Biometric acquisition security process

Abstract

Method for securing biometric acquisition The present invention relates to a method for securing biometric acquisition comprising steps of: - determining a first set of characteristic values ​​defining a first set of light events of which at least one characteristic value per event is determined by random selection; - controlling the intensity of at least one lighting source so as to apply the first set of events during a first biometric acquisition, - the first biometric acquisition, by exposing the acquisition surface (3) linearly according to a predetermined dimension for a predetermined exposure time, emitted in the form of an acquisition matrix; - characterizing a first observed lighting pattern; - evaluating a correspondence index depending on the first observed pattern and the first imposed pattern;and - decision of presence or absence of fraud by comparison of the correspondence index with a correspondence threshold. Figure for the abstract: Fig. 1;

Description

Title of the invention: Method for securing biometric acquisition Technological background

[0001] The present invention relates to the field of securing biometric acquisition terminals. Indeed, the biometric data are secured within a terminal, and during exchanges with a server for managing a fleet of terminals, but it is also appropriate to secure the flow of images between the optical device for acquiring the biometric trait by contact, and the embedded processor of the terminal passing over a dedicated layer, so as to protect against listening and / or replay by a fraudster seeking to usurp the identity of a legitimate user by mimicking the signal passing through the layer by injecting a signal emulating a previous biometric acquisition of the legitimate user (called in English: “Injection Attack”). Presentation of the invention

[0002] The invention aims to remedy at least in part these drawbacks and preferably all of them, and aims in particular to propose a method for securing biometric acquisition which is robust to injection fraud, easy to implement on existing terminals and accessible to all, without significant impact on the authentication or enrollment times including the acquisition time.

[0003] According to one aspect of the invention, a method is proposed for securing the acquisition by contact of a biometric trait of a user comprising steps of: - determination of a first set of characteristic values ​​defining a first set of light events to be applied to an acquisition surface of the biometric trait, said first set of events describing a first lighting temporal sequence represented in a matrix in the form of a first imposed lighting pattern, said values ​​characterizing for each event of the first set a lighting typology of the acquisition surface and an instant of application of said event, at least one characteristic value per event, among the instant of application and the lighting typology, being determined by random selection; - intensity control of at least one lighting source, emitting in a first wavelength, so as to apply the first set of events during a first biometric acquisition, - first biometric acquisition, by exposing the acquisition surface linearly according to a predetermined dimension for a predetermined exposure duration, emitted in the form of an acquisition matrix; - characterization, from the acquisition matrix of the first biometric acquisition, of a first observed lighting pattern; - evaluation of a correspondence index based on the first observed pattern and the first imposed pattern; and - decision on the presence or absence of fraud by comparing the correspondence index to a correspondence threshold so as to continue the process with a biometric enrollment step or a biometric authentication step in the event of satisfaction of the correspondence threshold.

[0004] This method makes it possible to add a light challenge during biometric acquisition, the challenge being random by its lighting typology and / or application time, and its verification being implemented by analyzing the observed pattern of the image acquired by exposing the acquisition surface linearly along the direction of the predetermined dimension. Thus, if the observed pattern does not correspond to the imposed pattern, i.e. the correspondence index is strictly lower than a correspondence threshold, the method is interrupted, not allowing enrollment or authentication on the basis of the acquired image, and an alert can be given. This method therefore makes it possible to address the drawbacks listed above and is preferably applied from a biometric trait contact acquisition terminal within a biometric access control system.It thus makes it possible to secure the acquisition both in an enrollment context (for example for the creation of an access account), or in an authentication context (for example to access a given area, a building or a real or virtual space, or a service). Indeed, the random nature (including pseudo-random) of the emitted light signal constitutes a challenge and makes it possible to verify that the image acquired at the time of acquisition by the optical acquisition device of the terminal actually comes from the terminal at said time and not from a third-party object, in particular by means of a recording of an image acquired by the terminal at a time other than said time of acquisition. In addition, this method makes it possible to achieve anti-fraud and secure the communication bus between an optical acquisition device and a data processing device without requiring the extraction of the times of application of events as such from the acquired images.

[0005] Equivalently, a non-correspondence index can be determined and in this case the threshold condition authorizing the continuation of the method applies if the non-correspondence index is lower than a non-correspondence threshold.

[0006] Preferably, the intensity control applying the first set of events of the lighting time sequence consists of at least one switching on and / or at least one switching off, in particular per channel.

[0007] According to advantageous and non-limiting characteristics: - Said evaluation of a correspondence index comprises a comparison of the first observed pattern with the first imposed pattern, the correspondence index depending on a ratio between the first observed pattern and the first imposed pattern. - Said method further comprises steps of: - determination of a second set of characteristic values ​​defining a second set of light events to be applied to the acquisition surface, said second set of events describing a second lighting time sequence represented in a matrix in the form of a second imposed lighting pattern, said values ​​characterizing for each event of the second set a lighting typology of the acquisition surface and an instant of application of said event, at least one characteristic value per event, among the instant of application and the lighting typology, being determined by random selection; - intensity control of the lighting source so as to apply the second set of events during a second biometric acquisition, - second biometric acquisition, by exposing the acquisition surface linearly according to the predetermined dimension for a predetermined exposure time equal to or different from the predetermined exposure time, emitted in the form of an acquisition matrix; - characterization, from the acquisition matrix of the second biometric acquisition of a second observed lighting pattern; - the evaluation of the correspondence index is a function of the second observed pattern and the second imposed pattern. - Said at least one characteristic value per event determined by random drawing for the same terminal and / or user are stored in an exclusion register in association with an identifier of said terminal and / or a biometric identifier of the user to whom they have been applied, which makes it possible not to apply them twice, at least for a given time, to the same terminal and / or user. - The evaluation of a correspondence index involves a comparison of the observed patterns with the imposed patterns, the correspondence index depending on a ratio between the first observed pattern and the second observed pattern, divided by a ratio between the first imposed pattern and the second imposed pattern. The method according to the invention comprises a step of reconstituting an image of the biometric trait from the first and second acquisition matrices, in particular by merging from said acquisition matrices; which makes it possible to use the acquired images for biometric authentication, without having to acquire them again. Biometric authentication or enrollment includes: - construction of a biometric template. Biometric authentication includes a biometric recognition step (matching) from at least one acquisition matrix of the at least one biometric acquisition and in relation to enrolled biometric data. The method comprises an initialization step triggered by detection of the presence of an object, such as a finger or palm of an individual, in contact with the acquisition surface. The method according to the invention is implemented by computer, in particular by a central processing unit of a biometric trait contact acquisition terminal. The biometric feature is a finger or palm dermatoglyph. The value characterizing the instant of event application designates a rank of the imposed lighting pattern represented in a matrix. The value of the lighting typology designates an imposed state of lighting, or a modification of lighting, for example among a modulation of the intensity of the lighting by the lighting source, a cutting off of the lighting source or a switching on of the lighting source; which makes it possible to create events by altering the lighting. Lighting typology also refers to the lighting source, which makes it possible to distinguish between several lighting sources, particularly those of different wavelengths, thus complicating the challenge. Each event set includes at least two events, which adds to the challenge. Said instants of application of each event of the first or second acquisition are defined relative to the start of exposure, specific to said acquisition, of the first rank in the predetermined dimension of the acquisition matrix; which makes it possible to synchronize the lighting events relative to the start of exposure of each acquisition, then to easily calculate the ranks affected by the events in the imposed pattern, in order to be able to compare them to those of the observed pattern. - The value of the lighting typology designates the imposed lighting state, said instants of application of each event of the first or second acquisition characterizing the instant of start of application of the imposed lighting state specific to said event. - At least one of the sets of determined characteristic values ​​contains for at least one of the events a value characterizing an end of application of the imposed lighting state specific to said event. Advantageously, said value characterizing an end of application of the imposed lighting state specific to said event is a duration of application. Alternatively, said value characterizing an end of application of the imposed lighting state specific to said event is an instant of end of application. - The application period is greater than one third of the acquisition period. - Each imposed pattern is expressed in the form of a theoretical average brightness of each rank of its acquisition matrix, each imposed pattern being determined from the temporal sequence of lighting which characterizes it and the predetermined exposure duration of the acquisition matrix. - The characterization of the observed pattern is carried out, for each biometric acquisition, by: - a calculation of an average brightness per channel, in particular monochrome, red, green, or blue, of each rank of the acquisition matrix in the predetermined dimension. - The step of evaluating the correspondence index comprises a calculation of a ratio of the observed patterns rank by rank, in particular in the form of a vector, between the average brightness of each rank of the acquisition matrix of the second acquisition and the average brightness of each line of the acquisition matrix of the first acquisition and a ratio of the imposed patterns rank by rank, in particular in the form of a vector, between the theoretical brightness of each rank of the second imposed pattern and the average theoretical brightness of each rank of the first imposed pattern.

[0008] According to another aspect, there is provided a biometric access control system comprising: - a terminal for acquiring a biometric trait by contact, said terminal comprising: - an optical contact acquisition device comprising a sensor, an acquisition surface, configured to be in contact with the biometric trait, and a rolling shutter configured to expose the acquisition surface linearly according to a predetermined dimension for a predetermined exposure duration, said optical acquisition device being configured to emit a signal representative of the acquired biometric trait in the form of an acquisition matrix, - a lighting source emitting in a first wavelength and arranged behind the acquisition surface and emitting in the direction of the acquisition surface; - an intensity control member for at least one lighting source emitting in a first wavelength so as to apply the first set of events during a first biometric acquisition, - a communication bus between an optical acquisition device and a data processing device; - the data processing device, comprising: - a module for determining a first set of characteristic values ​​defining a first set of light events to be applied to an acquisition surface of the biometric trait, said first set of events describing a first lighting temporal sequence represented in a matrix in the form of a first imposed lighting pattern, said values ​​characterizing for each event of the first set a lighting typology of the acquisition surface and an instant of application of said event, at least one characteristic value per event, among the instant of application and the lighting typology, being determined by random selection; - a module for characterizing an observed lighting pattern of said acquisition matrix; - a module for evaluating a correspondence index based on the observed pattern and the imposed pattern and for deciding on the presence or absence of fraud.

[0009] Said system has the same advantages as the method according to the invention.

[0010] Advantageously, the sensor is total reflection.

[0011] Advantageously, the sensor is monochrome or multi-channel (RGB).

[0012] Advantageously, the data processing device comprises a central processing unit local to the terminal controlling the control member and comprising a high-precision internal clock.

[0013] Advantageously, the data processing device comprises a random number generator.

[0014] Advantageously, the biometric access control system implements the method according to the invention.

[0015] Advantageously, the terminal comprises another lighting source emitting in another wavelength, each set of characteristic values ​​comprising per event a value designating the lighting source among the lighting sources of the terminal; which makes it possible to constitute more complex, multi-colored challenges.

[0016] In one embodiment, the data processing device includes: - a memory, storing enrolled biometric data, in particular in the form of a template; - a biometric recognition module based on the first biometric acquisition or a reconstructed image of the biometric trait and the enrolled biometric data.

[0017] Advantageously, said system comprises a module for reconstituting, at least partially, an image of the biometric trait, which makes it possible, in the case of multiple biometric acquisitions, to reconstitute, in particular by fusion, a complete quality image of the biometric trait.

[0018] According to another aspect of the invention, a computer program is provided comprising instructions adapted to the implementation of each of the steps of the method according to the invention when said program is executed on a computer.

[0019] According to another aspect of the invention, there is provided a means for non-transitory storage of information, removable or not, partially or totally readable by a computer or a microprocessor, comprising code instructions of a computer program for the execution of each of the steps of the method according to the invention. Presentation of figures

[0020] The invention will be better understood, thanks to the following description, which relates to embodiments and variants according to the present invention, given as non-limiting examples and explained with reference to the appended schematic drawings, in which:

[0021] [Fig-1] [Fig.l] illustrates a person bringing his finger close to a terminal biometric acquisition according to a possible embodiment of the invention,

[0022] [Fig.2] [Fig.2] shows a schematic diagram of steps implemented in the security method, according to a possible embodiment of the invention;

[0023] [Fig.3] [Fig.3] illustrates a principle diagram according to a mode of implementation of the security method;

[0024] [Fig.4] [Fig.4] illustrates an example of the structure of a processing device data of a system according to the invention; and

[0025] [Fig.5] [Fig.5] represents an example of signals calculated during the implementation of the method according to an embodiment of the invention.

[0026] Identical references will be used from one figure to another to designate elements that are identical or similar, in their form or in their function.

[0027] For the sake of brevity, the term substantially designates values ​​within plus or minus 10%. Detailed description

[0028] The invention can be applied in different contexts of enrollment or authentication for access by means of a biometric trait contact acquisition terminal.

[0029] The method according to the invention can be used in various applications for detecting injection fraud during enrollment or authentication, the fraud detection being based on an evaluation of a correspondence index depending on the pattern observed on the acquired biometric image and the imposed pattern of illumination of the acquisition surface, the imposed pattern depending on a random draw.

[0030] The invention can be used in the case of user access to a vehicle or to a restricted area, in particular, a building or a space, such as a port area.

[0031] For reasons of simplicity and in an illustrative and non-limiting manner, the invention will be presented below in the context of a biometric method for authenticating a dermatoglyph, but the teachings can be used for any application involving the authentication of a venous network. Similarly, in the illustrated embodiment the dermatoglyph is a finger dermatoglyph but alternatively, or the dermatoglyph can be a palm dermatoglyph.

[0032] The term random drawing designates the drawing of random or pseudo-random numbers.

[0033] The term authentication refers to one-to-one or one-to-n authentication, also called identification.

[0034] With reference to [Fig. 1], the authentication method can be implemented by means of a biometric access control system 100 comprising a contact biometric acquisition terminal 1 to which a user 103 presents his finger to affix his finger dermatoglyph (papillary print). The biometric acquisition terminal 1 comprises a sensor provided with a rolling shutter, and a lighting source 5, arranged at the rear of the acquisition surface 3 so as to illuminate it, said acquisition surface 3 being configured to be in contact with the dermatoglyph 2 of the user 103.

[0035] The acquisition surface 3 is for example all or part of the upper surface of a blade, also called a prism, (in particular of transparent material such as polymethyl methacrylate (PMMA)) forming a light propagation medium, or of a TFT (Thin-Film-Transistor) panel.

[0036] A lighting source 5, arranged at the rear of the acquisition surface 3 refers for example to: - to a lighting source arranged under the acquisition surface 3 and emitting directly towards the acquisition surface; or - to a lighting source arranged laterally under the acquisition surface 3 provided with a diffuser under the acquisition surface 3 to guide the light emission along this optical path towards the acquisition surface.

[0037] During a biometric acquisition, the rolling shutter exposes the acquisition surface 3 linearly according to a predetermined dimension, preferably vertically: line by line, for a predetermined exposure duration, the biometric acquisition being carried out by the optical acquisition device and emitted in the form of an acquisition matrix.

[0038] The lighting source 5 comprises, for example, red light-emitting diodes (LEDs).

[0039] The sensor, for example with total reflection, is arranged so as to receive the light diffused by the finger placed on the acquisition surface and its acquisition field covers all or part of the acquisition surface. The light emitted by the lighting source 5 passes through an optical path between the acquisition surface 3 and the sensor. The sensor is for example arranged at the rear of the acquisition surface 3 and can in particular be positioned offset from the acquisition surface (CMOS sensor for example), or for example be combined with the acquisition surface 3 (sensor in the form of a TFT panel for example). Alternatively, the rolling shutter could expose horizontally: column by column.

[0040] A printed circuit board (PCB) (not shown) is for example arranged at the rear of the sensor and connected to the on-board data processing device 106 of the terminal 1 by a ribbon cable (not shown). Alternatively, the sensor could be soldered onto the same printed circuit as the central processing unit (CPU) of the data processing device 106.

[0041] Each image acquired by the optical acquisition device and more precisely by the sensor passes, raw or after application of transformation, through a bus on the sheet during its communication to the onboard data processing device 106 of the biometric acquisition terminal 1. It is therefore a question in particular of securing the biometric information passing via this sheet by monitoring it, so as to protect against fraudulent disconnection of the bus and in particular against injection fraud which would consist of an attacker having listened to biometric data replaying it later. The biometric acquisition terminal comprises an information processing device 106 capable of implementing all or part of the steps of the method according to the invention. In the illustrated embodiment, the biometric access control system 100 comprises a remote data processing device 101, such as a server, and the data passing between the biometric acquisition terminal 1 and the remote device 101 preferably pass in encrypted form, in particular on an Ethernet network, or even on an internet network. Preferably, the remote device 101 is used to carry out the biometric tasks of biometric template comparisons during authentication of the biometric trait.

[0042] The biometric acquisition terminal 1 may be a mobile authentication terminal, such as a mobile identity control terminal in an airport, or a mobile identity control terminal of a voting station, or a fixed terminal such as a fixed terminal dedicated to identity checks at borders for example. The biometric acquisition terminal 1 may also be an electronic subsystem embedded in a vehicle forming a connected system for driver recognition, or for access to applications for the driver or the passenger. The biometric access control system 100 may comprise multiple terminals 1.

[0043] The data processing device 106 comprises at least one processor and one memory, and makes it possible to execute a computer program for implementing the method according to the invention.

[0044] When the user 103 wishes to identify himself to the biometric acquisition terminal 1 in order to access a service or a restricted access area, he first submits an identification request to said biometric acquisition terminal 1, for example, simply by placing his finger on the acquisition surface 3 of the contact sensor. According to another example, the request can be submitted using a human-machine interface, “HMI”, with which the biometric acquisition terminal 1 can be equipped.

[0045] Once the request has been submitted, the biometric acquisition terminal 1 proceeds to acquire a biometric trait of the user 103 by applying the security method according to the invention so as to detect in particular the occurrence of injection fraud and to interrupt authentication in the event of detected fraud. The biometric trait is chosen from at least one finger dermatoglyph, one palmar dermatoglyph, one finger vein network, or their combination. Advantageously, whether in the event of authorization to continue (absence of fraud) or in the event of interruption (fraud detected) of the authentication method, this status is time-stamped and recorded in a local register or in a remote register of the biometric access control system 100.Advantageously, this register is monitored so that if the number of failed attempts for the same biometric identifier over a given time is greater than a predetermined failure threshold, then a system alert is generated so as to be communicated to an agent in charge of managing all or part of the biometric access control system.

[0046] In the event of no detected fraud, the biometric authentication process continues and the biometric acquisition is sent to the remote data processing device 101. Alternatively, the steps of the security method according to the invention may include steps implemented on the remote device 101 and the biometric acquisition has then already been sent to said remote device 101 before the decision of presence or absence of fraud. The biometric acquisition received by the remote device 101 then constitutes the authentication test (resulting from prior enrollment), in particular put in the form of a biometric test template according to an encoding scheme. From then on, the remote device 101 compares the authentication test to one (so-called one-to-one authentication) or several (so-called one-to-n authentication) reference biometric templates stored in a database of biometric templates.Alternatively, the authentication steps can be carried out on the biometric acquisition terminal 1 without a remote server being necessary, in particular in the case of a limited biometric template database or in the case of multi-factor authentication, which makes it possible to carry out one-to-one authentication, preferably local in the case, for example, of a multi-factor terminal 1 comprising a smart card reader, the chip of the card comprising an encoding of the biometric trait of the cardholder, i.e. his reference biometric template, or an access key to this reference biometric template in the memory of the biometric acquisition terminal 1.

[0047] If there is a match between the authentication test and at least one authorized reference biometric template from the biometric template database, or in the case of one-to-one authentication between the authentication test and the reference biometric template, the user 103 is authenticated. He is then authorized to access the service or the restricted access area. Otherwise, the user 103 is not authenticated and access is refused. The biometric access control system 100 can notify the user 103 of the authentication status, namely the success or failure of the authentication, using a light signal, an audible signal, a message, or a combination thereof. In both cases, the authentication status is time-stamped and supplements the status preferably already recorded in the local register or in the external register.

[0048] With reference to [Fig. 2], the method according to the invention is described in the form of a flowchart presenting the steps implemented in the security method, according to a possible embodiment of the invention. The user 103 submits an identification request to said biometric acquisition terminal 1 by placing his finger on the acquisition surface 3, i.e. by presence detection. A biometric authentication method is then initiated and calls the security method P according to the invention in order to detect attempts at fraud by injection and, in the event of such fraud, prevent further authentication and thus prevent access.

[0049] The initialization step E0 of the security method P corresponds to the reception, in particular by the information processing device 106 of the terminal 1, of said request and in particular of an image acquired without lighting during this presence detection phase. Thus, the variations in acquisition conditions according to the method P are only applied when a finger is detected on the sensor, in order not to disturb the user by an erratic visual aspect, and in order to reduce the electrical consumption of the terminal 1. Furthermore, by considering the finger motionless on the acquisition surface 3, a variation in lighting between two acquisitions makes it possible to estimate for each rank (here line) of the acquisition matrix the brightness multiplier coefficient between the two acquisitions, because the two acquired signals are identical apart from the lighting differences.In this embodiment, the initialization step E0 comprises an estimation, with the lighting off, of the average brightness of each line MLI_ext from the image acquired by the optical acquisition device during the presence detection phase, so as to evaluate the environmental brightness perceived by the sensor to eliminate it from subsequent calculations (by subtracting from the subsequent image(s) acquired with lighting said image acquired without lighting) and improve their precision by only considering the light induced by the lighting of the controlled source(s). However, said environmental brightness being negligible, this estimation remains optional.

[0050] The securing method P then continues with the implementation, by the information processing device 106, of the instructions E1 for determining a first set of characteristic values ​​defining a first set of light events to be applied to an acquisition surface of the dermatoglyph, said first set of events describing a first temporal lighting sequence represented matrix-wise in the form of a first imposed lighting pattern, said values ​​characterizing for each event of the first set a lighting typology of the acquisition surface and an instant of application of said event, at least one characteristic value per event, among the lighting typology and the instant of application, being determined by random selection.The representation in matrix form of the lighting pattern is translated for example by a table per lighting source advantageously having as number of lines the same number of lines as the acquisition matrix and storing in each line an imposed light intensity value and this representation makes it possible to then relate it to an acquisition matrix, that is to say to the signal resulting from a biometric acquisition by exposure of the acquisition surface linearly according to a predetermined dimension during a predetermined exposure duration. The table. can also consist of a single column since the same intensity value is averaged across the entire row, in which case a multiplication by an identity table of the width of the acquisition matrix is ​​applied, for example, for the purpose of evaluating the correspondence index E5. This implementation mode also allows noise to be filtered.

[0051] In the embodiment illustrated here, the value of the lighting typology designates a modification of lighting among a cutting of the lighting source or a switching on of the lighting source, which makes it possible to create events by alteration of lighting.

[0052] The value of the lighting typology also designates the lighting source if the terminal 1 comprises several lighting sources 5 capable of illuminating, directly or indirectly, the acquisition surface and emitting in different wavelengths such as a first lighting source composed of a set of red light-emitting diodes (also called "backlight" in English) combined for example with another lighting source consisting of an isolated red light-emitting diode (LED), and / or a second lighting source composed for example of a green light-emitting diode and / or a third lighting source composed of a blue light-emitting diode.This diversity in the nature and wavelength of the lighting sources makes it possible to complicate the challenge so that a set of events includes at least 2 events (for example: an ignition and an extinction) and preferably between 4 and 6 (in particular with multiple lighting sources).

[0053] For each event, a time of application of the event is randomly drawn, in a range of values, for example between 0 and the predetermined acquisition duration, and / or a typology of lighting of the acquisition surface from a list of values ​​each designating the lighting modification to be applied and the associated lighting source 5, which list is advantageously dynamic, in that it depends on the current state of each lighting source 5, in particular as a function of the previous event for each lighting source, so as to constitute feasible combinations.

[0054] Preferably, an instant of application of an event is defined relative to the start of exposure, specific to the acquisition concerned, of the first rank in the predetermined dimension of the acquisition matrix, which makes it possible to synchronize the lighting events relative to the start of exposure of each acquisition, then to easily calculate the ranks affected by the events in the imposed pattern, in order to be able to compare them subsequently to those of the observed pattern. The value characterizing the instant of application of the event therefore designates a rank, here a line, of the imposed lighting pattern represented in a matrix.

[0055] Alternatively, if the value of the lighting typology designates an imposed lighting state, the instant of application of an event characterizes the instant of start of application of the imposed lighting state specific to said event and advantageously, the set of determined characteristic values ​​contains for each event a value characterizing an end of application of the imposed lighting state specific to said event, in the form of a duration (in number of ranks or in time from the start of the exposure of the sensor) or an instant of end of application. Similarly, the instant of end of application of an event is preferentially defined relative to the start of exposure, specific to the acquisition concerned, of the first rank in the predetermined dimension of the acquisition matrix, the value characterizing the instant of end of application of the event therefore makes it possible to designate a rank, here a line, of the imposed lighting pattern represented in a matrix.

[0056] For this first lighting time sequence, the theoretical average brightness MLT of each line of the imposed lighting pattern is therefore calculated from the characteristics of its events. Alternatively, the calculation of the theoretical average brightness MLT of each line of the imposed lighting pattern can be implemented during the execution step E4, by a determination module of a central processing unit, here, of the data processing device 106 internal to the terminal 1, of characterization instructions E4 of an observed lighting pattern.

[0057] Whether the value of the lighting typology designates a modification of lighting or an imposed lighting state, the duration between two events affecting the same lighting source or respectively the duration of an event, is preferably expressed in units of time, and allows a calculation of correspondence in number of ranks. This application duration is preferably non-zero and less than the predetermined exposure duration, which corresponds to an exposure of a number of ranks of the rolling shutter, for example between 100 and 600 lines for a 1000-line shutter and in particular equal to half the exposure duration, i.e. 500 lines in the example. This involves in particular better distinguishing the contribution of the lighting compared to the noise. The application duration is in particular chosen according to the subsequent use or not of the acquired images.Indeed, if the acquired images are only used for anti-fraud, the application times can be shorter than if the acquired images also serve as support for the biometric authentication algorithm, their quality in terms of image clarity can then be sought. For example, in the case of backlighting, signal cuts on average less than a third of the exposure time (i.e. the time between two image acquisitions) are preferred, so as not to significantly affect biometric authentication algorithms. Similarly, in the case of blue or green light-emitting diodes, lighting for at least half. the exposure time is preferred. Random drawing is then constrained within the pre-selected ranges.

[0058] The draw described here is random and in particular configured so as not to reproduce the same challenge for the same person and in particular on the same terminal. To do this, the values ​​drawn at random are recorded and time-stamped in a memory in association with the identifier (preferably anonymized) of each user for whom the method was applied and in particular the identifier of the terminal 1 on which the acquisition took place, thus creating a register of exclusions for the next draws and this exclusion register is consulted during the determination step E1. Preferably, this register of exclusions is hosted in the same memory as the register of time-stamped statuses.If the exclusion register is hosted in the local memory of the data processing device 106 of the terminal 1, the user identifier alone may be sufficient, and if it is hosted in a memory of a remote server 101 (in particular in the case of multiple terminals), the identifier of the terminal 1 is for example communicated to the remote server as metadata during each connection to the remote server. The identifier (preferably anonymized) of the user is for example created and stored, preferably by the remote server, by encryption of a biometric template of the acquired dermatoglyph obtained on the basis of the images acquired in step E2. Thus, at the next draw for the same terminal, the draw will exclude from the list or range the values ​​of the parameters already applied for this user, in particular on said terminal. This mode of implementation makes it possible to block the repetition of the same challenge on the same terminal for the same user.Advantageously, each exclusion from the exclusion register is temporary.

[0059] Advantageously, in the case of a biometric access control system comprising several terminals for biometric acquisition of dermatoglyphs by contact, the set of characteristic values ​​defining a set of light events to be applied to the dermatoglyph acquisition surface comprises the identifier of the terminal having received said identification request.

[0060] The intensity control instructions of the at least one lighting source emitting in a first wavelength so as to apply the first set of events during a first biometric acquisition are determined by the central unit 601 of the processing device 106 on the basis of the values ​​of the first set of characteristic values. If the values ​​of the first set of characteristic values ​​are determined locally by a central unit 601 of the processing device 106 of the biometric acquisition terminal 1, and the latter also comprises the control member, no remote transmission of these values ​​is required, however, if the values ​​of the first set of characteristic values ​​are determined by a central processing unit hosted in a remote device 101, i.e. external to the biometric acquisition terminal 1, said values ​​are then transmitted via a communication network and in particular in a secure manner, preferably encrypted, to the control unit of the biometric acquisition terminal 1, consisting for example of the printed circuit PCB of the sensor of the terminal 1.

[0061] The securing method P then continues with the execution, by the control member, of the control instructions E2 in intensity of the at least one lighting source emitting in a first wavelength so as to apply the first set of events during a first biometric acquisition. In the embodiment illustrated here, the lighting source 5 is composed of a set of red light-emitting diodes and emits in a single wavelength and the temporally variable control signal applies the first set of events composed for example of a cut-off of the lighting source then a switching on of the lighting source at the application instants drawn at random. The switching-off instant being drawn at random between 0 and the exposure duration, and the switching-on instant being drawn at random between the switching-off instant and the exposure duration.The execution of the E2 control instructions in intensity of the lighting source applies the pattern imposed during the E3 biometric acquisition. Advantageously, it can be provided that any acquisition begins with emission of the lighting source 5 and if the imposed pattern does not provide for the extinction of the light source an extinction command is applied at the end of the acquisition.

[0062] The execution of the control instructions E2 is implemented jointly with the biometric acquisition E3 since in the embodiment described here the instant 0 corresponds to the start of the biometric acquisition, that is to say to the start of the exposure of the acquisition surface linearly by the rolling shutter according to the vertical dimension during the predetermined exposure duration. In the embodiment illustrated here, and in a non-limiting manner, the rolling shutter exposes line by line, and the raw signal emitted by the sensor is directly in the form of an acquisition matrix, also called raw image. Preferably, the raw signal emitted by the sensor is converted into the form of an acquisition matrix. Similarly, in the case of a color sensor, a demosaicing operation (Bayer matrix conversion into RGB image) is preferentially carried out on the raw data (signal from the sensor) before its transmission.The raw signal may also be subject to minor transformation before transmission, in particular transformations that do not impact subsequent calculations. The exposure time is very short and the user's finger is assumed to remain motionless during the acquisition, which lasts for example 60 ms. Note that this assumption is easily verifiable by finger detection algorithms.

[0063] The security method P then continues with the execution, by a characterization module of a central processing unit, here, of the data processing device 106 internal to the terminal 1, of characterization instructions E4 of an observed lighting pattern, from the acquisition matrix of the biometric acquisition. In an extreme case with an image acquisition every 60ms, corresponding to the total acquisition time, and an exposure time of each pixel of 30ms, that is to say half the acquisition time, a light flash will not influence a few lines but all the lines, in variable proportion, and rather than detecting more or less bright lines (as for example in the case of an exposure time less than a tenth of the acquisition time), the average brightness per line will vary progressively over the entire image.The E4 characterization of the observed lighting pattern is then based on the detection of variations in average MLI brightness per line of the acquisition matrix obtained from the signal representative of the acquired biometric trait, in other words from the image acquired by the optical acquisition device. In a non-limiting manner, this E4 characterization step could result from the implementation of a neural network, in particular a convolutional one, previously trained on acquisition databases and observed patterns.

[0064] Having, in this embodiment, the estimation, with lighting off, of the average brightness of each line MLI_ext from the image acquired by the optical acquisition device during the presence detection phase, the average brightness of each line MLI_ext from the image acquired by the optical acquisition device during the presence detection phase is subtracted from the average brightness MLI per line from the image acquired by the optical acquisition device during the light variations. This subtraction makes it possible to remove from the acquisition matrix the light which is not due to the lighting of the sensor.

[0065] These steps E1, E2, E3 and E4 can be repeated for a second set of events during a second acquisition, the finger being assumed to be immobile.

[0066] Once the observed lighting pattern(s) have been characterized, the method P continues with the execution, by an evaluation module of a central processing unit, here, of the data processing device 106 internal to the terminal 1, of evaluation instructions E5 of a correspondence index as explained later in connection with [Fig.3]. Alternatively, the characterization E4 of the first pattern may also be latent and underlying the step E5 of evaluating the correspondence index, in particular if the latter is implemented by a neural network.

[0067] Once the correspondence index has been evaluated, the method continues with the execution, by a decision module of a central processing unit, here, of the data processing device 106 internal to the terminal 1, of decision instructions E6 of presence or absence of fraud by comparison of the correspondence index with a threshold of correspondence so as to continue the process with a biometric enrollment step or a biometric authentication step in the event of satisfaction of the correspondence threshold. Indeed, the sensor being by construction quite insensitive to external light, external disturbances are low, thus, if the observed pattern(s) do(es) not correspond(s) to the imposed pattern(s), which is notably evaluated by comparing the calculated correspondence rate to a threshold, the process is interrupted, notably with the emission of an alert; otherwise the process continues here with a biometric authentication step of the user. A biometric recognition (matching) of dermatoglyph from the acquired image and in relation to biometric data (for example in the form of a biometric template) enrolled and recorded in memory, locally, is implemented.In both cases the time-stamped status, success (absence of fraud) or failure (fraud detected), is preferably stored in a local RAM register or in an external register.

[0068] Several acquisitions can be carried out and analyzed one after the other according to the method described, for example for several dermatoglyphs, the final decision step E6 then being common and based on the multiple correspondence indices calculated. That is to say that the two conditions according to which the comparison of each correspondence index with each correspondence threshold (or the same correspondence threshold) must be respected to continue the biometric authentication method, and a time-stamped status of absence of fraud recorded in the register linked to the security method whereas otherwise the biometric authentication method is interrupted and a time-stamped status of presence of fraud is recorded in the register linked to the security method.

[0069] Intermediate data processing steps may be implemented before generating the enrolled or authenticated biometric data, from the acquired raw images, for example to transform them, in particular before generating the reconstructed image of the biometric trait and / or the biometric template. The intermediate processing may consist of one or more of the following image processing operators: - pixel-to-pixel (or point-to-point) modification operators. These include, for example, color, hue, and gamma correction; - local operators, in particular those for managing local blur or contrast, a local operator based on a neighborhood of the pixel, i.e. more than one pixel but less than the entire image; a local operator allows, from a neighborhood of an input pixel, to obtain an output pixel; - operators in frequency space (after image transformation). Involving one or more operators in frequency space opens the way to various possibilities for analog or digital noise reduction, such as reduction compression artifacts, improved image sharpness, sharpness or contrast.

[0070] [Fig.3] illustrates a principle diagram according to another mode of implementation in the method P. In the embodiment illustrated in relation to this figure, the steps: - determination El of the first set of characteristic values ​​defining a first set of luminous events including in particular the calculation of the theoretical average luminosity MLT1 of each line of the first imposed pattern, - E2 control of the intensity of the lighting source so as to apply the first set of events during a first biometric E3 acquisition, - first biometric E3 acquisition and - characterization E4 of a first observed lighting pattern and in particular of the average brightness MLI1 of each line of the acquisition matrix of the first acquisition; are for example the same as those previously described in relation to [Fig.2]. In this mode of implementation the steps already described are again implemented: - a step E1' of determining a second set of characteristic values ​​defining a second set of light events to be applied to an acquisition surface of the dermatoglyph, including in particular the calculation of the theoretical average brightness MLT2 of each line of the second imposed pattern. The second set of events describes a second lighting time sequence represented in a matrix in the form of a second imposed lighting pattern, said values ​​characterizing for each event of the second set a lighting typology of the acquisition surface and a time of application of said event, at least one characteristic value per event, among the lighting typology and the time of application, being determined by random selection.This determination step El' is represented here after the characterization step E4 of the first observed pattern, however it could also be carried out immediately after the determination step El of the first set of characteristic values, in particular so as not to reproduce the same events of the first set of events; then. - a step E2' of controlling the intensity of at least one lighting source so as to apply the second set of events during a second biometric acquisition; then - a second biometric E3' acquisition step, by exposing the acquisition surface of the sensor linearly according to the predetermined dimension for a predetermined exposure time (here the same as the exposure time applied during the first biometric E3 acquisition), issued in the form of an acquisition matrix; - a characterization step E4', from the acquisition matrix of the second biometric acquisition E3' of a second observed lighting pattern and in particular the average brightness MLI2 of each line of the acquisition matrix of the second acquisition.

[0071] Then the step E5 of evaluating the correspondence index is a function of both the first and second observed pattern as well as the first and second imposed pattern. For example, a ratio CMLI of the observed patterns line by line is calculated, in particular in the form of a vector, between the average brightness MLI2 of each line of the acquisition matrix of the second acquisition and the average brightness MLI1 of each line of the acquisition matrix of the first acquisition: such that CMLI = MLI2 / MLI1 and a ratio CMLT of the imposed patterns line by line, in particular in the form of a vector, between the theoretical brightness MLT2 of each line of the second imposed pattern and the average theoretical brightness MLT1 of each line of the first imposed pattern, such that: CMLT = MLT2 / MLT1, these ratios correspond to multiplying coefficients. For example, if we have MLI1 = [1,2,3,4,5] and MLI2 = [2,4,3,4,5], then CMLI = [2,2,1,1,1]), the same to calculate CMLT.Preferably, in the case where lines have values ​​close to 0, that is to say that the lines are too dark, so as to avoid a division by 0, these lines of the CMLI ratio of the observed patterns and respectively of the CMLT ratio of the imposed patterns are ignored from the calculation or deleted, within a limit of n% (for example 30%) of the lines, n being dependent on the sensor and knowing that a dark line corresponds to a line where the finger is not present. Then the CMLI ratio of the observed patterns and the CMLT ratio of the imposed patterns are compared, by calculating for example the norm p of the vector V such that V = CMLI -CMLT, so as to evaluate here a non-correspondence index.. .

[0072] The decision step E6 of the presence or absence of fraud is then implemented by comparing the correspondence index with the non-correspondence threshold so as to continue the method with a biometric enrollment step or a biometric authentication step in the absence of fraud. The preceding calculation giving as a result a positive number representative of the error, i.e. T non-correspondence index, if it is greater than the non-correspondence threshold it indicates fraud while an absence of fraud results in a non-correspondence index strictly lower than the non-correspondence threshold. For example, for an average error of 5% tolerated on the vector V relative to the average of CMLT (preferred to CMLI for reasons of robustness), this would correspond, for a norm 1 (p=l), to a non-correspondence threshold value for an image of 1000 lines of (0.05 x number of lines =) 50. Note that the high p-value for the p-norm will penalize extreme values, such as isolated errors. During this decision step E6, the results of the step are also recorded in the register, in particular in the form of the status of absence or presence of fraud.

[0073] Advantageously, the acquisition of at least two images with illumination makes it possible to implement a complementary step E7 of reconstitution of an image of the dermatoglyph from the first and second acquisition matrices, in particular by fusion from said acquisition matrices acquired consecutively. This reconstitution step E7 could alternatively be implemented during biometric authentication or enrollment. This reconstitution then makes it possible to improve the reliability and performance of biometric recognition. Thus, in the case of an application of the method for the purpose of enrolling a user, the fusion makes it possible in particular to obtain a complete image without an altered area (i.e. less illuminated).In one embodiment, if during the reconstitution step E7, the merging of the common areas without alteration shows divergences, linked to a movement of the print for example, a new implementation of steps E1 to E6 of the security method may be required. Once the image has been reconstituted, the latter may be provided to the biometric algorithms, for example for the purpose of generating a template and recording it in a biometric enrollment database. Similarly, in the case of an application of the method for identification or authentication, from the first acquisition E3, biometric algorithms may search for the presence of characteristic points of sufficient quality and at a sufficient distance from the altered areas, in order to find a reliable correspondence (matching).If the reliability is insufficient (number of characteristic points lower than a predetermined threshold for example) we can merge several acquired images until we obtain a correspondence with sufficient reliability.

[0074] It will be noted that the imposed lighting pattern is random in nature and that once the relevant image has been acquired, the calculations can be carried out subsequently, in particular remotely, thus steps E4, E5 and E7 can be carried out at any time after steps E1 / E2 / E3, both locally and remotely.

[0075] Alternatively, the exposure duration of the second biometric acquisition E3' may be different from the exposure duration of the first biometric acquisition E3, these values ​​being known to the system, their ratio will then be taken into consideration during step E5 of evaluating the correspondence index.

[0076] [Fig.4] represents an example of the structure of a data processing device 106 for implementing one or more embodiments of the invention. The data processing device 106 typically comprises one or more central processing units (CPU) 601 and / or one or more graphics processors (GPU) 605, a physical communication module (NET) 604, one or more physical input / output modules 607 for exchanging data with external devices (such as the optical acquisition device) (communication bus not shown), a transient storage medium 602 such as random access memory (RAM), a non-transient recording medium 603 (FLASH), and communication buses (not shown) for transferring data between the internal components of the data processing device 106.

[0077] The data processing device 106 allows the execution of one or more program modules comprising instructions which, when the program module(s) are executed, cause the data processing device 106 to implement the method according to the invention. The program module(s) may be written in any programming language, compiled or interpreted. They may be part of a software solution, i.e. a collection of executable instructions, codes, scripts or others and / or databases.

[0078] The data processing device 106 comprises the following elements, connected together via a communication bus: - a central processing unit (CPU) 601, such as a microprocessor, and including in particular a high-precision internal clock used to: record the precise time when the sensor returns to the end of transmission of the previous image. and execute each change at the scheduled time. Similarly, a TRNG (True Random Number Generator) random number generator drawing random values ​​is included in the CPU 601; - a transient memory 602, for storing the executable code of the method for implementing the invention as well as the registers adapted to record variables and parameters necessary for implementing the method according to embodiments of the invention; the memory capacity of the device is preferably supplemented by an optional RAM 602 connected to an expansion port, for example; - a non-transitory memory 603 for storing the computer programs and calibration data for implementing the embodiments of the invention; the stored computer programs include in particular a computer program comprising instructions adapted to the implementation of all or part of the steps of the method according to the invention when said program is executed on the processing device 106, said non-transitory memory 603 is then an example of a non-transitory means of storing information, removable or not; - a communication module 604 comprising a network interface 604, is connected to a communication network on which digital data to be processed are transmitted or received; The network interface 604 may be a single network interface, or composed of a set of different network interfaces (e.g., wired and wireless interfaces, or different types of wired or wireless interfaces). Data packets are sent over the network interface for transmission or are read from the network interface for reception under the control of the software application running in the processor 601; - a user HMI interface, including in particular a graphics processor 605, for receiving inputs from a user or for displaying information to a user, in particular guidance information (visual and / or vocal); - an input / output module 607 for receiving / sending data from / to external devices such as hard disk, removable storage media or others.

[0079] The executable code may be stored in the non-transitory memory 603, for example a flash memory or a read-only memory, or on a removable digital medium such as for example a disk. According to a variant, the executable code of the programs may be received by means of a communication network, via the network interface 604, in order to be stored in one of the storage means of the data processing device 106, such as the memory 603, before being executed.

[0080] The central processing unit 601 is adapted to control and direct the execution of the instructions or portions of software code of the program or programs according to one of the embodiments of the invention, instructions which are stored in one of the aforementioned storage means, such as the non-transitory memory 603. After power-up, the CPU 601 is capable of executing instructions from the transient RAM memory 602, relating to a software application. Such software, when executed by the processor 601, allows the execution of the method according to the invention.

[0081] In one embodiment, the apparatus is a programmable apparatus that uses software to implement the invention. Alternatively, the present invention may be implemented in hardware (e.g., as an application-specific integrated circuit (ASIC) or as a field programmable gate array (FPGA)).

[0082] According to one embodiment, the data processing device 106 is only hosted locally in the biometric acquisition terminal 1, which is for example the preferred architecture in the case of a fixed terminal, for example a fixed terminal dedicated to identity checks. The information processing device 106 may alternatively be external to the terminal 1, or distributed and comprise multiple processing sub-units, in particular at least partly external to the terminal 1 and communicating with each other via the network interface 604. Similarly, depending on the nature of the terminal in particular, all or part of the memory can be physically relocated, hosted for example on a remote server 101. For example, for a fixed terminal 1 in particular, the terminal is master and initialization, acquisition and control modules are hosted locally in terminal 1 but the other modules may not be, or only partly, hosted locally but in a physically remote slave processing entity, such as a remote server 101, this sharing of calculations between local terminal 1 and remote server makes it possible to send to the remote server 101 only the information necessary for decision-making and thus to minimize the response time linked to the exchange of data and the network throughput, without compromising client-side security linked to reverse engineering. It is even possible to have redundant calculations, the remote server verifying all or part of what terminal 1 has done.Alternatively, the remote server 101 is the master and the user terminal 1 the slave, so that the random draw is executed by the remote server, then the imposed pattern transmitted in real time by the remote server 101 to the terminal 1 so that the latter implements the control while being agnostic of the randomly drawn values ​​characterizing the challenge, which makes it possible to maximize the security of the terminal 1 and to prevent a replay on the side of the user terminal, the latter not deciding unilaterally on the challenge. Similarly, the terminal 1 can then send the encrypted raw acquired signals directly to the remote server 101 to minimize local calculations and reduce the risks associated with reverse engineering or, on the contrary, transmit the information directly necessary for decision-making (for example the correspondence index) to minimize the network load and the response time associated with the exchange of data and dependent on the network throughput. Preferably, the information exchanged, in particular from the remote server 101 to the terminal 1, is encrypted to improve the security of the exchanges.

[0083] The simplified example in [Fig.5] illustrates the temporal sequence of application in time t of a first set of light events comprising here three events defined as: - illumination B of blue wavelength, by a blue light-emitting diode, starting at time tbd and ending at time tbf; - a green wavelength V illumination, by a green light-emitting diode, starting at time tvd and ending at time tvf; - a red wavelength R illumination, by a red light-emitting diode, starting at time trd and ending at time trf.

[0084] In this embodiment only the light-emitting diodes of an additional RGB source of red green blue lighting are used but the main lighting source of red light-emitting diodes (backlight) could also be used in combination.

[0085] In the example illustrated, the time t equal to 0 corresponds to the start of exposure of the first row in the predetermined dimension, here line 1 L1, of the acquisition matrix, the exposure time is 30 ms for a total acquisition duration Tacq of 60 ms, i.e. an acquisition frequency of 15 images per second (fps), which means that each of the lines from L1 to LZ will be exposed for 30 ms, i.e. half an acquisition period. The first line L1 is finished exposing at te and at the same time its line vector sent on the communication bus between the optical acquisition device and the data processing device 106, and so on for the following lines up to the last line LZ whose exposure ends at the same time as the end of the acquisition Tacq. In this embodiment, an RGB sensor is used, which means that 15 images per second are acquired for each R, G, B channel.

[0086] The graph located under the lines represents the average Lum brightness per color channel and per line upon reception on the bus, the blue channel (for example between 455 and 465 nm) being represented by a closely spaced dotted line, the green channel (for example between 515 and 525 nm) being represented by a spaced dotted line and the red channel (for example between 620 and 630 nm) by a continuous line. This representation in matrix form illustrates the average brightness of each MLI line of the observed pattern. Indeed, the useful information is composed of an average Lum brightness value per line, and here per color channel since an RGB color sensor is used, and these average Lum brightness values ​​are calculated from the acquisition matrix. The graph as illustrated represents the case of a uniform image (uniformly illuminated sensor, in particular without a finger placed) for clarity.In fact, when a finger is placed, the graph is modified, however this does not affect the calculations because we compare two successive images, the finger being immobile.

[0087] The pattern matching evaluation is based on the fact that the sensor is of the rolling shutter type, the lines being exposed one after the other, and this in a predictable manner. Similarly, the duration between the end of transmission of an image and the start of exposure of the first line of the sensor, as well as the duration between two exposures of successive lines, all these durations being respected to the nearest microsecond and measured by the high-precision internal clock, are known in advance, predetermined by the sensor's rate settings. The high-precision internal clock of the CPU 601 therefore makes it possible to measure the time precisely between the instants of modification of illumination and the end of sending of the image. Thus, any minor change in intensity of the main lighting source results in an average change in brightness A starting at line L.Since A is known and L can be deduced from the time between the end of reception of the previous image and the instant at which the lighting change was triggered, the average brightness can be determined. theoretical MLT imposed per line in order to be able to compare it to the average brightness MLI observed per line from the image acquired by the optical acquisition device and thus verify whether the received image contains proof of the challenge. Similarly, any brief cut-off of the main lighting source will result in the presence of N underexposed lines from line L, N and L being able to be calculated precisely, which makes it possible to verify whether the received image contains proof of this change. Similarly, any brief switching on of a red, green or blue LED of the auxiliary RGB lighting source with a color C results in N overexposed lines in color C, from line L. C being known and N and L being able to be deduced from the duration between the end of reception of the previous image and the instant at which said changes were controlled, then one can verify whether the received image contains proof of these changes.

[0088] Advantageously, the security method P comprises an additional phase of control of the terminal, prior to the initialization step, in particular at the start of the terminal 1 or recurring and / or occurring at regular intervals when the terminal 1 is in standby, implementing steps E1 to E5 of the method according to the invention (without finger) and if the correspondence index obtained is lower than a predetermined control threshold (preferably equal to the correspondence threshold, or slightly lower to increase the tolerance), it is considered that unexpected alterations occur, reflecting a malfunction of the terminal and one or more of the following actions can be carried out: raising an alert, locking the product, returning the terminal to its factory settings.This additional control phase is particularly useful for optical acquisition devices in which part of the acquisition surface, called the working area, has total or almost total reflection, the sensor having a wider field and not restricted to this working area.

[0089] In an embodiment in which the optical acquisition device of the terminal 1 is not RGB color but monochrome, the sensor then acquires a single image, in gray levels for example, per each acquisition period but remains capable of detecting changes in brightness, and a fine intensity calibration makes it possible to differentiate the color of the overexposed lines, in particular by means of colored markers of the optical acquisition device arranged in the acquisition field of the sensor outside the working area.

[0090] The invention therefore makes it possible to secure biometric acquisitions, in particular by monitoring the connection between the biometric line contact biometric sensor and the data processing device 106 (local or remote) which receives and processes the data acquired by the sensor for the purpose of enrollment or authentication.

Claims

Claims

1. Method (P) for securing the acquisition by contact of a biometric trait of a user comprising steps of: - determining (El) a first set of characteristic values ​​defining a first set of light events to be applied to an acquisition surface of the biometric trait, said first set of events describing a first temporal lighting sequence represented in a matrix in the form of a first imposed lighting pattern, said values ​​characterizing for each event of the first set a lighting typology of the acquisition surface and an instant of application of said event, at least one characteristic value per event, among the instant of application and the lighting typology, being determined by random drawing;- intensity control (E2) of at least one lighting source, emitting in a first wavelength, so as to apply the first set of events during a first biometric acquisition (E3), - first biometric acquisition (E3), by exposing the acquisition surface linearly according to a predetermined dimension for a predetermined exposure duration, emitted in the form of an acquisition matrix; - characterization (E4), from the acquisition matrix of the first biometric acquisition, of a first observed lighting pattern; - evaluation (E5) of a correspondence index depending on the first observed pattern and the first imposed pattern; and - decision (E6) of the presence or absence of fraud by comparing the correspondence index with a correspondence threshold so as to continue the method with a biometric enrollment step or a biometric authentication step in the event of satisfaction of the correspondence threshold.;

2. Method according to claim 1, in which the evaluation (E5) of a correspondence index comprises a comparison of the first observed pattern with the first imposed pattern, the correspondence index depending on a ratio between the first observed pattern and the first imposed pattern.

3. Method according to any one of the preceding claims further comprising steps of: - determining (ET) a second set of characteristic values ​​defining a second set of light events to be applied to the acquisition surface, said second set of events describing a second lighting time sequence represented in a matrix in the form of a second imposed lighting pattern, said values ​​characterizing for each event of the second set a lighting typology of the acquisition surface and an instant of application of said event, at least one characteristic value per event, among the instant of application and the lighting typology, being determined by random selection;- intensity control (E2') of the lighting source so as to apply the second set of events during a second biometric acquisition, - second biometric acquisition (E3'), by exposing the acquisition surface linearly according to the predetermined dimension for a predetermined exposure time equal to or different from the predetermined exposure time, emitted in the form of an acquisition matrix; - characterization (E4'), from the acquisition matrix of the second biometric acquisition of a second observed lighting pattern; - the evaluation (E5) of the correspondence index is a function of the second observed pattern and the second imposed pattern.;

4. Method according to the preceding claim, in which the evaluation (E5) of a correspondence index comprises a comparison of the observed patterns with the imposed patterns, the correspondence index depending on a ratio between the first observed pattern and the second observed pattern, divided by a ratio between the first imposed pattern and the second imposed pattern.

5. Method according to any one of claims 3 to 4, comprising a step of reconstituting (E7) an image of the biometric trait from the first and second acquisition matrices, in particular by merging from said acquisition matrices

6. A method according to any preceding claim, wherein the biometric feature is a finger or palm dermatoglyph.

7. Method according to any one of the preceding claims, in which the value characterizing the instant of application of the event designates a rank of the imposed lighting pattern represented in a matrix.

8. Method according to any one of the preceding claims, in which the value of the lighting typology designates an imposed lighting state, or a lighting modification for example among a modulation of the intensity of the lighting by the lighting source, a switching off of the lighting source or a switching on of the lighting source.

9. Method according to any one of the preceding claims, in which said instants of application of each event of the first or second acquisition are defined relative to the start of exposure, specific to said acquisition, of the first rank in the predetermined dimension of the acquisition matrix.

10. Method according to any one of claims 8 to 9, in which the value of the lighting typology designates the imposed lighting state, said instants of application of each event of the first or second acquisition characterizing the instant of start of application of the imposed lighting state specific to said event.

11. Method according to claim 10, in which at least one of the sets of determined characteristic values ​​contains for at least one of the events a value characterizing an end of application of the imposed lighting state specific to said event.

12. Method according to any one of the preceding claims, in which each imposed pattern is expressed in the form of a theoretical average brightness (MLT) of each rank of its acquisition matrix, each imposed pattern being determined from the lighting time sequence which characterizes it and the predetermined exposure duration of the acquisition matrix.

13. Method according to any one of the preceding claims, in which the characterization (E4) of the observed pattern is carried out, for each biometric acquisition, by:

14. - a calculation of an average brightness per channel, in particular monochrome, red, green, or blue, of each rank (MLI1,MLI2) of the acquisition matrix in the predetermined dimension. Biometric access control system (100) comprising: - a terminal (1) for acquiring a biometric trait by contact, said terminal comprising: - an optical contact acquisition device comprising a sensor, an acquisition surface (3), configured so as to be in contact with the biometric trait, and a rolling shutter configured to expose the acquisition surface linearly according to a predetermined dimension for a predetermined exposure duration, said optical acquisition device being configured to emit a signal representative of the acquired biometric trait in the form of an acquisition matrix, - a lighting source (5) emitting in a first wavelength and arranged behind the acquisition surface (3) and emitting in the direction of the acquisition surface (3); - an intensity control member for at least one lighting source emitting in a first wavelength so as to apply the first set of events during a first biometric acquisition, - a communication bus between an optical acquisition device and a data processing device (106); - the data processing device (106), comprising: - a module for determining a first set of characteristic values ​​defining a first set of light events to be applied to a surface for acquiring the biometric trait, said first set of events describing a first temporal lighting sequence represented in a matrix in the form of a first imposed lighting pattern, said values ​​characterizing for each event of the first set a lighting typology of the acquisition surface and an instant of application of said event, at least one characteristic value per event, among the instant of application and the lighting typology, being determined by random selection; - a module for characterizing an observed lighting pattern of said acquisition matrix; - a module for evaluating a correspondence index based on the observed pattern and the imposed pattern and for deciding on the presence or absence of fraud.

15. System according to the preceding claim in which the data processing device (106) includes: - a memory, storing enrolled biometric data, in particular in the form of a template; - a biometric recognition module from the first biometric acquisition or from a reconstructed image of the biometric trait and the enrolled biometric data.

Images