is influenced to a highly complex degree, can be obtained at the output of the second multimode interferometer, thereby allowing a robust key.

In accordance with an embodiment, the material is at least one of an electro-optical material, a magneto-optical material, a thermos-optical material and a voltage-optical material.

In accordance with an embodiment, the material is an electro-optical material and comprises at least one of beta barium borate, lithium niobate, lead lanthanium zirconate titanate, and a liquid crystal and a nitrobenzene material, which may provide a liquid having a marked Kerr effect. These materials allow precise driving with little aging effects and high precision.

In accordance with an embodiment, a method for generating a key comprises guiding light from an input side of a light path to an output side of the light path while influencing a locally varying refraction index of a material, having a controllable refraction index, of the light path. The method comprises providing influenced light at the output side and receiving the influenced light at the output side. Additionally, the method comprises performing an evaluation based on the influenced light and generating the key based on the evaluation.

DETAILED DESCRIPTION OF THE INVENTION

Before discussing below in greater detail embodiments of the present invention referring to the drawings, it is to be pointed out that identical elements, objects and/or structures or those of equal function or equal effect are provided with equal reference numerals in the different figures so that the description of these elements illustrated in different embodiments is mutually exchangeable or mutually applicable.

The following embodiments refer to a device comprising a multimode interferometer. A multimode interferometer can be configured to guide light from an input side to an output side of a light path. Within the light path, the light can propagate in several modes which each cause the light to be influenced. This influence can be determined by a variable amplitude, phase or intensity distribution at the output side of the light path.

The following embodiments are discussed in connection with an electro-optical material which may comprise a locally varying optical refraction index depending on an electrical field. Although the embodiments are discussed in connection with an electro-optical material, embodiments are not restricted to this, but also relate to different materials the refraction indices of which vary. Magneto-optical materials for making use of a magneto-optical effect based on a magnetic field, thermo-optical materials for making use of a thermo-optical effect based on a variable temperature and/or a voltage-optical material for making use of a voltage-optical effect based on an electrical voltage are among these. Alternative or additionally, multi-quantum-well structures can be used which are produced by GaAs/AlGaAs semiconductors, for example.

In order to influence the different modes in the, for example, electro-optical material, a locally varying change in the refraction index of the electro-optical material can be made use of. Embodiments provide for applying electrical fields at different positions in the electro-optical materials in order to influence, i.e. change, the refraction index of the electro-optical material at the position of the electrical field. A spatial defect in the electro-optical material can be obtained by the varied refraction index, which allows a characteristic influence of the corresponding mode. The mode here can be influenced by a wavelength of light propagating through the light path, a position and/or a spatial extension of the defect. This means that the influence of the modes can be dependent on the wavelength of the light.

FIG.1shows a schematic block diagram of a device10for generating a key12in accordance with an embodiment. The device10comprises a multimode interferometer14which may be coupled to a light source. Exemplarily, the multimode interferometer14comprises a light path16through which light can migrate or is guided and can be influenced. A light source or a light guide connected to the light source can be coupled or connected at an input side18in order to obtain light. The light path16can comprise different refraction indices n1and n2at different positions in its course from the input side towards an output side22, wherein there may be any number, position and/or geometry of regions24which comprise a different refraction index n2when compared to other regions. In particular, more than 2 mutually different refraction indices n1or n2can be implemented in the multimode interferometer14. This means that there may be more than two different regions having different refraction indices.

An electro-optical material configured to change its refraction index responsive to an electrical field applied can be arranged in the light path16. Examples of such an electro-optical material are beta barium borate, lithium niobate, lead lanthanium zirconate titanate, nematic liquid crystals and/or para-nematic liquid crystals. Alternatively, different materials can be used, the refraction index of which can be varied, like materials for making use of a magneto-optical effect (magneto-optical material), a thermo-optical and/or voltage-optical effect (thermo-optical material and voltage-optical material, respectively). Alternatively or additionally, multi-quantum-well structures which are generated by GaAs/AlGaAs semiconductors, for example, can be used. This means that other mechanisms for varying the refraction index can be used in the MMI. Although the following embodiments relate to an arrangement of electrodes configured to apply an electrical field to the electro-optical material, in accordance with other embodiments, a different material can be used, wherein, in these embodiments, other physical quantities are applied to the material in order to influence the refraction index, like a magnetic field, an electrical voltage or a temperature, for example, which can be done by means of electrodes or other producers of the physical quantity which are to be arranged in the respective cases.

Electro-optical materials may comprise different refraction indices n1and/or n2based on spatially varying electrical fields, thereby allowing a locally varying influence of light guided through the light path16, thereby allowing light to propagate in different or varying modes. Similarly, magneto-optical materials may comprise the different refraction indices n1and/or n2based on spatially varying electrical fields, voltage-optical materials based on spatially varying voltages or thermos-optical materials based on spatially varying temperatures. The modes formed may comprise or provide a phase distribution, amplitude distribution or intensity distribution in the influenced light on the output side22, which is different compared to the input side18.

The device10comprises receiving means26configured to receive the influenced light on the output side22. Exemplarily, receiving means26may comprise photodetectors or the like. Alternatively, other light-sensitive elements or materials can be used, like resistor elements which react to incident light with different resistance values.

In addition, the device10comprises evaluating means28configured to perform an evaluation based on the influenced light and generate the key12based on the evaluation. Exemplarily, the evaluating means28can be configured to evaluate a pattern of the light at the output side22based on the information obtained from the receiving means26, for example as regards phase distribution, amplitude distribution and/or intensity distribution. This pattern can be transferrable to the key12based on a predefined criterion. In accordance with an embodiment, the receiving means26may comprise a one-dimensional or two-dimensional array of photodetectors. The evaluating means28may perform, for each of the photodetectors, a threshold value decision as to whether the quantity detected in the respective photodetector, like an intensity, is to be transferred to a binary 0 or a binary 1. A bit sequence can be obtained as the key12by lining up the individual decisions.

Alternatively, other types of deriving a bit sequence are possible in order to obtain the key12. In addition to deriving a bit from a threshold value decision, it is also possible to further process such a bit sequence obtained, like by inverting, combination with other bits or quantities or the like.

FIG.2ashows a schematic top view of a device20in accordance with an embodiment, which may have a similar setup to the device10. The device20comprises the multimode interferometer14. When compared to the device10, the device20comprises a light source32which is connected to a light guide34configured to provide light provided by the light source32to the input side18. Alternatively, the light source32may also be connected directly to the input side18so that placing the light guide34is optional. The light guide34may, for example, be an optical waveguide or optical fiber or the like. The light source32may be any light source. However, it may be of advantage for the detection and/or evaluation to be performed by the receiving means26or evaluating means28based on narrow-band light. A wavelength range Δλ of at most 10 nm, advantageously at most 1 nm and particularly advantageously in a range of values of 1 to 10 pm may, for example, be considered to be narrow-banded. This may be done while considering that interferences in the multimode interferometer or light path are generated in dependence on the wavelength so that it may be of advantage for an interference between individual modes to use narrow-band light. A design criterion may consequently be for a coherence length of the light path to be implemented such that interferences at the output side22are obtained with good contrast so that an evaluation containing as few errors as possible can be performed.

A narrow-band light source, like a narrow-band light-emitting diode (LED) or laser, can be used for obtaining narrow-band light. Alternatively or additionally, the light source32may also comprise a filter configured to filter broad-band light of a light-generating element or light comprising at least higher a wavelength variation and to provide, at the output of the filter, the narrow-band light which can be guided to the multimode interferometer. Alternatively or additionally, it is also possible for the receiving means26to comprise such a filter configured to filter the influenced light and to filter out only a narrow-band signal from a potentially broad-band signal at the output side22in order to provide narrow-band filtered light at a filter output. The evaluating means28can be configured to perform the evaluation based on the narrow-band light.

The device20may comprise an electrode arrangement36configured to generate the locally varying refraction index based on locally varying electrical fields of the electrode arrangement36. Here, the electrode arrangement36may comprise a plurality or multitude of electrodes381to3816. The electrodes381to3816can be driven individually by driving means42of the device20. The driving means42can be configured to drive an individual electrode381to3816at one point in time, or any combination of at least two, at least three or a higher number of or even all electrodes381to3816. Here, the driving means42can apply an equal voltage but also mutually different electrical voltages to the electrodes381to3816, so that, while considering or neglecting an electrode distance of the electrodes381to3816to a respective or common reference electrode, an equal field, but also mutually differing electrical fields, can be generated in the electro-optical material of the light path of the multimode interferometer.

Although the device20is illustrated such that the electrode arrangement36comprises 16 electrodes, any other number of electrodes can be used, like at least one, at least two, at least five, at least ten, at least 16, at least 64, at least 256 or a higher number of electrodes.

The electrodes381to3816can be arranged to be spatially separate from one another, which means electrically insulated from one another. Individually driving the electrodes381to3816allows spatially separately influencing the refraction index of the electro-optical material of the multimode interferometer, like at the respective position of the electrodes381to3816. Influencing here may relate to the presence of an electrical field, the intensity thereof and/or quality when compared to a situation when there is no electrical field, i.e. when the respective electrode381to3816is not driven. Alternatively, influencing can be achieved by obtaining a different field strength, like changing a voltage value at the electrode381to3816from a first value unequal to 0 to a second value unequal to 0. Alternatively or additionally, influencing may also be achieved by the absence of the electrical field, which means that the reference state may refer to the presence of the electrical field.

The driving means42can be configured to drive the electrodes381to3816of the electrode means36such that a respective pattern in the influenced light and/or the key12is unambiguously associated to each pattern of driven electrodes. Exemplarily, an input signal44of the driving means42may comprise a piece of information or bit sequence which indicates unambiguously which of the electrodes381to3816are to be driven and/or driven to what extent. If, for example, the driving means42is configured to drive a number of 16 electrodes381to3816in a binary way, this means changing between a first voltage value and a voltage value, like on/off, so that the input signal44may comprise a corresponding number of bits, like16. Each of the bits of the input signal44may thus be associated unambiguously to an electrode381to3816and/or a combination of at least two of the electrodes381to3816, and indicate whether and how these electrodes are driven. Based on obtaining an unambiguous pattern at the output side22based on each of the patterns of driving the electrodes381to3816, an unambiguous key12can be obtained, which exemplarily comprises an equal bit length as the input signal44. This means that a key12can be associated to each of the patterns on the output side22. A key12can exemplarily be associated to each input signal44and/or an input signal can be associated to each key. By introducing redundancies, codes or the like, a smaller number of bits can be obtained.

An asymmetry in the electrode arrangement36can be of advantage for an unambiguous association of each key12to an input signal44or pattern of driven electrodes381to3816. When, for example, considering exclusively the electrode3816in a theoretical experiment, an influence of the light obtained by driving it can be identical or at least almost identical, irrespective of where the electrode16is along an x direction adjacent to the light path of the multimode interferometer14. A position along the y direction, in contrast, may be of relevance and a changed position y of the electrode3816along the y direction may result in a changed influence. However, the influence may be symmetrical relative to a position where the light of the light source is guided into the multimode interferometer14, light a central axis. Based on such symmetry, an arrangement at a maximum y value and an arrangement at a minimum y value of the electrode3816may result in an identical or almost identical influence on the output side22. An asymmetry of the electrode arrangement36along the directions x and/or y may thus offer advantages with regard to an unambiguity of the pattern obtained at the output side22.

The electrodes381to3816can be arranged in a two-dimensional array and be formed to be asymmetrical with regard to at least one direction within the two-dimensional array.

The electrode arrangement36in accordance withFIG.2ashows a particular advantageous embodiment where the electrodes381to3816are formed to be asymmetrical along both directions x and y of the two-dimensional array. The electrodes381to3816can be arranged in rows and columns, wherein a row may exemplarily comprise the electrodes384,383,382and381;385,387,386and385;3812,3811,3810and389and3816,3815,3814and3813. A column may exemplarily comprise the electrodes384,385,3812and3816;383,387,3811and3815;382,386,3810and3814and381,385,389and3813.

Electrodes within a column can be arranged along a column direction, like y. Electrodes within a row can be arranged along a row direction, like x. It is to be understood that any other association to the directions can be obtained by any other designation of the directions in space and/or by rotating the device20in space.

Electrodes within a row may comprise a mutually differing dimension along the row direction x. Here, the dimension of the respective electrode along the row direction x can be unambiguous. Unambiguous here may refer to the fact that each electrode is implemented individually with regard to its dimension and exemplarily comprises a dimension x1, x2, x3or x4which are mutually different. Unambiguity, however, may also refer to the fact that the respective dimension x1to x4cannot be obtained by a combination of other electrodes within the respective row. Influencing the light in the light path may be dependent on a spatial extension of the defect generated by the electrical field, i.e. the varying refraction index. By means of such unambiguity, namely that a dimension x1to x4cannot be obtained by a combination of respective other values x1, x2, x3and/or x4, it can be avoided that a similar defect is obtained in the same row.

Alternatively or additionally, a dimension of electrodes within a column may comprise a mutually different dimension which is unambiguous within the column along the column direction y.

In accordance with a non-limiting embodiment, the following can apply:
x1<x2<x3<x4; and
y1<y2<y3<y4.

When comparing a dimension along the respective row direction x or column direction y of two adjacent electrodes, for example when computing a quotient, wherein the greater dimension is in the numerator and the smaller dimension is in the denominator, like y4/y3for the electrode pair3816;3812or x3/x2for the electrode pair3815;3814, a quotient comprising a quotient value can be calculated. In accordance with an embodiment, the following can, for example, apply: x4=2*x3, x3=2*x2and x2=2*x1and y4=2*y3, y3=2*y2and y2=2*y1. Here, a quotient value of 2 can be obtained, for example, which is constant within each column and each row. This means that the quotient of the dimension of any two adjacent electrodes along the row direction and/or column direction can comprise a uniform quotient value. The quotient value can, for example, by a value within a range of values of at least 1.5 and at most 10, at least 2 and at most 8 and/or at least 2 and at most 3, like 2, for example. For quotients having a value between 1 and 2, there may be values for which the sum of two electrode lengths corresponds to the length of a third electrode, which can be avoided with regard to unambiguity. For values greater than or equaling 2, the length of a third electrode can no longer be obtained by the sum of the lengths of other electrodes, so that values of at least 2 for the quotient are of advantage.

Although the electrode arrangement36is described such that the electrodes or the array are formed to be asymmetrical relative to both directions x and y, asymmetry with regard to one direction may be sufficient. Although the array of the electrode arrangement36is described such that the quotient is constant within a row and a column, in accordance with other embodiments, an array may be implemented such that electrodes within one row may comprise a mutually different dimension, unambiguous within the row, along the row direction x. Alternatively or additionally, the electrodes within one column may comprise a mutually different dimension, unambiguous within a column, along the column direction y. Alternatively, there may be a symmetrical arrangement or implementation of potentially identically implemented electrodes or the quantity influencing the refraction index.

In a generalized form, the electrode means38may be implemented with regard to the two-dimensional array such that the electrodes381to3816are formed to be asymmetrical with regard to at least one direction x or y of the two-dimensional array, with regard to the influence of the light guided by the light path. This may be implemented such that each electrode381to3816causes an unambiguous influence of the light at the output side22. The asymmetrical influence of the light guided through the light path, with regard to at least one direction x or y of the two-dimensional array, can be generated by mutually different electrode geometries and/or by mutually different electrical voltages at the electrodes381to3816.

A reference electrode for the electrodes381to3816is not illustrated inFIG.2a.

FIG.2bshows a schematic sectional side view of the device20along a section A-A′ of a sectional line illustrated correspondingly inFIG.2a. A reference electrode38Rcan be arranged to be opposite the electrodes384,388,3812and3816within the sectional line A-A′ so that the electro-optical material46of the multimode interferometer is arranged between the electrodes384,388,3812and3816or more or even all of the electrodes of the electrode arrangement36and the reference electrode38R. The reference electrode38Rmay be an individual electrode which is opposite the total electrode arrangement36, but may also be implemented in the form of several electrodes. In the illustration ofFIG.2b, an electrical voltage may, for example, be applied to the electrode3812, like by driving by means of the driving means42. In the region of an electrical field E obtained in this way, a refraction index of the electro-optical material46may be altered and exemplarily be n2, whereas a refraction index n1may be present outside the electrical field. The electro-optical material46may be enclosed by a cladding48which can reduce or prevent light from exiting the electro-optical material46, or allow electrical insulation of the electrodes of the electrode array36relative to the reference electrode38R. The device20can comprise a substrate52which supports the device20at least partly.

The output side22may comprise a similar geometry to the cross-sectional area of the electro-optical material46shown, wherein a different cross-sectional area can be obtained by changing the geometry of the electro-optical material46along the light path. Along such an area, the evaluating means28can be configured to determine a local intensity distribution of the influenced light or the filtered light and to generate the key12based on the local intensity distribution.

In other words, a suggested cryptographic multimode interferometer (crypto MMI) transfers an electrical n-bit input signal into an electrical m-bit output signal, wherein n≥m can apply and, thus, n=m, like n, m=16. At first, each bit of the n-bit input signal is, for example, converted, by means of the driving electronics, to a voltage value corresponding to the respective bit value, by means of which an electrode associated to the respective bit is driven.

As can be seen fromFIG.2bwhich shows a section through the MMI, the refraction index of the electro-optical core material changes due to the electrical field E between the electrodes and a counter electrode so that the result in the MMI is a refraction index distribution which results from the entirety of the electrode distribution driven. The refraction index distribution within the MMI, similarly to a hologram, when being passed, influences the phase of the light incident in the MMI from a laser (light source) so that the result for each digital input signal is a characteristic intensity distribution at the output of the MMI. This intensity distribution at the output of the MMI is detected by a device for detecting (receiving means) and transferred to an n-bit output signal by means of evaluating electronics (evaluating means). Such an MMI exhibits a characteristic suitable for encryption. Thus, when suitably selecting the electrode shape and arrangement, transferring the input signal to the output signal is unambiguous. This may be achieved by an asymmetrical arrangement of the electrodes. The transfer function can consequently not be extrapolated by means of statistical methods. Furthermore, computing models of the crypto MMI are too imprecise and/or too complicated as far as calculation is concerned, for mapping its behavior.

FIG.2cshows a schematic sectional side view of a device20′ modified when compared to the device20along the sectional line A-A′. The device20′ is modified in that the electrodes38iand at least one counter electrode are arranged on the same side of the material46. As is the case in the device20, a common counter electrode38Ris provided, for example. Alternatively, several common or individually controllable counter electrodes38Riare provided. In accordance with an embodiment, one counter electrode38Rieach is associated to an electrode38iso that there are corresponding numbers of drive electrodes38and counter electrodes38R. In this way, one bit of the key can be represented each as a pair of an electrode38iand a counter electrode38Riand be formed by driving the pair.

FIG.3shows a schematic illustration of the output side22which is subdivided into mutually different sub-regions541to5416. The output side22can be a cross-sectional area of the light path which is evaluated with regard to the locally varying influence. The evaluating means of the device, like the evaluating means28, can be configured to perform a local influence, like the local intensity distribution, in mutually different sub-regions541to5416of a total region of the light path, i.e. the output area22. The key generated may comprise a plurality of key portions. Each key portion can comprise at least one bit but also a higher number of bits. The key12exemplarily comprises a number of 16 bits B1to B16. Each key portion B1to B17can be associated to a sub-region541to5416, which means that, based on the local evaluation of each sub-region541to5416, at least one bit of information can be derived for the key12. If a binary threshold value decision is, for example, performed for each sub-region541to5416, one bit of information can be gained for each sub-region541to5416. If there is a multi-stage threshold value decision, a higher number of bits can be arranged per sub-region.

Although the output side22is represented as a rectangle, it may comprise any other shape, like a round, elliptical, polygonal shape, a free-form area or combination thereof. Each of the sub-regions541to5416may be formed to be round, angular, polygonal, elliptical or a free-form area and may comprise a same dimension or dimension differing from other sub-regions. In particular, a position and type and shape of the sub-regions541to5416can be adjusted to the light pattern obtained.

Although the sub-regions541to5416are illustrated such that they form a single-line array at the output side22, any arrangement can be selected, like a two-line or multi-line array or any other geometrical arrangement which matches the effects to be detected at the output side22.

As is discussed in connection withFIGS.2aand2b, the multimode interferometer can be configured to vary the refraction index of the electro-optical material in a locally varying manner based on a bit sequence in the input signal44. The input signal44may comprise a first number of bits, like16. The evaluating means can be configured to provide, for the key12, a bit sequence having a corresponding number of bits for the key12.

FIG.4shows a schematic block diagram of a device40in accordance with an embodiment, which communicates with another device57. The device40can be of a similar setup to the device20and can comprise a physical or logical signal input56. At the signal input56, the device40can receive a bit sequence which exemplarily comprises the input signal44, in a wireless or wired manner. The device40can comprise a logical or physical signal output58and be configured to transmit the key12using the signal output58. The signal input56and the signal output58may be parts of separate or a common communication interface. The device57can be configured to transmit the input signal44to the device40. Based thereon, the device40can form the key12and transmit same back to the device57. Based thereon, the device57can check whether the device40knows the shared secret in order to generate the matching key12based on the input information of the input signal44. Instead of the key12, the device40can also be configured to transmit a message encoded or decoded using the key12to the device47. In this case, the encrypted or unencrypted message can be the shared secret.

Alternatively or additionally, the device40can be configured to transmit the input signal44with the signal output58and receive the key12responsive thereto. The device40can thus be configured to provide the bit sequence of the input signal44at the signal output58and can receive, at the signal input56, an input signal which comprises a reference key, i.e. the key12. In this case, the device40can be configured to compare the reference key to the self-generated key and to evaluate an identity of the device57based on a result of the comparison.

Again making reference toFIGS.2aand2b, the device20can be configured to guide first light and second light, differing from the first one, through the light path in temporally differing time intervals. The mutually differing light settings may, for example, be mutually different wavelengths. Based on each of the mutually different wavelengths, a mutually differing interference pattern can be obtained at the output side22of the light path so that different keys can be provided by the evaluating means28based on different wavelengths. The evaluating means can be configured to combine the keys obtained in this way to form a total key, like by joining or connecting the individual bits to one another.

FIG.5ashows a schematic top view of an electrode arrangement36aof a multimode interferometer14awhich can be used in inventive devices. The electrode arrangement36acomprises a number of at least 256 electrodes, arranged in four columns, wherein the dimensions of the electrodes within the respective column are equal and can, depending on the column, be x1, x2, x3or x4. A number of 256 electrodes may, for example, be driven by means of a signal comprising at least 256 bits. A different number of electrodes can be used with no limitation caused, like more than 10, more than 50, more than 100 or more than 256. The electrodes of a row, like the electrodes381,382,383and384, can comprise different dimensions in the x direction and equal dimensions along the y direction. Additionally, the electrodes within one column, like electrodes384,388,3812, . . . ,38256, can comprise an equal dimension along the y direction, like the dimension y1. This means that, compared toFIG.2a, the asymmetry may be present also in only one direction x or y. Optionally, the electrodes384,388,3812, . . . ,38256may comprise the same width, but, with reference to the MMI, be nevertheless arranged to be asymmetrical, like by an offset or a different dimension in y1or the like.

FIG.5bshows a schematic top view of an electrode arrangement36bof a multimode interferometer14vwhich can be used in devices in accordance with embodiments described herein. The electrodes381, to3816are asymmetrical with regard to their surface geometry and each comprise a free-form surface. Electrode lengths (dimension along the x direction) and electrode widths (dimension along the y direction) may vary individually within the electrode and result in an individual surface geometry of the respective electrode381, to3816.

FIG.5cshows a schematic top view of an electrode arrangement36cof a multimode interferometer14cwhich can be used in devices in accordance with embodiments described herein. The electrode arrangement36ccomprises electrodes381to3812symmetrical with respect to the geometry used. The electrodes381to3812can be arranged in a number of rows621to623, like three rows. The rows621to623can be arranged symmetrical relative to an axial symmetry axis64, which may describe, for example, a center light propagation path of the light path16. Thus, the rows621and623can be arranged off the symmetry axis64but comprise an identical dimension y1relative to the electrode widths, i.e. extension along the y direction. In addition, rows621and623can be arranged with the same distance to the symmetry axis64on both sides thereof. Row622may, for example, be arranged on and, consequently, in symmetry to the symmetry axis64and comprise an equal dimension along the y direction or a respective deviating dimension y2. The driving means42can be configured to apply mutually different electrical voltages to the rows621and623, for example, to obtain an asymmetry in this way. This means that the electrodes381to3812can also be formed to be geometrically symmetrical and an asymmetry can be obtained by asymmetrically driving the electrodes.

FIG.5dshows a schematic top view of an electrode arrangement36dof a multimode interferometer14dwhich can be used in devices in accordance with embodiments.

The electrodes381to38256can be arranged in a two-dimensional array having rows and columns, wherein all the electrodes can comprise an equal dimension along the x direction, i.e. x1, and along the y direction, i.e. y1. An asymmetry can be obtained by electrically mutually differently driving the electrodes381to38256. Alternatively or additionally, an asymmetry can be obtained by arranging the electrodes asymmetrically relative to the symmetry axis64. It may be sufficient to provide only the electrodes in differing columns661to664with mutually different electrical potentials.

Further electrodes can be arranged, like symmetrically relative to the electrodes381to38256illustrated and relative to the symmetry axis64. In this case, it may also be of advantage to provide the then symmetrically additionally arranged electrodes with different electrical potentials, in particular relative to an electrode, symmetrical thereto, within the same column661to664.

In other words, electrodes can be provided with different voltages, in correspondence with their rows, so as to break the symmetry, in case ofFIG.5c, and obtain an asymmetry. In the case ofFIG.5d, electrodes can be provided with different, for example increasing, electrical voltages, in correspondence with their column, which can generate a similar or equal effect as is generated by continuously longer electrodes along the negative x direction inFIG.2a.

FIG.6shows a schematic top view of a device60in accordance with an embodiment, which can be similar in setup to the device20. The device60can comprise driving means42′ configured to drive a corresponding number of digital-to-analog converters681to6816, the analog output signals721to7216of which are used for driving the electrodes381to3816, based on the input signal44having a number of 16 Bits, for example. Waveguides741to7416can be connected to the multimode interferometer14at the output side22, and configured to couple out a respective light signal from the output side22. Exemplarily, one of the waveguides741to7416each can be connected to a sub-region541to5416in accordance withFIG.3. Calculating means76of the device60can comprise the receiving means26which may, for example, comprise an array of photodetectors781to7816to receive one of the signals of the waveguides741to7416each. Alternatively, a smaller number of photodetectors may be arranged and these may, for example, be used in time-multiplex.

The calculating means76may comprise a number of analog-to-digital converters821to8216of which one each can be coupled to a photodetector781to7816, wherein multiplexing concepts may be used here, too. Based on converting the light signal of an individual waveguide741to7416or a subrange, a respective Bit B1to B16of the key12can be obtained.

Forming the bits B1to B16can take place in the calculating means which can assemble the bit values.

FIG.7shows a schematic top view of a device70in accordance with an embodiment, comprising at least a first multimode interferometer141and a second multimode interferometer142, which may each be formed to be equal or mutually different, and which may each be formed in accordance with the discussions in the context of the multimode interferometer14,14a,14b,14cand/or14d, for example. At its input side182, the multimode interferometer142is coupled to an output side221of the multimode interferometer141, wherein the multimode interferometer142can obtain the interference patterns at the output side221either partly or completely. The connection between the output side221and the input side182may be via the waveguide74. This means that the multimode interferometer142can obtain already influenced light as an input signal and continue to influence this light. One or several further multimode interferometers143can be provided optionally and be arranged in parallel to the multimode interferometer142.

Exemplarily, the multimode interferometer143can obtain a different portion of the local intensity distribution than the multimode interferometer142, like based on mutually different sub-regions58in accordance withFIG.3. The third multimode interferometer143which can be coupled to the output of the light path16in parallel to the second multimode interferometer142can thus be configured to obtain a local intensity distribution of the light path differing from the second multimode interferometer142. This means that different sub-regions of the output side221can be coupled to different input sides182or183of different multimode interferometers. In other words,FIG.7shows cascading of multimode interferometers.

FIG.8shows a schematic top view of a device80in accordance with an embodiment where the multimode interferometer141is arranged to be interleaved in another multimode interferometer142. The multimode interferometer141can be implemented such that the light path comprises two or more outputs841to843. These may output different spatial intensity distributions of the light path based on mutually different sub-regions of the output side22and couple to an input of the multimode interferometer142at different lateral positions or couple into the light path162thereof. This means that different spatial intensity distributions of the inner multimode interferometer142can be coupled in at laterally different positions of the light path162of the outer multimode interferometer142. The light path162can be driven by a special array or a special electrode arrangement362in order to influence the sub-signals obtained from the outputs841to843. The position of coupling the waveguide to the respective multimode interferometer can be varied and be in the center, but this is not necessarily the case.

The devices70and80, i.e. the combination of several multimode interferometers, allows increasing a robustness of the keys since the potential influence and, thus, the calculating operations to be considered increase and reproducing or extrapolating the key is becoming correspondingly more complicated.

Although the serial connection or cascading of MMIs inFIG.7and the interleaving inFIG.6are described to be mutually separate implementations, the two may also be arranged in a combined manner, i.e. cascaded MMIs can be interleaved and/or interleaved MMIs be cascaded.

Apart from employing a multimode interferometer, embodiments also comprise multimode interferometers which can be connected to the light source, like a laser, with no waveguide being present. Additionally, embodiments also refer to crypto MMIs which comprise successively cascaded MMIs, see device70, and/or interleaved MMIs, see device80.

Embodiments refer to a cryptographic hardware key comprising electro-optically programmable multimode interferometers as the core component. The embodiments allow a component which is able to convert an electrical digital input signal to an electrical digital output signal in an unambiguous manner. Here, the method for converting, i.e. encrypting, the input signal may be based on physical effects, which is of advantage when compared to software- or hardware-based algorithms. The embodiments exhibit an electro-optically programmable multimode interferometer (MMI) as a core component of a cryptographic hardware key, which may also be referred to as crypto MMI.

FIG.9shows a schematic flow chart of a method100in accordance with an embodiment. Step110comprises guiding light from an input side of a light path to an output side of the light path under the influence of a locally varying refraction index of a material of the light path which comprises a controllable refraction index. This may comprise controlling the refraction index so that the material varies locally in its refraction index. Step120comprises providing influenced light at an output side of the light path. Step130comprises receiving the influenced light at the output side. Step140comprises performing an evaluation based on the influenced light. Step150comprises generating a key based on the evaluation.

Although embodiments described herein disclose an arrangement of 12, 16 or 256 electrodes and using a corresponding number of bits for driving, a different number of electrodes can be used and/or a number of bits differing from one bit can be used for driving an electrode.

Although some aspects were described in connection with a device, it is to be understood that these aspects also represent a description of the corresponding method so that a block or element of a device is to be understood to be also a corresponding method step or feature of a method step. In analogy, aspects described in connection with or as a method step also represent a description of a corresponding block or detail or feature of a corresponding device.