Method for the recognition and monitoring of fibrous supports, and applications of said method in information technology

ES 2 394 444 T3

DESCRIPTION

Procedure for the recognition and monitoring of fibrous substrates, as well as the applications of said procedure in particular in the field of computer science

The present invention relates to a method for recognizing and monitoring fibrous media, as well as the applications of said method in particular in the field of computing, and more precisely for controlled access to secure data.

Fibrous media such as different types of paper, cardboard, non-woven materials are commonly used as information media, product packaging, technical products, etc. Fibrous media today constitute media for sensitive or valuable data (banknotes, checks, identity documents, etc.), they are used in sensitive environments (States, banks, etc.), others are the product continents sensitive or valuable (packaging, envelopes, etc.), others also fulfill sensitive technical functions (filtration, conservation, etc.).

The procedures of the prior art for the recognition or authentication of fibrous materials, applied mostly to papers, are carried out in most cases by adding security elements such as physical elements (reactive tablets or not, security thread , etc.) and / or chemical substances (reagents) and specific manufacturing methods (watermarks, fiber selection, texturing, surface treatment, etc.). These security elements incorporated into the initial material make it more difficult to falsify the fibrous support, but they do not allow each support to be individually recognized and, above all, considerably multiply the production costs of this type of materials.

Document US 6 584 214 describes a method for authenticating an element with a complex multidimensional internal structure, in which the three-dimensional structure contains, for example, bubbles, cavities or particles permanently attached to a polymer matrix.

Document US 4 677 435 describes a method of identifying a document, which comprises a step that consists of initially scanning the structure of microscopic grains inside the place of the seal formed on the grain in a document, and a stage that consists of subsequently scanning the structure of some microscopic grains and comparing the data pixels obtained in these two stages to determine if there is a correlation between the grains, which indicates that the document scanned later is the same document as the document. scanned initially.

Patent application EP 1 202 225 proposes using the unique physical attributes of a document to generate in a first step a key associated with the document that is encoded in the form of an image, printed on the document. To verify the authenticity of a document, a key is generated in a second stage, under the same conditions as in the first stage, which is encoded in the form of an image and the image obtained is compared with that which is printed on the document. If the image generated during authentication is identical to the image printed on the document, the document is considered to be authentic; otherwise, it is considered to be inauthentic.

However, the inventors have found that when under "normal" conditions of use of the process, the document could have been slightly modified, for example, having been folded, perforated, printed, etc. throughout its use, after the registration of the key printed in the form of an image in the latter. In addition, the document remains "alive" during use and variations in the environment (humidity, light, etc.) may slightly vary its dimensions or coloration and, consequently, the generated key and associated image.

On the other hand, the key extraction conditions are difficult to reproduce exactly if different acquisition or lighting materials are used, for example.

Consequently, this authentication method that uses an exactly identical comparison cannot be considered satisfactory, since it systematically eliminates any document bearing a different image than the one obtained in the authenticity verification, without being taken into account. It takes into account the problems related to the life of the document, nor those related to the reproducibility of the measurements.

Having noted the shortcomings of the existing prior art techniques in the field of document authentication, the inventors propose to offer a process for accepting a candidate fibrous support, made of paper, cardboard or a non-woven material, as an authentic fibrous support, comprising the following stages:

- generation of at least one authentic digital signature from a structural feature extracted from a reference area of ​​a fibrous support qualified as authentic, said digital signature representing the fibrous structure of the reference area, and storage of this in a support of digital data;

- generation of a candidate digital signature from a structural feature extracted from an area of

ES 2 394 444 T3 reference of the candidate fibrous support, said digital signature representing the fibrous structure of the reference zone;

- comparison of the candidate digital signature with at least one of the previously stored authentic digital signatures, in such a way that a positive or negative decision to accept the candidate fibrous support is issued.

In the method according to the invention:

- the structural characteristic of the candidate fibrous support and of the authentic fibrous support (s) is obtained by examining a volume of the support;

- the comparison determines, according to a statistical method, a similarity index between the first candidate digital signature and the authentic digital signature, and compares that similarity index with a given acceptance threshold, selected to allow the issuance of a positive decision acceptance in those cases in which the candidate digital signature does not exactly match the authentic digital signature with which it is compared.

The statistical method used will allow, in particular, depending on the selected acceptance threshold and / or the application of a digital treatment to generate the digital signature, to recognize a fibrous support that has undergone voluntary or involuntary modifications such as printing, perforation , folds, spots, etc., throughout its existence or use.

Advantageously, the acceptance method according to the invention has one or more of the following characteristics:

the acceptance threshold is selected to allow the issuance of a positive acceptance decision in those cases in which the candidate fibrous support is an authentic fibrous support, whose reference area has undergone some modifications between the registration stage of the authentic digital signature and the generation of the candidate digital signature;

- the candidate fibrous support is an authentic fibrous support whose reference area has undergone some modifications between the stage of registration of the authentic digital signature and the generation of the candidate digital signature and a positive acceptance decision is issued; In particular, the candidate fibrous support is an authentic fibrous support whose reference zone has undergone printing, perforation, cutting, folding, handwriting, laminating, coating with a magnetic stripe, surface treatment, coloring, impregnation, embossing and, in in particular, a print or perforations, between the stage of registration of the authentic digital signature and the generation of the candidate digital signature;

- the candidate digital signature is generated by subjecting the detected structural characteristic to a complementary digital treatment that reduces the impact of the modifications that have occurred in the reference area between the stage of registration of the authentic digital signature and the generation of the signature digital candidate;

- the candidate digital signature is generated by subjecting the digitized structural feature to an erosion procedure and / or to filtering and / or thresholding;

- the reference zone of the authentic fibrous support (s) is (are) the whole or part of a blank fibrous support or a blank part of a fibrous support, and in particular a blank sheet of paper;

- the authentic digital signature and the candidate digital signature represent the chaotic, complex, unique and practically invariable structure over time of the reference area from which they are extracted;

- the structural characteristic of the authentic fibrous support (s) and of the candidate fibrous support detected corresponds to the internal porosity, the structural appearance to the light, or the three-dimensional organization of the fiber network to microscopic or macroscopic scale;

the structural characteristic of the authentic fibrous support (s) and of the candidate fibrous support is obtained by detecting the interaction of the fibrous support with visible light, by means of translighting;

- the structural characteristic of the authentic fibrous support (s) and the candidate fibrous support is subjected to at least one analog or digital treatment, selected from spatial or frequency filters, for example, passes high, low pass, band pass, Fourier transform, so-called wave transforms, descriptors, the algorithms that allow to analyze and / or transform and / or reorganize and / or classify and / or threshold the raw data extracted from the structural characteristics (s), convolution / deconvolution operations, logical and arithmetic operations between images and / or signs; For example, a Fourier transform of a signal-image may be used, in particular a fast Fourier transform ("FFT") algorithm if the signal is discrete in nature, or a Fourier lens if the signal is optical in nature. ;

- the acceptance threshold is selected in such a way as to allow the issuance of a positive acceptance decision in those cases in which the fibrous medium is an authentic fibrous medium, even though its digital signature does not exactly match the authentic digital signature of said authentic support with which it is compared, due to differences in the conditions for obtaining the digital signature, and in particular for measuring the structural characteristic;

- the candidate digital signature is compared with several authentic digital signatures previously stored in a database;

- the issuance of a positive acceptance decision is accompanied by the identification of the digital signature

Authentic ES 2 394 444 T3 offering the best similarity index;

- it comprises, on the other hand, a stage of calculating and issuing a confidence index of the issued acceptance decision;

- the digital signatures are dynamic and are obtained from fibrous supports in displacement with respect to a sensor for measuring the structural characteristic.

In accordance with another of its aspects, the object of the present invention is a device adapted for the application of the acceptance procedure as defined above, comprising:

- means for extracting by means of candling a structural characteristic of a reference zone of a candidate fibrous support, representing the fibrous structure of the reference zone, comprising a sensor such as a camera, CCD or CMOS;

- means for digitizing and possibly processing / encoding the structural characteristic measured in a candidate digital signature;

- means for calculating, according to a statistical method, a similarity index between the candidate digital signature and an authentic digital signature previously stored on a digital data medium;

- means for comparing the similarity index obtained with a given acceptance threshold, selected to allow the issuance of a positive acceptance decision in those cases in which the candidate digital signature does not exactly match the authentic digital signature with the that this is compared;

- means of issuing a positive or negative acceptance decision.

The means for treating the structural characteristic may comprise algorithms that make it possible to reduce the impact of the modifications, produced in the fibrous support after the generation of its authentic digital signature and its registration as authentic, for example printing, perforations, folds, stains, in particular means of treatment by erosion, filtering and / or thresholding.

In accordance with another of its aspects, the invention proposes to offer new secure access computing solutions, which present a high degree of security, security linked to the complexity and uniqueness of the structure of fibrous supports and great reliability through the application of the statistical acceptance procedure described above.

In the same way, the invention also has as its object a procedure to carry out a secure access control and to give or not an access authorization, which applies an acceptance procedure as defined above, the issuance of a positive decision of acceptance conditioning the access authorization.

In particular, the invention provides, within the framework of particular applications of the acceptance procedure, in some secure access control procedures to sensitive information, comparing the candidate digital signature with several authentic digital signatures, each authentic digital signature having previously been associated by indexing to sensitive information and having been stored in a database, and accompany the issuance of a positive acceptance decision to the identification of the authentic digital signature that offers the best similarity index, and to the access to the information associated with said authentic digital signature.

The present invention will be better understood from the description that follows, with reference to the accompanying figures.

Figure 1 illustrates the different stages of a variant of the method according to the invention.

Figure 2 illustrates a stage of the method according to the invention and shows the similarity indices that are obtained when comparing an authentic digital signature (of an authentic paper) stored with the one obtained later for the same authentic paper, the same real paper modified by printing and a different paper.

Figure 3 graphically shows different distribution curves of the number of samples (n) as a function of a similarity index (IS) calculated using the method of the invention.

One of the objects of the present invention is to offer a general method for the recognition of a fibrous medium, based on a statistical test of similarity between a digital signature extracted from said fibrous medium and a previously stored "authentic" digital signature of a fibrous medium. on a digital data carrier, and in particular a database. The first stage of the procedure consists of detecting a structural characteristic of an authentic fibrous support, generating an authentic digital signature from said characteristic and registering and keeping it permanently. The true digital signature is stored on a digital data carrier and does not need to be printed on the true fibrous carrier. A digital data carrier can be in the form of any computer data storage unit. In the same way, several digital signatures can be generated from different authentic and stored fibrous media, advantageously in a database. This step can be carried out, for example, on blank fibrous supports, before they have undergone any transformation. The second stage consists of detecting, again, a structural characteristic from a candidate fibrous support and generating a candidate digital signature, and in

ES 2 394 444 T3 comparing it with the one or more authentic digital signatures previously stored in order to verify whether the candidate fibrous support has been recognized as authentic despite the modifications it may have undergone.

Finally, depending on the result of the comparison, a positive or negative decision of the candidate fibrous support is issued as the true fibrous support.

The methods of the invention use digital signatures that are obtained from at least one structural characteristic of a fibrous support made of paper, cardboard or a non-woven material, selected for its structural characteristics. As a consequence, all the methods according to the invention comprise at least one step of obtaining at least one digital signature from a fibrous support of paper, cardboard or a non-woven material, more precisely a step of detecting al least one structural characteristic of the material of paper, cardboard or a non-woven material, in order to generate at least one digital signature. The fibrous support of paper, cardboard or a non-woven material used has a complex, chaotic, unique structure that is practically unchanged over time. According to its classical meaning, a "digital signature" of an area of ​​the fibrous support designates a representation, a digital characterization that is specific to the observed area. According to the invention, a digital signature is extracted from the structure of the material that constitutes the fibrous support, this is obtained from at least one characteristic of the material that represents its structure. Advantageously, the digital signature has a random character. In particular, digital signatures can be presented in the form of a digital image of the structure of the material element, as illustrated in figure 2.

By fibrous support made of paper, cardboard or a non-woven material is meant a physical element made of paper, cardboard or a non-woven material; This physical element can be presented in multiple ways: film, sheet, box, envelope, card, etc.

The present invention applies to any type of paper, cardboard or non-woven material, cellulose fibers, fiberglass, carbon, plastic, ceramic, asbestos, or a mixture of these fibers, etc., optionally combined with mineral fillers and other additives (binders, for example). Classical cellulose fiber paper and board types are preferred. These fibrous supports are made up of an extremely complex, anisotropic and heterogeneous porous material in which the fibers can come together to form aggregates. Therefore, all fibrous supports, whatever their type, have a unique, complex, chaotic, three-dimensional fibrous structure that is practically unchanged over time. Fibrous substrates are generally made in continuous strips and traditionally in sheet form, although other forms also exist. Fibrous supports made of paper, cardboard or a non-woven material have characteristics that are both deterministic and random. It will be convenient, within the framework of the invention, to extract its random part in the best possible way. Those fibrous media are also generally non-copyable / non-reproducible.

Authentic fibrous supports according to the invention can be of any type, they do not need to be printed, nor do they need to have undergone a particular transformation. The process according to the invention can be applied to existing fibrous supports, whether they have been transformed or not, whether they have been used or not. The fibrous backing does not have to be manufactured exclusively for this use.

Authentic fibrous media can be a material that has just come off the production line, a blank sheet, a printed document, an envelope, a label, a flat or corrugated cardboard box, an access card, a credit card. identification, a used ticket, etc. Any type of paper can be used with reactive or non-reactive tablets and / or security threads and / or physicochemical (reactive) substances and / or watermarks and / or a particular texture and / or surface treatment and / or coloring and / or bleaching agents, etc. But, within the framework of the invention, it is the fibrous structure of the fibrous support that is essential for generating the digital signature. True fibrous media can be a moving media, in which case the generated digital signature, as will be described below, is "dynamic".

Fibrous supports made of paper, cardboard or a non-woven material, and in particular paper supports, combine a very complex structure, a chaotic appearance on different scales, a uniqueness of each area and, if they have not been subjected to a Excessive external aggression that could lead to the total destruction of its structure, are practically invariable over time due to very slow aging under normal external conditions. By chaotic it is understood that its fibrous structure cannot be reproduced or predicted, it is random. In this way, when examining the structure of a reference zone of said fibrous support, one or more characteristics can be extracted that represent the chaotic, complex, unique and in principle quite stable structure of the examined zone. The method according to the invention uses an extracted structural feature to generate, after digitization and possibly processing / encoding, one or more digital signatures. Each digital signature that is obtained with a reference area of ​​a fibrous support cannot be obtained with another fibrous support, not even with another area of ​​the same fibrous support. Obviously, the reference zone from which the structural feature is extracted can extend to the whole of the fibrous support.

Paper, cardboard or non-woven material is a three-dimensional material and cannot be reduced or assimilated to a simple layer or a surface. Many fundamental properties are a consequence of its thickness, such as porosity / permeability, compressibility, rigidity, etc. The uniqueness of the structure of a material of this type is guaranteed by the random entanglement in several layers of elementary fibers that have been subjected to a

ES 2 394 444 T3 physicochemical treatment prior to use. The very structure of that network is the most permanent element of a paper, cardboard or non-woven material, and it cannot be destroyed without destroying the material itself. In the sense of the invention, a structural characteristic is extracted from the fibrous material, that is, a characteristic representing the internal chaotic organization of the material. By examining the structure of the material and thus by observing a volume of the latter, the invariance of the extracted structural feature is practically guaranteed. In addition, the structural characteristic detected, since it offers the properties of the material at a certain depth, allows complex information to be extracted. In the extracted structural characteristic and in the generated digital signature, the complex, chaotic, unique and practically invariable character over time of the structure of the material that constitutes the fibrous support is found.

The detection of characteristics of the surface of a paper, cardboard or a non-woven material is not adapted in practice to the applications considered in the present invention, since:

- the condition of the surface of a paper, cardboard or non-woven material can be easily modified, intentionally or unintentionally, by smoothing it (eg with the fingernail), embossing it (mechanical marking of the surface), etc .; these modifications preclude any attempt to retrieve pertinent information from the candidate fibrous support; while the volume structure of the material after these transformations remains practically unaltered;

- transformations such as printing, perforations, writing, etc., alter the characteristics of the surface of the material more than its volume characteristics;

- in practice it is impossible to laminate a paper, cardboard or non-woven material and still retain the initial surface characteristics, while the internal volume structure remains accessible and intact; and yet, plasticization is a simple and effective means of protecting and thus increasing the useful life of a fibrous material.

Thus, within the framework of the invention, it is not limited to detecting a state of the surface of the reference zone, but is, on the contrary, a representative characteristic of the three-dimensional fibrous structure in a certain volume, a certain thickness. The fibrous structure of a fibrous support can be examined in multiple ways, observing a volume of the fibrous support: internal porosity, structural appearance to the light, three-dimensional organization of the fiber network on a microscopic or macroscopic scale, coloration or impression that represents the internal structure of the material, optical, magnetic or physical markers that the fibrous structure bears, which are all unique characteristics that can be extracted. Different properties of the fibrous support can also be observed together. For example, in the case of security paper, it is possible to detect, on the one hand, the structural appearance against the light and, on the other hand, the particular elements contained in the paper.

The measurement of said structural characteristic of a fibrous support that represents its chaotic, complex, unique and practically invariable structure, and its digitization, eventually followed by a digital treatment to obtain a digital signature can be carried out in different ways. Typically, the characteristic is measured in a reference zone that corresponds to a localized part of the fibrous support or the entire fibrous support.

The structural characteristic is generally extracted with a measurement sensor that makes it possible to obtain a representation, a characterization of the internal organization of the fibrous support, in a volume of the latter. For example, non-contact methods (optical and / or electromagnetic) can be used in which an electromagnetic wave or radiation interacts through reflection and / or absorption and / or transmission and / or diffusion and / or refraction and / or diffraction and / or or interference with fibrous media, which use an optical / electronic sensor to perform measurement and acquisition, and even digitization. The sensor (s) used can then be placed in any position with respect to the observed fibrous support and with respect to the radiation source (s). Traditionally, the radiation used can be visible and / or infrared (IR) and / or ultraviolet (UV) light and / or laser or beta rays and / or gamma rays and / or X and / or other rays. The choice of radiation (s) and the sensor (s) used can be influenced by the application of the procedure, the type of fibrous support chosen, the selected measurement scale, the cost of the application, etc. The sensor (s) used can be fixed with respect to the source and / or the fibrous support, or be in relative motion. Advantageously, the detection / digitization of the structural feature is obtained by detecting the interaction of the fibrous support with visible light, by transmission, in particular by means of a sensor, such as a camera, CCD or CMOS.

Methods with contact between the fibrous support and the measurement sensor (s) can also be used. The sensor is then of the probe type, eventually integrating, in addition to the mechanical dimension, electromagnetic (magnetic behavior) or other dimensions. In this case, a relative movement of the stylus and the fibrous support is necessary. Another alternative is to use the fibrous support as a support for an ultrasonic wave or another voltage (electrical, thermal, chemical, biological, etc.) and record the behavior in different orientations, that is, the response of the fibrous support subjected to that wave, of applied voltage.

Fibrous materials can be in relative motion with respect to the sensor (for example a reel of paper unwinding in front of a fixed camera, or a disk of rotating paper with a sensor in radial displacement, etc.): the structural characteristic is then measure continuously in such a way that a signature is generated

ES 2 394 444 T3 digital «dynamic».

The extraction of the structural characteristics of the fibrous support can be carried out on one or more scales, from the microscopic level to the macroscopic level, that is, from 1 pm to several cm in general, and even on other scales. Its structure can be examined by means of candling, and this at the level of the fibers, elements of approximately 100 pm to several mm in length, and approximately 10 to 20 pm in width in the case of paper, or even at the level of fiber aggregates, traditionally on the order of 1 to 10 mm² in the case of paper. The complexity of the digital signature depends on the scales, on the orientations that are selected, therefore, depending on the application considered.

The detection, in the fibrous support, of a structural feature that reflects its complex and unique structure is performed by examining a volume of the support and after digitization, the digitized structural feature can be found in 1D, 2D or 3D form. The structural characteristics represent the structure of the material that constitutes the reference zone of the fibrous support. As mentioned previously, these are obtained by observing internal characteristics, and possibly the surface in a volume of the latter. Detection can also be done independently of time or "in real time". In the latter case, the structural characteristic is sampled over time. In the same way, dimensions can be added to this detection phase, observing the fibrous support with different orientations or illuminations, in color, shades of gray or in binary form. The image considered can also be an image, real or complex (amplitude and phase) in terms of image processing and analysis.

The digital signature (s) used in the method of the invention correspond to said digitized structural characteristic, optionally subjected to digital processing or coding according to one or more algorithms. Traditionally, digital is understood as a representation of data or physical dimensions in the form of any type of signals (including real or complex images, amplitude and / or phase components) with discrete values, for example in the form of figures (whatever the base: binary, decimal, hexadecimal, etc.) or in the form of any set of symbols (alphabet, predefined grammar, etc.). Digital systems often use analog-digital or digital-analog converters. Thus, such a digital signature is presented, for example, in binary form, or in the form of one or more color or grayscale images, of one or more images, real or complex.

The acquisition and shaping / conditioning, and even the digitization of one or more structural features of the fibrous support is carried out by means of one or more sensors with or without contact with the fibrous support. These sensors are normally followed by an analog processing unit (optical or electronic, for example) or digital (capture card connected to any or automatic computer platform).

One or more digital signature (s) are generated from the extracted and conformed / conditioned structural characteristics. Encoding (in analog and / or digital form) can be performed followed or preceded by digitization if the extracted structural features are not already in digital form, the nature of these treatments may vary depending on the type of fibrous support selected and the application for which the procedure is carried out.

In this way, numerous methods can be considered to generate, from the detected structural characteristics, one or more digital signatures, and it is not reasonable to mention them all. The techniques listed below are not, therefore, an exhaustive list.

The structural characteristics are advantageously subjected to an analogue treatment, or to a digital treatment then carried out after digitization. Of course, the known methods of signal or image processing and analysis can of course be used directly. The electronic or algorithmic treatments that are used are then based, in analog or digital form, on spatial and / or frequency filters (high pass, low pass, band pass, etc.), and / or Fourier transform and / or the so-called wave transforms, and / or descriptors, and more generally any type of algorithm that allows analyzing and / or transforming and / or reorganizing and / or classifying and / or thresholding the raw data (among these, signals and images) extracted from the structural feature (s). Convolution / deconvolution operations, as well as logical and arithmetic operations between images and / or signals can be applied to obtain said signatures. By way of illustration, the Fourier transform of an image-signal can be applied either by means of a fast Fourier transform ("FFT") algorithm if the signal is of a discrete nature, or by means of a Fourier lens. if the signal is optical in nature.

Whatever the coding or the treatment to which the structural characteristic is subjected, the digital signature obtained reflects the chaotic fibrous structure of the reference zone from which it has been extracted. If the digitized structural characteristic is in the form of a grayscale image obtained by means of a CCD camera and that it is decided to binarize, there is no doubt that information will be lost, therefore discrimination capacity in the acceptance decision, but the speed of calculation and access time to the different signatures will be improved. Therefore, it is necessary to find and select a balance as a function of the applications made of the acceptance method according to the invention. It is evident that all the value of the

ES 2 394 444 T3 invention is revealed by using one or more digital signatures that preserve a random and complex character characteristic of the unique and stable structure of the material, in the reference area, despite the treatment applied to the structural characteristics used to generate the digital signature.

The measurement of a structural characteristic of a fibrous support and its encoding / digitization in a digital signature is applied in the two essential phases of the procedure, as shown in figure 1; first, during the creation of an authentic digital signature or a family of authentic digital signatures, from one or a family of fibrous media considered authentic, and then during the obtaining of the candidate digital signature of the submitted candidate fibrous media to the acceptance procedure. Obviously, it is important that the measurement and coding procedures applied in these two phases of the procedure are similar, that is, that they repeat the same essential steps. More precisely, taking into account the uniqueness of the structure, the "true" measurement and the subsequent measurement of the structural characteristic must be carried out in two reference zones, which have at least one common part, and which will advantageously be located at the same point on the support; This can be precisely located by means of a delimitation or plastic protection, for example, or correspond to the entire support. The same structural characteristic must be measured. Advantageously, the coding procedures that are eventually applied must be the same. However, obtaining a candidate digital signature can apply complementary digital treatments, which seek to suppress some modifications made to the fibrous medium, after registering its digital signature as authentic. This last point will be described in detail below.

On the other hand, the conditions of measurement of the structural characteristic (atmosphere, degree of humidity, illumination, orientation of the fibrous support with respect to the sensors, devices used, etc.) in the two phases of the procedure (measurement of the "authentic" and measurement of the "candidate") will hardly be identical. In addition, the measurement and digitization procedures applied to the authentic fibrous substrates and the candidate fibrous substrates may be slightly different, in particular the equipment settings may be different.

Therefore, in most cases, the digital signature of a proven fibrous medium, even though it corresponds exactly to an authentic fibrous medium that has served to generate an authentic digital signature, will not exactly match said authentic digital signature.

Furthermore, although the fibrous structures do not change or change little in time and space, and if certain structural characteristics are measured at a given moment, very similar, if not intact, these same characteristics can be found at a later point in time. However, between the registration of its authentic digital signature and its subsequent submission to the acceptance procedure, a fibrous medium may experience:

- deliberate transformations: printing by any means, perforation and micro-perforations by any means, cutting, folding, gluing, handwriting or drawing, lamination, coating with a magnetic strip or other surface treatment, coloring, impregnation with any substance including resin, lamination with other materials, embossing, etc .;

- unintentional transformations caused by external aggression: stains, dirt, aging, dimensional variations caused in particular by temperature, humidity, tears, folds, etc.

Thus, the digital signature of a fibrous medium obtained after these modifications will not coincide with the authentic stored signature that was obtained before the modification with that same medium.

Indeed, it can be considered to protect a fibrous support from possible external aggressions (scratches, perforations, optical deterioration, etc.) in order to keep it as invariable as possible. This protection can be carried out by inserting the fibrous support permanently in an external casing or a resin that does not in any way impede access to its internal characteristics. This outer wrapper can contain, in addition to the support itself, other elements (a photograph, for example, in the case of an identification card), and be intimately bound to the paper in such a way that the opening of the wrapper implies the destruction of all or part of the fibrous support. The type of protection to be provided to the paper depends on the selected application (frequent reading in the case of an access card, application sensitivity, etc.). However, the presence of this plastic wrap modifies the detected structural characteristic of the fibrous support and, therefore, its digital signature.

The acceptance procedure according to the invention makes it possible to recognize it as an authentic fibrous medium despite those different modifications that have occurred between the stage of registering the authentic digital signature of a fibrous medium and the generation of a candidate digital signature of the same support whose reference area has undergone some transformations.

Thus, one of the essential characteristics of the present invention is that it performs a statistical test of similarity between a digital signature of a fibrous support candidate for acceptance and a previously stored authentic signature. The potential of this method is that it can determine whether or not the candidate fibrous support can be considered authentic and can allow, by means of a confidence index, to quantify the probability of having made an error in the decision (for example, that paper is considered authentic with a probability of having

ES 2 394 444 T3 made a decision error of 1 / 1,000,000). Therefore, an initial database can be created with any authentic fibrous supports and resorted to this throughout the useful life of the fibrous supports and / or their transformation. The database of authentic signatures can also contain all kinds of data, including these adjustment parameters, thresholds, confidence index, chosen scale, etc. The degree of tolerance of the method according to the invention makes it possible to confirm the identity of a paper support that has been transformed, subsequent to its registration as authentic, in particular that it has been printed, laminated or cut. This statistical sensitivity also offers a better tolerance to the mode of extraction of the digital structural characteristic considered, which can be carried out from that moment with different devices at different times, and at lower cost.

Furthermore, with the method according to the invention, the reference area and in particular its structure can be modified between the generation of the authentic signature and the obtaining of the candidate signature. The method according to the invention can be applied with a candidate fibrous support whose reference zone is different from the reference zone of an authentic fibrous support, whose authentic signature is stored. Advantageously, the latter will be recognized as a true fibrous support, if its reference area has at least 0.1 mm³, for example an area of ​​1 mm² for a thickness of approximately 100 pm, common and not modified with the reference area that has been used to generate the authentic digital signature.

Different statistical methods can be used to calculate similarity indices depending on the class of the digital signature: the local entropy correlation of images, the correlation of images, the Hamming distance of binary images, the Euclidean distances can be cited , etc. The acceptance threshold selected as a decision criterion is a limit value of the similarity index, which determines the limit between the candidate fibrous substrates that will be accepted and, therefore, will be considered authentic, and those that will be rejected. and therefore they will be considered imposters. There are then four families of cases based on that selected decision criterion:

- Authentic Accepted (AA)

- Accepted Imposters (IA)

- Authentic Rejected (AR)

- Imposters Rejected (IR)

Therefore, the decision criterion can be selected such that the amounts of AA and IR are maximized, and the amounts of IA and Ar are minimized. The decision criterion can be adjusted depending on the degree of security desired in the application under consideration: the question that arises is whether, in the application under consideration, it is preferable to reject the authentic ones or accept the impostors.

It is also possible to establish a quantification of the probability of cases of each of the events AA, IA, AR and IR, which allows to develop a strategy for defining the selection criteria based on the probability of cases of one or more of the following events. The acceptance threshold is anyway selected in such a way as to allow the issuance of a positive acceptance decision in those cases where the candidate digital signature does not exactly match the authentic digital signature with which it is compared. Within the framework of the invention, the acceptance threshold is preferably selected in such a way as to accept as authentic fibrous media that have been used to generate the authentic digital signatures, but whose reference area has subsequently undergone some modifications. , in particular printing, punching, cutting, folding, handwriting, lamination, coating with a magnetic stripe, surface treatment, coloring, impregnation or embossing.

On the other hand, during the generation stage of the candidate digital signature, the digitized structural characteristic can also be subjected to a digital treatment that aims to reduce or eliminate the impact of these modifications. In the case of an impression, an additional treatment by an erosion procedure may be considered in terms of image treatment. You can also filter the treatment in such a way that the text is eliminated. In the case of perforation, filtering, for example with a band pass filter and / or thresholding can constitute the pre-treatment. However, after filtering, the digital signature on a printed fibrous medium will not exactly match its digital signature obtained before printing.

The acceptance threshold and the eventual treatment are selected according to the transformations that the reference zone of the support can undergo.

Figure 2 illustrates, by means of a simple example, the interest of the fibrous support recognition method according to the invention. A characteristic acquisition method (seen against the light in daylight), a coding method (band-pass filter in this case) and a similarity index (arithmetic mean of the XOR of the two binary images) are selected to perform the comparisons. The image viewed against the light of an area of ​​a true blank paper is stored digitally on a computer. The blank paper is then printed and thus converted into a document. A different blank paper is then selected. The acquisition of the characteristic of the authentic printed paper is carried out, a pre-treatment is carried out to eliminate the effect of printing (erosion in terms of image treatment), this sample is coded and compared according to

ES 2 394 444 T3 the methods selected with the stored characteristic, a similarity index of 25,603 is obtained. The same characteristic acquisition and coding operations are carried out (unnecessary pre-treatment in that case, since the paper is not printed) with a different blank paper support and it is compared in a similar way with the characteristic of the authentic support registered: a similarity index of 135,208 is obtained. By choosing an acceptance threshold of 50, for example, you can easily differentiate the authentic paper support, even transformed (similarity index lower than the selected threshold) from the different paper support / imposter (similarity index greater than the selected threshold) .

To select the acceptance threshold, the type of different signatures to be accepted is determined: differences due to the different conditions for generating the digital signature and / or differences due to transformations or modifications that occur in the reference area. The acceptance threshold will preferably be selected in such a way that it accepts in at least 99% of the cases, preferably in at least 99.9% of the cases, and preferably in at least 99.99 % of the cases, the different signatures that you want to accept that, although different from the authentic stored digital signatures, correspond to an authentic fibrous medium.

The method according to the invention can be used in any type of existing machine, for example printers, fax, photocopier, scanner, etc. that allow digital information to be transmitted and / or stored and / or reproduced. In particular, the device comprises:

- means for extracting a structural characteristic of a reference zone of a candidate fibrous support, which represents the unique, complex, chaotic and practically invariable structure over time of the reference zone, and in particular a sensor such as a camera CCD or CMOS;

- means for digitizing and possibly processing / coding the structural characteristic measured in a candidate digital signature, which may include algorithms that make it possible to reduce the impact of the modifications produced in the reference area such as printing, perforations, folds, stains, etc.;

- means for calculating, according to a statistical method, a similarity index between the candidate digital signature and an authentic digital signature previously stored on a digital data medium;

- means for comparing the similarity index obtained with a given acceptance threshold, in such a way as to allow the issuance of a positive acceptance decision in those cases in which the digital signature does not exactly match the authentic digital signature with the one that is being compared;

- means of issuing a positive or negative acceptance decision.

The acceptance procedure according to the invention can be used in different applications. Obviously, it can be used to verify the integrity or authenticity of documents that carry sensitive or valuable information.

Next, the process of the invention is especially useful for the traceability of fibrous substrates. In particular, it can make it possible to monitor the life of a fibrous support, and in particular of a paper, of its selection as an authentic support for its use, passing through some transformation stages (printing, engraving, perforation, folding, cutting, etc. .). At different stages of its life, a candidate digital signature may be generated from said support to verify its authenticity. The method according to the invention with an acceptance threshold allows such traceability, in contrast to a method based on exact comparison. A database of authentic digital signatures can be built from authentic blank supports (sheets or cards, or envelopes or packaging, or otherwise). The candidate digital signatures obtained from the media that have undergone some subsequent transformation due to their use can then be compared, for example, with the authentic digital signatures stored in the database.

Another application is to physically join the fibrous support to a product, an object, and even a living being, and use it as a label. The direct monitoring of the fibrous support by means of the method of the invention makes it possible to indirectly monitor the product, the object or the living being. The physical bond must be such that any attempt to separate the fibrous support from the product, object or living being results in the destruction of the label, for example a non-repairable tear, in the case of an adhesive bonded paper.

Other essential applications of the acceptance procedure according to the invention relate to the field of secure access control procedures. The invention also has as its object a method to carry out a secure access control and to give or not an access authorization in which an access key formed by a fibrous support of paper, cardboard or a non-woven material is used, which comprises the following: following stages:

- a characteristic of the fibrous structure of the fibrous support is detected in a reference volume of the fibrous support and a digital signature representing the fibrous structure of the reference volume of the fibrous support is generated from said detected structural characteristic;

- this digital signature is compared according to a statistical method with a previously registered digital data and a positive or negative acceptance decision is issued;

- an access authorization is issued if the comparison has allowed the issuance of a positive acceptance decision.

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The previously stored digital data corresponds to an authentic digital signature. All variants of the acceptance procedure such as those described above can therefore be applied directly for access control. In the case of a secure access control procedure, the issuance of a positive acceptance decision conditions the access authorization. The fibrous support can then be used directly as a physical and material key that allows access control to premises, machines, computers or hotel rooms, of the information indexed in the authentic digital signature. When, through the application of the acceptance procedure, the physical key is accepted, its user is then given permission to access or enter a room, a machine, a computer or a hotel room. The application of the method according to the invention makes it possible to obtain a degree of security superior to that obtained with the iris of the human eye, which is the best biometric characteristic used in the industry today.

In applications that require secure access to sensitive information, a database of authentic signatures is generated and an additional virtual link is made that associates each digital signature with at least one sensitive information, by means of a database, of safe preference. Each digital signature is therefore indexed to sensitive information. In this case, the issuance of a positive acceptance decision is accompanied by the identification of the authentic digital signature that offers the best similarity index, and access to the information associated with said authentic digital signature. This type of secure access control procedure finds application in multiple fields, such as activity monitoring, traceability, control and information security. By means of the method according to the invention, the desired degree of security can be selected depending on the application, in particular by playing with the complexity of the digital signature and the selected acceptance threshold. The use of a "dynamic" signature allows access to information, for example, as long as the candidate material examined is considered authentic.

It may also happen that the authentic digital signatures, stored in the database, are not the digital signatures of different fibrous media, but rather different digital signatures of the same fibrous media, which allow different levels of access.

The fibrous medium acts as an access key and can be presented, among others, in the form of cards and physically associated with an electronic chip and / or a magnetic strip. The support can also integrate information transmission means or integrate one or more elements that are used in an information transmission, in particular elements sensitive to radio frequencies (active or passive antennas, for example ), which guarantee wireless and remote data communication.

Sensitive information can be structured in a different database or shared with the database of authentic signatures, made secure or not by any known means, fully or partially, and contain:

- data on the settings of the sensors and of the installation for digitization, extraction and generation of digital signatures (scale, algorithms used, etc.);

- general administrative data (identity of persons, personal codes, etc.);

- biometric data;

- data contained in part on paper (printed or otherwise);

- data on other papers (digital signature of an envelope, for example, present at the same time as that of the letter paper).

Another application directly considered by the present invention is the joint use of the acceptance procedure according to the invention with any biometric method of identification of a human being, an animal or a plant. Indeed, the acceptance procedure according to the invention can be adapted to computer platforms developed for the comparison of biometric characteristics, such as fingerprints in particular. This joint use makes it possible to solve difficult problems of identification of the bearer of a paper document (access card, identification card, etc.) by integrating the authentic fibrous medium, together with the authentication of said document, and associating these two identifications / authentications to the information regarding the bearer and / or his paper document.

The method according to the invention can also be used, for example, to make e-mail secure, to provide remote access to data, through a telecommunications network (for example, the Internet) or to carry out electronic signatures.

The examples that follow illustrate the invention and are in no way limiting.

EXAMPLE 1

A digital signal in grayscale image is extracted from a paper seen against the light, by means of a CCD camera, in which filtering algorithms (Gabor 2D, Laplace, FFT band pass, etc.) are applied at different frequencies. spatial (scales), and even in different directions. These operations are carried out to create the family of "authentic" signatures and during the application of the acceptance procedure on a candidate paper sample.

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From that moment on, the images resulting from these algorithms can be compared directly in grayscale or in binary form, that is, in the form of matrices of 0 and 1. The digital signatures of the authentic ones or of the candidate sample must be, preferably the same size.

When using binarization and thresholding, the comparison of this digital signature A in the candidate sample with the authentic digital signatures B of the same size is done by a bit-by-bit logical XOR (exclusive-OR) operation. A resulting C image file is obtained with the same size as the compared digital signatures. The phase of calculating the similarity index is carried out by calculating the arithmetic mean of the bits of the image C (Hamming distance (DH)). In this case, two signatures obtained from the same paper will have a DH close to zero, two signatures obtained from two different papers will have a DH close to 0.5. The acceptance threshold must belong to the interval] 0; 0.5 [. When grayscale images obtained after the encoding phase are directly compared, image correlation can be used as a comparison method. Identical papers will have a correlation close to 1 and different papers a correlation close to 0 (positive or negative). You can select an acceptance threshold belonging to the interval] 0; 1 [as a decision criterion, below which the compared papers will be considered to be different (impostors) and above which they will be considered to be identical (authentic).

Therefore, the acceptance threshold criteria are completely tied to the coding methods and the selection of a similarity index for comparison. In addition, each application will have security requirements that will determine the strategies for the selection of coding methods, similarity indices and particular acceptance thresholds.

A double test was carried out on different sheets of paper extracted from the same production (mother reel), examining the translucency of the samples. First, the characteristic digital signatures of all the different samples were compared and, having previously selected a relevant similarity index, a distribution of the number of samples based on their similarity index (eg correlation) could be established. A given sample was then selected and measured a large number of times, thus causing errors in the measurement process, and it was possible to establish, using the same index of similarity, a distribution of the number of measurements as a function of their index of similarity.

The distribution curves have then been represented graphically. The abscissa axis corresponds to the similarity factor between the tested signatures and the ordinate axis corresponds to the number of images at a given level of the similarity index.

The representation in the same graph of the three distribution curves obtained shows that the procedure according to the invention allows two different papers to be discriminated.

Figure 3 shows the variation of a distribution curve that is obtained as a function of the deliberate or accidental modification of a paper throughout its life. Curve (a) is the distribution curve of digital signatures obtained with different papers. Curve (b) is the distribution curve of digital signatures obtained with the same paper in different acquisitions.

The digital signatures of a modified paper with different modifications (successive measurements) are represented in the distribution curve (c) = it seems that this curve (c) is displaced with respect to the distribution curve (b) obtained from the same paper (without modification, successive measurements) and approaches the distribution curve (a) obtained with different papers. The distribution curve (c) also has a tendency to widen. However, it is clear that curves (a) and (c) are far enough apart to select a reliable acceptance threshold (SA), below which the compared papers are considered to be identical (the candidate is authentic), and above which they will be considered to be different (the candidate is an imposter). The choice of the acceptance threshold is determined according to the use to be made of the papers, when establishing the application considered. The choice of a discriminating acceptance threshold (SA) in the overlap zone will necessarily imply a probability of error as regards the recognition of the paper subject to the method according to the invention. This threshold is adjusted according to the application, selecting whether it is better to make an error of the type rejection of an authentic paper or of the type acceptance of an imposter, and a confidence index of the decision will be issued based on the selected threshold. In the case of paper, this probability of error is extremely low. The probability of error is less than 1 in 10¹⁵ measurements if the calculated intersection point between the two curves (a) and (c) is selected as the acceptance threshold level and is represented in figure 3. In practice, the overlap between these two curves is almost zero or extremely low .

EXAMPLE 2

As mentioned above, FFT (fast Fourier transform) frequency band pass filtering can be performed on a candled paper image, which can then be binarized. For well-determined values ​​of this band-pass filter, a random binary image is obtained, an image of the chaotic structure of the paper that can be assimilated to a fingerprint. Therefore, algorithm platforms specially created for the management of fingerprint identification were used, the tests of which are based on the

ES 2 394 444 T3 recognition of singularities in footprint patterns (small details (orientations, curvatures, relative position, center, etc.).

The advantage of this type of method is that the aspects of the generation of digital signatures, comparison and decision of acceptance or rejection of the paper candidate for identification are already present in these platforms, and are even combined with the calculation of a confidence index. The algorithms in question are multiple and most of them are very fast in the calculation and query phases of the reference database. They also present interface management with the scanners used for fingerprint recognition. They were used, after minor adaptations, on the paper supports.

In this way, it is possible to carry out recognition tasks of the holder of an identification card simultaneously with the recognition of the physical support (paper) of the identification card, or to give access permission to a place, a machine, an activity or information if the person is recognized as authentic and if he is indeed in possession of a correct fibrous medium (access card).