ANTI-COUNTERFEITING PROTECTION FOR DIGITAL FILES

Abstract

The invention relates to securing an original digital file against forgery and falsification of its associated data, particularly data related to its membership in a specific batch of digital files, while enabling offline or online verification of the authenticity of a secured digital file and the conformity of its associated data with that of a genuine original digital file. The invention is particularly useful for securing print-ready digital files.

Description

ANTI-COUNTERFEITING PROTECTION FOR DIGITAL FILES FIELD OF INVENTION The present invention relates to the technical field of protecting digital data against falsification or manipulation, and the traceability of digital files. BACKGROUND OF THE INVENTION The problems of digital file forgery and manipulation are well-known, serious, and on the rise. The example of forging data marked on an original digital document, such as a digital identity document or a digital version of a diploma, is commonplace, and the problem is even worse when considering a digital copy of the original (possibly genuine) document. Simply tracking identifiers such as serial numbers, or even including some digital watermarks, is generally a weak response, since forgers can easily copy such numbers or digital watermarks. Another drawback of most conventional methods for ensuring the authenticity of digital files, or securing your digital data, is that they tend to view files in isolation, even if they are part of a well-defined group, such as a batch of digital documents, for example. This ignores valuable authentication information. Therefore, it is an object of the invention to secure a printable digital file against counterfeiting and falsification of its associated data, and particularly data related to its membership in a specific batch of digital files. It is also an object of the invention to enable offline verification of the authenticity of a printable digital file secured according to the invention and the conformity of its associated data content with that of a genuine digital file. The invention also aims to secure printable digital files in such a way that it is easy to verify the authenticity of the data content of both the printable digital files and their printed versions.In particular, one objective of the invention is to ensure print-ready digital files, a print-ready digital file being known as a print file that meets the following criteria: all possible RGB images are converted to CMYK color, the file is in a correct format such as PSD, EPS, AL, high-resolution JPG, PDF or TIF, and the final image has sufficient resolution (i.e., 300 dpi or higher). BRIEF DESCRIPTION OF THE INVENTION According to one aspect, the invention relates to a method of securing an original digital file belonging to a batch of a plurality of original digital files against falsification or manipulation, each original digital file including its own digital data, characterized in that it comprises the steps of: - for each original digital file in the batch, calculate by means of a one-way function an associated digital file signature of its digital data; - forming a tree based on the plurality of digital file signatures calculated for the original digital files in the batch and comprising nodes arranged according to a given node order in the tree, said tree comprising node levels starting from the leaf nodes, which correspond to the plurality of digital file signatures respectively associated with the plurality of original digital files in the batch, to the root node of the tree, each non-leaf node of the tree corresponding to a digital signature by means of the one-way function of a concatenation of the respective digital signatures of its child nodes according to a tree concatenation order, the root node corresponding to a reference root digital signature, i.e., a digital signature by means of the one-way function of a concatenation of the digital signatures of the nodes of a penultimate level of nodes in the tree according to said tree concatenation order; - associate with the given original digital file a corresponding digital verification key that is a sequence of the respective digital signatures, from the leaf node level to the penultimate node level, of each other leaf node that has the same parent node in the tree as the leaf node that corresponds to the digital file signature of the given original digital file, and successively at each subsequent level in the tree, of each non-leaf node that has the same parent node in the tree as the same previous parent node considered at the previous level; - to make available to a user the root digital signature of the reference tree; and - include in the original digital file a corresponding digital security mark comprising a machine-readable representation of its digital data and its corresponding digital verification key, thereby obtaining a marked original digital file whose digital data is secured against counterfeiting or manipulation. Therefore, if the digital security mark included in the digital file is printable as a barcode, the printed document (including the printed barcode) obtained by printing the secured digital file (using a conventional printer) is also secured; that is, its printed data is secured against counterfeiting or tampering. The reference root digital signature of the tree's root node can either be published on a user-accessible medium, stored in a user-accessible, searchable root database, or on a blockchain, or in a user-accessible database secured by a blockchain. Therefore, the reference root digital signature becomes immutable. Therefore, according to the invention, the conflict of digital signatures of all original digital files in a batch, due to the tree structure and use of robust one-way functions to calculate the tree node values, together with the root digital signature of the tree being made immutable and the inclusion of the digital data and its associated digital verification key in a digital security mark included in the corresponding original digital file, allows the tracking and traceability of the marked files and their copies, as well as their printed versions, with a high level of reliability while preventing data forgery and falsification of the marked files. The original marked digital file may additionally comprise root node access data included therein and containing sufficient information to allow the user to access the reference root digital signature of the root node of the tree corresponding to the batch of original digital files, such information being a link to an operable access interface to receive from the user a root request containing digital data, or digital file signature, obtained from a digital security mark of a marked original digital file, and send back a reference root digital signature of the corresponding tree, the PPP access interface allowing, respectively, one of the following: - the medium in which the reference root digital signature is published; - the searchable root database in which the reference root digital signature is stored; and - the blockchain, or respectively the database secured by a blockchain, in which the reference root digital signature with timestamps is stored. According to the invention, it is also possible that: - a virtual digital file is counted as belonging to the batch of original digital files, including said virtual digital file its own virtual digital data, and an associated virtual digital file signature obtained through the one-way function of the virtual digital data, said virtual digital file not being the actual file but used only to generate the associated virtual digital file signature from its virtual digital data; and - the root digital signature of reference associated with said batch of original digital files being calculated from a tree having all the digital file signatures of the original digital files in the batch, including the virtual digital file signature, as leaf nodes. To have shorter signatures, the one-way function can be a slice function, and a digital signature of an original digital file can be a sequence of a given plurality of smaller weight bits selected from the slice value bits of the corresponding digital data. In the above method, additional digital data corresponding to the digital data associated with the original marked digital file can be stored in a searchable information database accessible by the user through an operable information database interface to receive from the user an information request containing digital data, or a digital file signature, obtained from a digital security mark of an original marked digital file, and send back corresponding additional digital data. The digital data in the original marked digital file may additionally include reference data for a corresponding unique physical characteristic of an associated object or individual. Furthermore, the unique physical characteristic of the associated object or individual may be, respectively, a material-based security mark applied to the associated object or a biometric identifier of the associated individual. In the previous method, the sequence of digital signatures in the digital verification key included in the digital security mark can be arranged according to a sequence order of the nodes that differs from the corresponding node order defined by the tree concatenation order, and the digital security mark can additionally include an order code associated with that sequence order. These features increase the level of security against code-breaking attacks. According to the invention, in the case where the digital data of the respective original digital files of the batch are distributed among given fields common to all digital files of the batch, specific digital data related to these fields may not be included in the digital data, but may be grouped into a separate field data block associated with the batch, in which: i) the digital file signature of an original digital file is calculated using the one-way function of a concatenation of the corresponding digital data and the field data block; and i) the reference root digital signature is made available to the user together with the associated field data block. Another aspect of the invention relates to a method of verifying the authenticity of a digital file secured according to the above securing method, or the conformity of a copy of such a secured digital file with respect to the original, comprising the steps of, after processing a test file that is said digital file or said copy of the digital file by means of a processing unit connected to a memory: - having stored the test file in memory; - read a digital data representation and a digital verification key into a digital security mark of the stored test file, and respectively extract corresponding digital test data and digital verification key from said read representation; - having stored in memory a root digital signature of reference to a root node of a tree of the batch of original digital files, and having programmed in the processing unit the one-way function to calculate a digital signature of digital data and a concatenation of digital signatures according to the order of nodes in the tree and the tree concatenation order; - verify whether the extracted digital test data and the associated digital test verification key actually correspond to the stored reference root digital signature by performing the following steps: - calculate with the one-way function a digital proof signature of the extracted digital proof data, said digital proof signature corresponding to a proof leaf node in a proof tree that corresponds to the digital security mark of the proof file; - extract from the sequence of digital signatures in the test digital verification key, a digital signature from each other leaf node of the test tree that has the same parent node as the test leaf node and calculate a digital signature of a concatenation of the test digital signature and the digital signature extracted from each other leaf node, thus obtaining a digital signature from the same parent node of the test leaf node; - successively at each subsequent level in the test tree and up to the penultimate level of nodes, extract from the sequence of digital signatures in the test digital verification key, a digital signature from each other non-leaf node of the test tree that has the same parent node as the same previous parent node considered in the previous stage and calculate a digital signature from a concatenation of the digital signature of said each respective other non-leaf node and the digital signature obtained from said same previous parent node, thereby obtaining a digital signature from said same parent node of said same previous parent node; - calculate a digital signature from a concatenation of the digital signatures obtained from the non-leaf nodes corresponding to the penultimate level of nodes in the test tree, thus obtaining a candidate root digital signature of the root node of the test tree; and - check if the candidate root digital signature obtained matches the stored reference root digital signature, so that, in the case where these root digital signatures match, the digital data in the test file is that of a genuine digital file. If the original marked digital file is secured while having the aforementioned separate field data block, the processing unit's memory can additionally store the associated field data block, and the stage of calculating a test digital signature corresponding to a test leaf node in a test tree corresponding to the digital security mark in the test file can comprise calculating with the one-way function a digital signature of a concatenation of the extracted test digital data and the stored field data block. If the digital file has been secured by storing the reference root digital signature in a user-accessible, searchable root database as mentioned above, and the processing unit is further connected to an operable communication unit to send and receive data back via a communication link, the above verification method may comprise the preliminary stages of: - send a request to said root database via the communication unit through the communication link, and receive back the reference root digital signature; and - store the received root digital signature in the memory. If the secured digital file comprises root node access data as explained above, and the processing unit is further connected to an operable communication unit to send and receive data via a communication link, the above verification method may comprise the preliminary stages of: - Read the root node access data included in the test file; - send with the communication unit through the communication link a root request to said access interface containing digital data, or a digital signature of said digital data, obtained from the digital security mark in the test file, and receive back a corresponding reference root digital signature of the associated batch; and - store the received reference root digital signature in memory. If the marked digital file has additional digital data stored associated with it in a queryable information database as mentioned above, the image former may be further equipped with operable means of communication to send to the information database interface an information request containing digital data, or a digital file signature, obtained from the digital security mark of the test file, and receive back corresponding additional digital data. In the case where the secured digital file includes reference feature digital data as mentioned above, and the image former is further equipped with an operable sensor to detect a unique physical feature of an associated object or individual, and the processing unit is programmed to extract corresponding feature digital data from a detection signal received from the sensor, the image former having stored in memory reference feature digital data CDD corresponding to said unique physical feature of the associated object or individual, the verification method may comprise the further steps of, after visualizing a subject that is said associated object or individual: - detect with the sensor a unique physical characteristic of the subject and extract corresponding candidate digital characteristic data CDDc; - compare the candidate CDD obtained digital feature data with the stored reference CDD digital feature data; and - In the case where the candidate CDDcs digital feature data are similar to the stored reference CDD digital feature data, within a given tolerance criterion, the subject is considered to correspond respectively to a genuine object or individual validly associated with a genuine digital file. Another aspect of the invention relates to a digital file belonging to a batch of multiple original digital files and secured according to the aforementioned securing method. Each original digital file in the batch has its own digital data and corresponding digital verification key, and the batch has a corresponding root digital signature. The digital file comprises a machine-readable security mark that includes a representation of its digital data and verification key. The digital data of the digital file may additionally include reference feature digital data (RFD) of a corresponding unique physical characteristic of an associated object or individual. Furthermore, the unique physical characteristic of the associated object may be a material-based security mark applied to the associated object. Another aspect of the invention relates to a system for verifying the authenticity of a digital file, or the conformity of a copy of such a digital file, with respect to a marked original digital file belonging to a batch of original digital files secured according to the aforementioned securing method, comprising an image former having an image former unit, a processing unit with memory, and an image processing unit, the memory storing a root digital signature referencing a tree corresponding to the batch of original digital files, and the processing unit being programmed with the unidirectional function to calculate a digital signature of digital data and a concatenation of digital signatures according to the node order of the tree and the tree concatenation order, said system being operable for: - having stored in memory a test file which is said digital file or said copy of the digital file; - read a digital data representation and a digital verification key into a digital security mark of the stored test file, and respectively extract corresponding digital test data and digital verification key from said read representation; - verify whether the extracted digital test data and the digital test verification key actually correspond to the stored reference root digital signature by performing the programmed operations on the processing unit of: - calculate with the one-way function a digital proof signature of the extracted digital proof data, said digital proof signature corresponding to a proof leaf node in a proof tree that corresponds to the digital security mark of the proof file; - extract from the sequence of digital signatures in the test digital verification key, a digital signature from each other leaf node of the test tree that has the same parent node as the test leaf node and calculate a digital signature of a concatenation of the test digital signature and the digital signature extracted from each other leaf node, thus obtaining a digital signature from the same parent node of the test leaf node; - successively at each subsequent level in the test tree and up to the penultimate level of nodes, extract from the sequence of digital signatures in the test digital verification key, a digital signature from each other non-leaf node of the test tree that has the same parent node as the same previous parent node considered in the previous stage and calculate a digital signature from a concatenation of the digital signature of said each respective other non-leaf node and the digital signature obtained from said same previous parent node, thereby obtaining a digital signature from said same parent node of said same previous parent node; - calculate a digital signature from a concatenation of the digital signatures obtained from the non-leaf nodes corresponding to the penultimate level of nodes in the test tree, thus obtaining a candidate root digital signature of the root node of the test tree; and - check if the candidate root digital signature obtained matches the stored reference root digital signature, so that, in the case where these root digital signatures match, the system is configured to deliver an indication that the digital data in the test file is from a genuine digital file. In the previous system, if the original marked digital file has an associated field data block as mentioned above, additionally storing the associated field data block in the processing unit's memory, the programmed operations of calculating a test digital signature corresponding to a test sheet node corresponding to the digital security mark of the test file then comprise calculating with the unidirectional function a digital signature of a concatenation of the extracted test digital data and the stored field data block. In the case where the marked original digital file belongs to a batch of secured original digital files including reference feature digital data of a corresponding unique physical characteristic of an associated object or individual as mentioned above, the above system being further equipped with a sensor connected to the processing unit and operable to detect a unique physical characteristic of an associated object or individual, and the processing unit being programmed to extract corresponding feature digital data from a detection signal received from the sensor, the system having stored in memory reference feature digital data CDD corresponding to said unique physical characteristic of the associated object or individual, the system may be further operable to: - to detect with the sensor a unique physical characteristic of a subject that is said object or associated individual, and to extract corresponding candidate digital characteristic data CDDc; - compare the candidate CDD obtained digital feature data with the stored reference CDD digital feature data; and - in the case where the candidate CDDcs digital feature data are similar to the stored reference CDD digital feature data, within a given tolerance criterion, provide an indication that the subject is considered genuine. The present invention will be described more fully hereafter with reference to the accompanying figures in which similar numbers represent similar elements throughout the different figures, and in which prominent aspects and features of the invention are illustrated. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a schematic view of a general concept for securing a batch of original digital files according to the invention. Figure 2A illustrates a secured digital biometric passport as an example of a secured digital biometric identity document according to the invention. Figure 2B illustrates a check of an individual who has the secured digital biometric passport from Figure 2A by an authorized agent. Figure 3 illustrates a batch of digital documents related to components of an aircraft insured according to the invention. DETAILED DESCRIPTION OF THE INVENTION The present disclosure is described in detail at this point with reference to non-limiting modalities illustrated in the figures. Figure 1 illustrates a general concept of the invention related to securing a batch of digital files and a method for calculating a verification information encoding that can be associated with each digital file. Figure 1 illustrates a group or batch of digital files Ai,...,As, which contain a digital representation of a machine-readable security mark 110 (illustrated here by a 2D barcode). In what follows, the expression "digital security mark 110" actually means a digital representation of a machine-readable security mark 110. Figure 1 illustrates a group or batch of digital files and its associated tree in which, for simplicity, only eight original digital files are shown: Ai.....Aa. Also for simplicity, the tree associated with the batch of files Ai,..,.,Ab is here a simple binary tree.A digital file can refer to a manufactured item or its packaging, a physical document or image, a package containing several items (such as a blister pack of medication), or a container holding pallets of packaged goods, etc. Not only an object, but even a person can be associated with a digital file within the meaning of the modalities of the invention; for example, authorized attendees at an event, members of a group, or members of a herd or flock could carry some form of ID credential or be physically marked with a mark containing data recorded in a corresponding digital file. A batch of digital files could refer, for example, to a common manufacturing cycle, items delivered by a particular supplier, items manufactured or shipped during a period of time, a set of related images, a group of people, a herd or flock, or any other user-defined grouping of any object for which a digital file A (having digital content D) can be defined. Any of the items shown in Figure 1 could be a virtual item Av, which is an optional software construct that can be included to enable the encoding of selected data. This is explained further below. For example, one of the eight items, say item As, could actually be a virtual item Av that is counted as belonging to the batch of eight items and is treated like any of the other seven real items since it can be processed in substantially the same way (even though it does not correspond to a real object). Of course, a plurality of virtual items Avi, Av2, ..., Avk can be used to encode digital data and produce more robust digital item signatures (see below). For each item Ai, A2, ..., A7, As in the batch (possibly with As ∩ Av), the respective digital item data Di, D2, ..., D7, Ds (possibly with Ds ∩ Dv) are associated or extracted (or, in the case of the virtual item Av, created) using any appropriate method. This data could be some measurement of physical characteristics, textual data such as completed forms or product information, a serial number or other identifier, contents indications, a digital representation of an image, or any other information that the system designer chooses to associate with an item. The digital item data Di can be extracted from human-readable data (e.g., alphanumeric data) marked on an associated item (e.g., printed on the item or on a label affixed to the item) by means of a reader capable of producing corresponding digital data from a digital file Ai.Additional digital data (e.g., instructions for use of the associated item or safety instructions, etc.) may be associated with the extracted data to constitute the digital data of item D!. For the virtual item Av, the associated digital data can include, for example, a batch identification number, the number of items in the batch, a (pseudo) random number for the sake of increased security by increasing data entropy, date and / or time information, etc. Other forms of associated data could be indications of permissible or impermissible operating rules, expiration dates, etc. In short, the digital data Dv can be anything that can be represented digitally. For each item in the batch, its respective digital item data Di,D2,...,D7,D8 are preferably mathematically transformed in such a way that they are essentially hidden, although this is not an absolute requirement for any modality. This transformation, applied to the digital item data D1 of an item A1, serves to create a corresponding digital signature x1. This digital signature is produced by means of a one-way function, that is, a function that is easy to compute but difficult to reverse (see S. Goldwasser and M. Bellare, Lecture Notes on Cryptography, MIT, July 2008, http: / / www-cse.ucsd.edu / users / mihir). One advantageous transformation is, for example, applying a slicing function H() = slicing() to digital data, which generally has the property of returning an output of a known bit length regardless of the input size. This technical effect is particularly useful for creating a digital signature of digital data from a digital file (e.g., associated with an item) regardless of the size of the digital data and the size of the batch of corresponding digital files. The slicing function is a well-known example of a one-way function. If a cryptographic slicing function is used, such as the SHA (Secure Slicing Function Algorithm) class of functions, for example, SHA-256, then there are the additional benefits that the function is practically irreversible and collision-resistant; that is, the probability that two different inputs will lead to the same result is negligible.As will be understood from the description below, this is also not a requirement of the invention, although it is advantageous for the same reasons as in other applications. As shown in Figure 1, the values ​​xi,X2,xs.....xs are the slice values, i.e., the associated digital article signatures, of the respective article data sets, i.e., Xj = H(Dj), for j=1 ,...,8 (in the case where As ξ Av, then Ds ξ Dvy xs ξ xv= H(DV)). To shorten the signature, the digital signature of item x¡ of item A¡ can even be just a sequence of a given plurality of less weight bits selected from the bits of the snipping value H(Dj): for example, with the SHA-256 snipping function of the SHA-2 family, it is sufficient to retain only the 128 least weight bits from the 256 bits of the signature to still have a robust signature with respect to a codebreaking attack. Figure 1 shows a batch of eight original items marked A1, ..., A1, each having a corresponding security mark 110 applied to it. It illustrates the method of securing the items and their respective associated digital item data D1, ..., D2 (symbolically represented in the A1 files in Figure 1 by a sequence of 0 and 1 bits) by means of a digital signature tree of the digital data. Trees associated with digital signatures are well known (binary hacking trees, n-ary hacking trees, or Merkle trees). They generally have base nodes, or leaf nodes, which are used to construct subsequent (intermediate) level nodes by digitally signing a concatenation of the digital signatures associated with the leaf nodes according to a certain grouping of the leaf nodes.In the case of a binary tree, the digital signatures associated with the first intermediate-level nodes are calculated by digitally signing (for example, with a one-way slice function H, or a one-way elliptic curve function...) a concatenation of the digital signatures associated with two consecutive leaf nodes. In the case of a nary tree, the values ​​of the first intermediate-level nodes are obtained by concatenating the values ​​of n consecutive leaf nodes. A tree can also have a more complex structure (mixed trees) since the concatenation of leaf nodes can be performed by pairs of consecutive nodes for certain leaf nodes, by triplets of nodes for other consecutive leaf nodes, and so on. For simplicity, Figure 1 shows a simple binary tree with eight leaf nodes: the respective values ​​of the eight leaf nodes a(1,1),...,a(1,8) of the tree, respectively correspond to the digital signatures of article xi = H(Di).....xs = H(Ds). The value of the first index, that is, 1, for all. IVIA / a / ¿U¿ l / UU 1400 leaf nodes indicates the first level (or base level) of the tree, and the second index ranging from 1 to eight indicates the order of nodes (leaf) of the tree. The values ​​of the next-level (non-leaf) nodes—that is, the four level-two nodes a(2,1), a(2,2), a(2,3), and a(2,4)—are obtained by digitally signing a concatenation (symbolically represented at this point by an operator using a slice function) of the values ​​of pairs of leaf nodes, i.e., pairs of their child nodes in the tree. This grouping of child nodes to obtain the values ​​of the next-level nodes defines the tree's concatenation order. To simplify the notation, we use the node symbol a(i,j) to also represent its associated value (i.e., its associated digital signature). At this point, the tree has only two intermediate levels above the leaf node level, with the root node at the top level.The root node level is, in fact, the last non-leaf node level in the tree. Therefore, the values ​​of the four non-leaf nodes at the next intermediate level are: a(2,1) = H(a(1,1)+a(1,2)), that is, a(2,1) = H(H(Di)+ H(H(D2)), (since a(1,1) and a(1,2) are the child nodes of node a(2,1)) a(2,2) = H(a(1,3)+a(1,4)) a(2,3) = H(a(1,5)+a(1,6)) a(2,4) = H(a(1,7)+a(1,8)) and, for the next, penultimate, node level (at this point, level three) there are two node values: a(3,1) = H(a(2,1)+a(2,2)) a(3,2) = H(a(2,3)+a(2,4)). We observe that it is possible to choose a different tree concatenation order for each non-leaf node: for example, instead of having a(2,4) = H(a(1,7)+a(1,8)) we could define a(2,4) = H(a(1,8)+a(1,7)), which provides a different node value. Finally, the value of the root node R of the tree, or reference root digital signature, is obtained as: R = H(a(3,1)+a(3,2)). Due to the cascading concatenations involved in a tree, it is virtually impossible to recover a root value if any bit of digital data in a node (particularly a leaf node) is changed. Furthermore, if some virtual items are included in the batch (whose virtual item digital data is known only to the system that produced the digital signatures of the tree's leaf nodes), a forger will not be able to recover the root digital signature even if they know the digital data of all the items produced (and tagged) in the batch. According to the invention, the reference root digital signature R of the batch of original digital files is made immutable and therefore tamper-proof by being published on a (public) medium accessible to a user who needs to verify the authenticity of an item (or its associated data), or by being stored in a user-accessible, searchable root database, or, in a preferred mode, by being stored on a user-accessible blockchain (or a blockchain-secured database). The user can then store the reference value R acquired from these available sources. For each original digital file A¡ in the batch, a corresponding digital verification key k¡ (or verification path) of the associated tree is then calculated as a sequence of the respective digital signatures, from the leaf node level to the penultimate node level, of each other leaf node that has the same parent node in the tree as the leaf node corresponding to the digital signature of the original digital file A¡, and successively at each subsequent level in the tree, of each non-leaf node that has the same parent node in the tree as the same parent node considered at the previous level. In the example in Figure 1, there are eight verification keys ki ks that correspond respectively to the eight items Ai As in the batch and their corresponding eight leaf nodes a(1,1),...,a(1,8): 1) For the leaf node a(1,1) = xi = H(Di) corresponding to article Ai, the verification key is ki = {a(1,2),a(2,2),a(3,2)}, from which the root digital signature value R can be recovered through the following steps (executed according to the node order in the tree and the tree concatenation order): i) starting from the leaf node a(1,1) = xi and leaf node a(1,2) = x2 in ki (a(1,2) is the other leaf node that has the same parent node, i.e., node a(2,1), as the leaf node that corresponds to the digital signature of article xi, i.e., node a(1,1)), the value of parent node a(2,1) is obtained by a(2,1) = H(a(1,1)+a(1,2)) (i.e., a(2,1) = H(xi + x2)), i) starting from the a(2,1) obtained and the next node value in ki, i.e., a(2,2) of the next level of non-leaf nodes, which is a non-leaf node that has the same parent node in the tree, i.e., node a(3,1), as the same previous parent node considered at the previous level, i.e., node a(2,1), the The parent node value a(3,1) is obtained by a(3,1) = H(a(2,1 )+a(2,2)), iii) from the a(3,1) obtained and the next node value in ki, that is, a(3,2) of the penultimate level of nodes, which is a non-leaf node that has the same parent node in the tree, that is, the root node,that the same parent node considered at the previous level, i.e., node a(3,1), the root node value R is obtained by R = H(a(3,1 )+a(3,2))., Note: In this example we have three stages i), i) yi¡¡), because the tree has three levels below the root node level and therefore the verification key contains three node values. Therefore, the value of the root node of the tree can be obtained as: R = H(H(H(a(1,1 )+a(1,2))+a(2,2))+a(3,2)). 2) For the leaf node a(1,2) = x2= H(D2) which corresponds to article A2, the verification key is k2= {a(1,1),a(2,2),a(3,2)}, from which the root value R can be recovered through the following steps (executed according to the order of nodes in the tree and the tree concatenation order): i) from a(1,2) = x2 and a(1,1) = xi in ki (a(1,1) is the other leaf node that has the same parent node, i.e., node a(2,1), as the leaf node that corresponds to the digital signature of article x2, i.e., node a(1,2)), the value of parent node a(2,1) is obtained by a(2,1) = H(a(1,1)+a(1,2)), i) from the a(2,1) obtained and the next node value in k2, i.e., a(2,2) of the next level of non-leaf nodes, which is a non-leaf node that has the same parent node in the tree, i.e., node a(3,1), as the same previous parent node considered in the previous level, i.e., node a(2,1), the value of parent node a(3,1) is obtained by a(3,1) = H(a(2,1 )+a(2,2)), iii) from the a(3,1) obtained and the next node value in k2, that is, a(3,2) of the penultimate level of nodes, which is a non-leaf node that has the same parent node in the tree, that is, the root node, as the same previous parent node considered in the previous level, that is, node a(3,1),The root node value R is obtained by R = H(a(3,1 )+a(3,2))., Therefore, the value of the root node of the tree can be obtained as: R = H(H(H(a(1,1 )+a(1,2))+a(2,2))+a(3,2)). 3) For the leaf node a(1,3) = X3 = H(Ds) corresponding to article A3, the verification key is ks = {a(1,4),a(2,1 ),a(3,2)}, from which the root value R can be recovered through the following steps (executed according to the order of nodes in the tree and the tree concatenation order): i) starting from a(1,3) = xa ya(1,4) = X4 in ks (a(1,4) is the other leaf node that has the same parent node, i.e., node a(2,2), as the leaf node that corresponds to the digital signature of article xs, i.e., node a(1,3)), the value of parent node a(2,2) is obtained by a(2,2) = H(a(1,3)+a(1,4)), i) starting from the a(2,2) obtained and the next node value in ks, i.e., a(2,1) of the next level of non-leaf nodes, which is a non-leaf node that has the same parent node in the tree, i.e., node a(3,1), as the same previous parent node considered at the previous level, i.e., node a(2,2), the value of parent node a(3,1) is obtained by a(3,1) = H(a(2,1 )+a(2,2)), iii) from the a(3,1) obtained and the next node value in ks, that is, a(3,2) of the penultimate level of nodes, which is a non-leaf node that has the same parent node in the tree, that is, the root node, as the same previous parent node considered in the previous level, that is, node a(3,1),The root node value R is obtained by R = H(a(3,1)+a(3,2))., Therefore, the value of the root node of the tree can be obtained as: R = H(H(a(2,1 )+H(a(1,3)+a(1,4)))+a(3,2)). 4) For the leaf node a(1,4) = X4 = H(D4) corresponding to item A4, the verification key is k4 = {a(1,3),a(2,1),a(3,2)}, from which the root value R can be recovered through the following steps (executed according to the order of nodes in the tree and the tree concatenation order): i) from a(1,4) = X4 ya(1,3) = xs in k4, the value of parent node a(2,2) is obtained by a(2,2) = H(a(1,3)+a(1,4)), iii) from the a(2,2) obtained and the next node value in k4, that is, a(2,1) of the next level of non-leaf nodes, the value of parent node a(3,1) is obtained by a(3,1) = H(a(2,1)+a(2,2)), iii) from the a(3,1) obtained and the next node value in k4, that is, a(3,2) of the penultimate level of nodes, the value of root node R is obtained by R = H(a(3,1 )+a(3,2)). Therefore, the value of the root node of the tree can be obtained as: R = H(H(a(2,1 )+H(a(1,3)+a(1,4)))+a(3,2)). 5) For the node a(1,5) = xs = H(Ds) which corresponds to the article As, the verification key is ks = {a(1,6),a(2,4),a(3,1)}, from which the root value R can be recovered through the following steps (executed according to the order of nodes in the tree and the tree concatenation order): i) from a(1,5) = xs ya(1,6) = χθ in ks, the value of parent node a(2,3) is obtained by a(2,3) = H(a(1,5)+a(1,6)), iii) from the a(2,3) obtained and the next node value in ks, that is, a(2,4) of the next level of non-leaf nodes, the value of parent node a(3,2) is obtained by a(3,2) = H(a(2,3)+a(2,4)), iii) from the a(3,2) obtained and the next node value in ks, that is, a(3,1) of the penultimate level of nodes, the value of root node R is obtained by R = H(a(3,1 )+a(3,2)). Therefore, the value of the root node of the tree can be obtained as: R = H(a(3,1)+H(H(a(1,5)+a(1,6))+a(2,4))). 6) For the node a(1,6) = χθ = H(De) which corresponds to the article Aθ, the verification key is ks = {a(1,5),a(2,4),a(3,1)}, from which the root value R can be recovered through the following steps (executed according to the order of nodes in the tree and the tree concatenation order): i) from a(1,6) = xs ya(1,5) = xs in ks, the value of parent node a(2,3) is obtained by a(2,3) = H(a(1,5)+a(1,6)), iii) from the a(2,3) obtained and the next node value in ks, that is, a(2,4) of the next level of non-leaf nodes, the value of parent node a(3,2) is obtained by a(3,2) = H(a(2,3)+a(2,4)), iii) from the a(3,2) obtained and the next node value in ke, that is, a(3,1) of the penultimate level of nodes, the value of root node R is obtained by R = H(a(3,1 )+a(3,2)). Therefore, the value of the root node of the tree can be obtained as: R = H(a(3,1)+H(H(a(1,5)+a(1,6))+a(2,4))). 7) For node a(1,7) = X7 = H(D?) corresponding to item A?, the verification key is k? = {a(1,8),a(2,3),a(3,1)}, from which the root value R can be recovered through the following steps (executed according to the order of nodes in the tree and the tree concatenation order): i) from a(1,7) = X7 ya(1,8) = xs in k7, the parent node value a(2,4) is obtained by a(2,4) = H(a(1,7)+a(1,8)), iii) from the a(2,4) obtained and the next node value in k7, that is, a(2,3) of the next level of non-leaf nodes, the parent node value a(3,2) is obtained by a(3,2) = H(a(2,3)+a(2,4)), iii) from the a(3,2) obtained and the next node value in k7, that is, a(3,1) of the penultimate level of nodes, the root node value R is obtained by R = H(a(3,1 )+a(3,2)). Therefore, the value of the root node of the tree can be obtained as: R = H(a(3,1 )+H(a(2,3)+H(a(1,7)+a(1,8)))). 8) For the node a(1,8) = xs = H(Ds) which corresponds to the article As, the verification key is ks = {a(1,7),a(2,3),a(3,1)}, from which the root value R can be recovered through the following steps (executed according to the order of nodes in the tree and the tree concatenation order): i) from a(1,8) = xs ya(1,7) = X7 in ks, the parent node value a(2,4) is obtained by a(2,4) = H(a(1,7)+a(1,8)), iii) from the a(2,4) obtained and the next node value in ks, that is, a(2,3) of the next level of non-leaf nodes, the parent node value a(3,2) is obtained by a(3,2) = H(a(2,3)+a(2,4)), iii) from the a(3,2) obtained and the next node value in ks, that is, a(3,1) of the penultimate level of nodes, the root node value R is obtained by R = H(a(3,1 )+a(3,2)). Therefore, the value of the root node of the tree can be obtained as: R = H(a(3,1 )+H(a(2,3)+H(a(1,7)+a(1,8)))). In general, to retrieve a root (candidate) node value starting from a given leaf node value and the node values ​​specified in the check key associated with that given leaf node, the following steps are performed: - extract from the sequence of node values ​​in the verification key, a node value (i.e., a digital signature value) from each other leaf node in the tree that has the same parent node as the given leaf node and calculate a digital signature from a concatenation of the given node value and, respectively according to the order of nodes in the tree and the tree concatenation order, the node value extracted from each other leaf node, thereby obtaining a digital signature from the same parent node of the given leaf node; - successively at each subsequent level in the tree and up to the penultimate level of nodes: .extract from the sequence of node values ​​in the verification key, a node value from each other non-leaf node in the tree that has the same parent node as the same previous parent node considered in the previous stage, and .calculate a digital signature from a concatenation of the node value of each respective other non-leaf node and the digital signature obtained from that same previous parent node, according to the order of nodes in the tree and the tree concatenation order, thus obtaining a node value from that same parent node of that same previous parent node; and - Calculate a digital signature of a concatenation of the node values ​​obtained from the non-leaf nodes that correspond to the penultimate level of nodes in the tree according to the order of nodes in the tree and the tree concatenation order, thus obtaining a root digital signature of the root node of the tree. As is obvious from the previous example, the root node value R can ultimately be retrieved from any leaf node value given by a digital signature of a concatenation of this leaf node value with only the node values ​​specified in the corresponding verification key. Therefore, the volume of data in the verification information required to retrieve the root node value is clearly much smaller than the volume of data required to calculate the reference root node value (i.e., based solely on the leaf node values, calculating all the non-leaf node values ​​of the intermediate levels of the tree): this is an advantage of the invention in view of the limited size constraint available in a security mark (such as a two-dimensional barcode). According to the invention, the digital security mark 110 of a digital file A¡ of a batch of items includes verification information V¡ that allows both online and offline verification operations of the authenticity of the marked file, of the conformity of its associated data with respect to that of the genuine marked file, providing a unique, immutable and tamper-proof link between the digital data D¡ of A¡ and the fact that the original marked digital file A¡ belongs to a given batch of genuine items, while maintaining a bit size of a digital representation of this verification information V¡ at a level compatible with a data content of a two-dimensional machine-readable barcode that can be easily read by a conventional reader: this verification information comprises the digital item data D¡ and the corresponding verification key k¡, V¡ = (D¡,k¡).The verification operations include retrieving the batch value, or reference root digital signature R, from the tree associated with the batch, by first reading the digital data D¡ and the corresponding digital verification key k¡ in the machine-readable security mark 110 of digital file A¡, then calculating a candidate digital signature X¡ by means of a one-way function of the digital data read D¡ as X¡ = H(D¡), and calculating a candidate root digital signature Rc as explained above from a digital signature of a concatenation of X¡ and tree node values ​​according to the sequence of node values ​​indicated in the digital verification key k¡.This security scheme, which has the advantage of not needing data encryption and therefore encryption / decryption key management (in particular, no cryptographic key is included in the digital security mark), is much more robust with respect to a code-breaking attack compared to conventional data encryption by means of public encryption key-private decryption key (such as the RSA Rivest-ShamirAdleman system, for example).As a result, the size of the digital data to be represented in the digital security mark according to the invention is compact, allowing the use of a conventional 2D barcode representation (e.g., a QR code) (particularly useful for print-ready digital files) and, therefore, conventional barcode readers (or even a simple programmed smartphone with a camera), while providing a very high level of robustness against code-breaking attacks. Furthermore, this security mark supports both online (via a server communicating with a code reader) and offline (via a programmed code reader) verification of the authenticity of a marked digital file and the conformity of its data to that of a genuine (original) digital file.Also, according to the invention, the digital data representation D¡ and the key data representation k¡ may differ, the data concatenation scheme and / or the one-way function may depend on the node level in the tree, which provide additional levels of robustness with respect to code-breaking attacks. Preferably, to further reduce the size of digital data (i.e., verification information V) to be included in a digital security mark, if the digital data D¡ of the respective original digital files A¡ of the batch are distributed among given fields that are common to all digital files in the batch, digital data related to these fields are not included in each digital data DI, but are grouped into a separate field data block FDB associated with the batch of digital files, and: - the digital signature x¡ of an original digital file A¡ of the batch is then calculated with the one-way function H of a concatenation of the corresponding digital data D¡ and the digital data of the data block of fields FDB, i.e. x¡ = H(D¡+FDB); and - The reference root digital signature R is made available to the user along with the associated field data block FDB (which also makes the field data block immutable). In one variant of the invention, the FDB field data block is made accessible to the user independently of the reference root digital signature. The aforementioned size reduction is possible in most cases, since most of the data associated with the digital files of a batch are classified according to certain fields to structure the data: for example, for a pharmaceutical product associated with a secure digital file, the serial number, expiration date, etc., are all included. IVIA / a / ¿U¿ l / UU 1400 associated with these fields are included in D¡ (e.g., 12603, May 2020, etc.) while common field names such as serial number, expiry data, etc. are in the FDB fields data block. There are many known methods for encoding information. Any such method can be used in implementations of any modality of this invention. A common form of marking is the well-known QR code (as a representation of a 2D image contained in a digital file). As is well known, for a given area, the more data a QR code is capable of encoding, the greater its module density (approximately, the density of black / white squares) and the higher the resolution required for its printing and reading. In addition to their density (in the number of modules in squares), QR codes are also generally classified according to the level of error correction they include. Currently, there are four different standard levels: L, M, Q, and H, each representing the degree of deterioration, i.e., data loss, from which the QR code image can be preserved or recovered.Levels L, M, Q, and H can maintain approximately 7%, 15%, 25%, and 30% deterioration, respectively. The following table shows at least approximate values ​​for different QR code versions: Version Size (in modules) Number of encodable bits ECC L-level ECC H-level 10 57x57 2192 976 25 117x117 10208 4304 40 177x177 23648 10208 Not all bits can be used to encode a data payload, however, as some modules are used for target scanning, masking, and error correction. Therefore, there is a trade-off between the amount of information a QR code (or whatever 110 mark is used) can encode and how much information is included in a V verification and must be encoded. For a chosen type of digital security mark 110 (such as a QR code), with a limited encoding capacity, a suitable one-way function H should therefore also be chosen: a function whose output is too large in terms of required bits may be unusable at all, and a function whose scope is too small may not be secure enough. Moreover, in many applications, scalability can be a problem. For example, some data security schemes involve signatures that grow as the number of members in a batch increases, which could unacceptably limit the size of a batch from the perspective of how many bits the digital security mark 110 can encode. This is why, according to a preferred embodiment of the invention, the type of function chosen is a one-way slicing function of the SHA-2 family. Preferably, a calculation module (not shown) is included within an assurance system to execute the provided code for performing the calculations to digitally sign the digital data of the original digital files in a batch, to determine the digital verification keys for the different digital files, and to calculate the root digital signature of the corresponding reference tree. The assurance system may also include suitable modules for inputting (pre-programmed) values ​​that correspond to the digital data (Dv) of the virtual digital file(s) (Av).It would also be possible to perform the file-related chunking calculations externally (for example, on a remote connected server), for example, wherever the digital files are created, thus avoiding the need to transmit raw digital data Di across a network from that site (or sites) to the assurance system, if that is a problem. For each digital file A¡, corresponding verification information V¡ is compiled and encoded (represented) in some form of machine-readable digital security mark 110, which is then included in the respective article. For any virtual digital file Av, its corresponding verification information Vv = (Dv, kv) can be internally associated with it through the assurance system. The verification information generally includes at least, for any digital file A¡ in a batch of digital files, the corresponding digital data D¡ and the corresponding digital verification key k¡: i.e., V¡ = (D¡, k¡). Additional digital data can be further associated with a digital file and may include, for example, the batch value, i.e., the reference root digital signature R, or any other information that the system designer (or system administrator) chooses to include, such as, for example, an associated item serial number, batch ID, date / time information, product name, a URL pointing to other online information associated with either the individual item (such as an image of the item, its labeling or packaging, etc.), the batch, or the manufacturer / supplier, a telephone number that can be called for verification, etc. The additional digital data can be stored in a searchable information database accessible by a user (via an information database interface). Once the digital verification k¡ of an original digital file A¡ has been calculated and included (i.e., through encoding or any chosen data representation), along with the corresponding digital data D¡, in the machine-readable digital security mark 110 in the digital file A¡, the resulting marked original digital file and its associated digital data are, in fact, secured against counterfeiting and tampering. A user, receiving a digital file such as Ai, for example, can scan (or otherwise read) the digital security mark of Ai with an imager (reader) and extract the digital data Di and the digital verification key ki (and any other information that may have been encoded in the mark). For the purposes of verifying the marked digital file Ai, the user must first retrieve the verification information Vi=(Di,ki) from the digital security mark 110 of Ai and, therefore, calculate the digital signature xi from the extracted digital data Di. To do this, the user must know the one-way function to use for calculating a digital signature, in this case the one-way function H() (for example, a SHA-256 hash), and then perform the operation xi=H(Di) to obtain the complete data (xi,ki) needed to calculate a corresponding candidate root digital signature Rc.The user can receive, for example, the one-way function securely (e.g., using a public / private key pair) or by requesting it from the digital archive provider or any entity that has created the signatures and keys, or that has already programmed it into a user's processing unit of their image former. Next, to calculate such a candidate root digital signature Rc, the user will need to know the type of data scheme (for concatenating node values ​​via H(a(i,j)+a(i,k)) to use. The user can obtain this information in any known way, either securely (for example, using a public / private key pair) or simply by requesting it from the digital file provider or any entity that created the verification data or has already programmed it into the user's processing unit. However, the concatenation scheme may, in fact, correspond to a simple conventional end-to-end join of the two digital data blocks that correspond to the two node values. In this case, no specific scheme should be transmitted to the user.In some variants, the concatenation scheme may additionally insert a concatenation block, which may contain data specific to the classification or level of the concatenated digital data blocks in the tree, with the result of making a code-breaking attack even more difficult. Knowing the data concatenation scheme, the user can then calculate (for example, through the appropriately programmed image former) the candidate root digital signature Rc, as explained above, by digitally signing, step by step, a concatenation of the digital signature xi and node values ​​according to the node sequence specified in the digital verification key ki (see Article 1) above related to node a(1,1), executed according to the node order in the tree and the tree concatenation order. At this point, the candidate root digital signature is obtained as follows (the node order in the tree being provided by the respective level indices (i,j) and classification at the level): Rc= H(H(H(a(1,1 )+a(1,2))+a(2,2))+a(3,2)). This candidate root digital signature Rccalculated should then be equal to the available (or published) reference R value: this value may have been previously acquired by the user and / or already stored in the image former's processing unit memory; it could also be a value requested by the receiver and received from the system administrator in any known way. If the candidate Rccalculated and the available reference root digital signatures R match, then this calculation verifies the information in the secure digital mark 110 and confirms that the digital file Ai is from the correct batch. A link to access the reference root digital signature R for the batch corresponding to the digital file Ai could be included in the digital security mark 110 (e.g., a web address, if R can be retrieved from a corresponding website), although this is not a preferred variant. A user, receiving a digital file such as Ai, can scan (or otherwise read) the digital security mark in Ai with a reader and extract the digital data Di and the digital verification key ki (and any other information that may have been encoded in the digital security mark). An example of a reader is a computer with a display, or even a (programmable) smartphone.For the purpose of verifying the marked file Ai, the user must first retrieve the verification information Vi=(Di,ki) from the digital security mark in Ai and, therefore, calculate the digital file signature xi from the extracted digital data Di. To do this, the user must know the one-way function to use for calculating a digital signature, in this case the slicing function H(), and then perform the operation xi=H(Di) to obtain the complete data (xi,ki) necessary to calculate a corresponding candidate root digital signature Bc. The user can obtain, for example, the one-way function securely (e.g., using a public / private key pair) or by requesting it from the digital file provider or any entity that created the signatures and keys, or that has already programmed it into the processing unit of a user's reader. Preferably, the reference root digital signature (i.e., batch value) R is stored in a queryable root database that can be accessed (via a communication link) by the user using their computer equipped with a communication device, such as a smartphone in the previous example. The user who needs to verify the digital file Ai can simply send a root request from their smartphone to the database address, via a database access interface. The request contains the digital data Di read from the digital security mark 110 of Ai (or the calculated digital signature xi = H(Di)), which allows the retrieval of the corresponding reference batch value R. The access interface then returns the reference root digital signature R to the smartphone.The database can be secured using a blockchain to strengthen the immutability of the stored root digital signatures. One advantage of the invention is that it makes the link between a physical object—that is, an original digital file stored in memory—and its attributes—that is, the associated digital data and its membership in a specific batch of digital files—virtually immutable through the corresponding root digital signature. The aforementioned verification process of a digital file A! can also be used to authenticate human-readable data content from A! in a corresponding printed version of the digital file A!. In fact, a user can read the corresponding digital data D! on a computer display as it is decoded from the digital security mark in the digital file A! by the computer, and visually verify that the displayed information is consistent with the data printed in the printed version of the digital file. In a preferred embodiment, the digital data Di additionally includes feature data (CDD) of a corresponding unique physical characteristic of an object, or an individual, associated with the original digital file marked Ai, which can be used to (materially) authenticate the associated object, or the associated individual, by comparing the feature data extracted from the digital security mark and corresponding detection data of the unique physical characteristic obtained from a suitable sensor. Therefore, CDDi being the feature data corresponding to the unique physical characteristic in a digital file Ai, the corresponding unique physical signature data UPSi can be obtained by encoding CDDi (preferably by means of a one-way function): for example, by taking a slice of the feature data CDDi, i.e., UPSi = H(CDDi).However, any other known encoding could be used instead: for example, for brevity, an elliptic curve digital signature algorithm could be used. As a highly simplified example illustrating digital feature data CDDi corresponding to a unique physical feature of an object OBJi associated with a digital file Ai, consider a simple digital image obtained by forming an image of the object OBJi (or a specific area in OBJi), for example, using a smartphone camera. The corresponding unique physical signature data UPSi, for example, is a slice of the digital image, UPSi = H(CDDi). The digital feature data CDDi that generated the signature UPSi is the reference digital feature data for Ai, and the resulting signature UPSi is the corresponding reference unique physical signature data for Ai.Preferably, UPS¡, i.e., the unique physical signature data referencing digital file A¡, is stored in a searchable database or blockchain (or a database secured by a blockchain) accessible to users (e.g., via a request containing the digital data D¡ read from the digital security mark in digital file A¡, or its corresponding digital file signature x¡). The stored UPS¡ thus becomes immutable. A copy of CDD¡ can also be stored in the memory of the user's smartphone (or reader or computer). In one variant, a copy of UPS¡ can also be stored in the memory of the user's smartphone (or reader or computer) to enable offline verification. An authenticity check of the digital file A¡ can be performed by extracting candidate feature digital data CDD¡c from the digital data D¡ read (at this point, with a decoding application running on the smartphone) in the digital security mark included in the digital file A¡, and comparing them with the reference feature digital data CDD¡ stored in the smartphone's memory: in case of match CDD¡c= CDD¡, the digital file A¡ is considered to be genuine (its digital content corresponds to that of an original digital file marked genuine).If the reference feature digital data CDD is not stored in the smartphone's memory, but instead the physical unique signature data UPS is stored in the smartphone's memory ((with the advantage of occupying much less memory compared to CDD), then the authenticity of A can still be checked by verifying that the candidate physical unique signature data UPS obtained by calculating the chunking value of the candidate feature digital data CDD extracted from the digital data D, i.e. UPSC = H(CDDC), matches the reference physical unique signature data UPS stored in memory. A user can further verify the authenticity of a received digital file A¡, even through an offline process (self-verification), by detecting this unique physical characteristic in the object or individual associated with the digital file A¡, using a sensor capable of making such a measurement (at this point, the smartphone camera), and obtaining candidate digital characteristic data CDD¡Ca from the detected characteristic (at this point, a digital image). IVIA / a / ZUZ l / UU I4OJ taken by the smartphone). Next, the user can compare (via the image processing unit of their smartphone, or visually on a smartphone display) the obtained CDD¡Contents with a copy of the reference CDD¡ (stored in the smartphone's memory): in case of a reasonable match CDD¡C= CDD¡ (i.e., the two digital data agree within some given tolerance or similarity criterion), the digital file A¡ is considered genuine (i.e., its digital content corresponds to that of an original digital file marked genuine). Furthermore, the user can also calculate the corresponding candidate physical signature data from the copy of the reference CDDs stored in the smartphone's memory as UPSC=H(CDDs), and compare it with the reference physical signature data UPSC stored in the smartphone's memory. If UPSC=UPSC, the digital file A is confirmed as genuine with an even higher degree of confidence (since a single bit difference is enough to cause a mismatch). In addition, if a match is found, the digital data D associated with A is also authenticated, having been verified as corresponding to that of a genuine digital file, as explained above by retrieving the corresponding batch value R from the verification information (D,k) read from the digital security mark in A. In one variant of this method, the verification of the authenticity of a digital file A¡ by a user can be carried out through an online process. In this case, the reference data, i.e., the digital characteristic data CDDi and / or the unique physical signature reference data UPSi, are stored in a user-accessible, searchable database in which the reference data related to a digital file A¡ are stored in association with, respectively, the corresponding digital data D¡ (included in the digital security mark in A¡) or with the corresponding digital file signature x¡ (which can be calculated by the user once the data D¡ is extracted from the digital security mark through the operation x¡=H(D¡)): the reference data can be requested by sending a query to the database containing, respectively, D¡ or x¡. A conventional method of securing an object is to apply a material-based security mark (possibly tamper-proof) to it—that is, a mark with a detectable intrinsic physical or chemical property that is very difficult (if not impossible) to reproduce. If an appropriate sensor detects this intrinsic property in a mark, the mark is then considered genuine with a high degree of confidence, and therefore so is the corresponding marked object. Many examples of such known intrinsic authentication properties exist: the mark may include particles, possibly randomly dispersed, or have a specific layered structure, possessing intrinsic properties of optical reflection, transmission, absorption, or even emission (luminescence, for example), or polarization, diffraction, or interference.This intrinsic property may be detectable under specific lighting conditions (light of a specific spectral content). This intrinsic property may result from the specific chemical composition of the marking material: for example, luminescent pigments (possibly not commercially available) may be dispersed in an ink used to print a pattern on the object and are used to emit a specific light (e.g., in a spectral window within the infrared range) after illumination with a specific light (e.g., light in the UV spectral range). This is used to secure banknotes, for example. Other intrinsic properties may be used: for example, the luminescent particles in the marking may have a specific luminescence emission extinction time after illumination with an appropriate excitation light pulse.Other types of intrinsic properties include the magnetic properties of embedded particles, or even a fingerprint property of the object itself, such as the relative positioning of inherently random fibers dispersed on a document's paper substrate in a given area. When observed at sufficient resolution, this can reveal a unique signature. Similarly, random printing artifacts of data printed on the object, when viewed at sufficient magnification, can also produce a unique signature. The main problem with an object's intrinsic fingerprint property is its robustness to aging or wear.However, a material-based security mark does not always guarantee the security of data associated with the marked object. For example, even if a document is marked with a material-based security mark, such as a logo printed with security ink in one area, the data printed on the remaining portion can still be forged. Furthermore, overly complex authentication signatures require significant storage capacity, involving external databases and communication links to query them, making offline authentication of an object impossible.According to the invention, an object marked by a material-based security mark and associated with a digitally marked file is secured by the fact that the digital characteristic data corresponding to the unique physical characteristic of the marked object, or its corresponding unique physical signature data, are immutably linked (through the publication or storage of the aggregated digital signature on a blockchain) to the digital data in the digital security mark and to part of the associated digital file. The invention can therefore be used to secure both a batch of objects and a corresponding batch of associated digital files. Of course, any other known intrinsic physical / chemical property can be used to obtain the digital characteristic data CDD related to a unique physical characteristic of an object OBJ, associated with a digital file A, and the corresponding unique physical signature data UPS. As another illustrative example, it is possible to print a 2D barcode that forms a material-based security mark on an object with a security ink that includes a luminescent pigment having its characteristic extinction time constant as well as its light excitation wavelength advantage and its luminescence emission wavelength window: the result is an ink that has a specific reference extinction time τ that serves as a material fingerprint of the ink.Simply illuminate the barcode with an excitation light within an illumination wavelength window that covers the pigment excitation wavelength window, and collect the resulting luminescence light from the barcode using a sensor capable of detecting the light intensity within the luminescence emission wavelength window to authenticate the barcode and, therefore, the object. For example, a user's reader might be equipped with a flash capable of emitting excitation light onto the barcode, a photodiode capable of collecting the corresponding luminescence light intensity profile l(t) (during a detection time interval) from the barcode, and the reader's CPU programmed to calculate an extinction time value from the collected intensity profile l(t).For example, the excitation wavelength window may be within the UV (ultraviolet) band and the emission wavelength window within the IR (infrared) band. If, during object verification, the luminescence light intensity collected by the user's imager shows a feature extinction over time corresponding to a candidate extinction time tc, then the ink, and consequently the object, is considered genuine if tc = τ (within a given tolerance range). In this case, the digital feature data CDD of an object marked OBJ includes at least the reference extinction time value τ (and possibly data related to the excitation wavelength window and the emission wavelength window).As is obvious from the examples above, including (unique) feature reference digital data in the verification information of a digital security mark of an associated digital file A¡ has the technical effect of providing a tamper-proof link between the digital data of the digital file and the authentication data of its associated object. Another illustrative embodiment of the invention relates to a batch of biometric identification documents, for example, biometric digital passports, as shown in Figure 2A. Each digital passport, as a digital file, is associated with a corresponding individual, i.e., the passport holder. For clarity, the digital data of Ai is represented in Figure 2A as equivalent textual and alphanumeric information (i.e., human-readable), for example, as it might be displayed from a digital PDF (Portable Document Format) file, and the digital security mark is shown as an equivalent two-dimensional pattern of a conventional QR code.This embodiment of the invention is particularly useful for creating printable digital files, such as print-ready digital files, to enable a printer to issue a printed secured document directly from a corresponding printable secured digital file (e.g., a digital file relating to an identity document, diploma, contract, etc.). In this example, we will use a hash function as a one-way function to sign digital passport data, preferably an SHA-256 hash function due to its well-known robustness. In fact, given a certain batch size, the hash function selected (which has its own list of slots) for the purpose of signing the digital passport data is, therefore, an example of a one-way encryption function, such that each distinct digital passport has its own distinct digital passport signature, thus making the signature unique.The domain of a slicing function (i.e., the set of possible keys) that is larger than its range (i.e., the number of different table indices) will correlate different keys with the same index, which could result in collisions. Such collisions can be avoided, when the batch size is known, by considering the slot listing associated with the slicing function table of a slicing function and retaining only a function that provides zero collisions, or by independently choosing a slicing table collision resolution scheme (e.g., such as combined slicing function, cuckoo slicing function, or hopscotch slicing function). Figure 2A shows an example of a digital biometric passport A1 secured with a machine-readable digital security mark 210 (hereinafter a QR code) encoded in Ai, and comprising digital passport data 230 containing conventional passport data, e.g., digital data representing a document title 230a (Passport), a set of biographical data of the passport holder 230b: surname (Doe), given name (John), sex (M), date of birth (March 20, 1975), nationality (United States), origin (Des Moines), place of birth (Oakland), a passport issue date 230c (February 24, 2018), and a validity period 230d (February 23, 2020). This digital passport data may further comprise some (unique) serial number(s) 235 assigned by the passport issuing authority (hereinafter 12345).The digital passport data also includes biometric data of the passport holder as characteristic digital data (CDD) corresponding to a unique physical characteristic of an individual associated with the digital passport. A machine-readable representation (e.g., alphanumeric) of data characterizing this unique physical characteristic (not shown), which corresponds to the biometric data, is associated with the digital passport data. A digital data representation is understood in a broad sense of the term: this data representation need only enable the retrieval of the original digital data. The machine-readable data representation (e.g., the biometric data) of the unique physical characteristic may correspond, for example, to fingerprint identification data or iris identification data of the digital passport holder.For example, biometric data 230e corresponding to a person's fingerprint can result from an analysis of a set of specific features of fingerprint characteristic points such as ridge completion, short ridges and bifurcation (according to the conventional Henry Classification System). Therefore, for a given digital passport A¡ from the batch of μ biometric digital passports issued, at this point with μ = 1024, the associated digital passport data D¡ includes the digital data 230a-230e mentioned above. In one variant of this modality, the associated digital passport data D¡ may include only the values ​​of the fields that are common to all issued passports, while the common fields, i.e., Passport, Surname, Sex, Date of Birth, Nationality, Origin, Place of Birth, Date of Issue, and Validity, are included in a separate field data block (FDB) as explained above: for example, D¡ contains only a representation of the field values ​​Doe, John, M, March 20, 1975, United States, Des Moines, Oakland, February 24, 2018, and February 23, 2020. Preferably, the additional digital passport data is associated with the aforementioned digital passport data 230. For example, a digital image of the passport holder's fingerprint pattern, or a digital identity photograph, etc. In one variant of this modality, this additional digital passport data is stored in a searchable information database 250, which can be accessed by submitting an information request containing certain passport data (for example, the holder's name, biometric data, security mark data, or unique serial number 235) to retrieve the corresponding fingerprint pattern data and receive it back.Preferably, a link to information database 250 is included, as information access data 240, in the digital passport: at this point this information access data is encoded in a digital representation of a QR code that contains a reference index to retrieve corresponding additional data in information database 250. However, in a variant of passport control operation that involves access to a remote information database (online operation), the QR code could contain, for example, the URL of the information database that is accessible via the web. Next, a digital passport signature is calculated using a one-way slicing function on the digital passport data D1 corresponding to the digital passport data 230a-230e of digital passport A, for example, by means of the robust SHA-256 slicing function mentioned above, to obtain the corresponding (unique) digital passport signature x1=H(D1). Similarly, digital passport signatures are calculated for all digital passports in the batch, for all different holders. From all the passport signatures in the batch, a reference root digital signature R is calculated according to a tree order and tree concatenation order of an associated (binary) tree, as explained above. Since there are μ = 1024 passports in the batch, the corresponding binary tree has 1024 leaf nodes a(1,1),...,a(1024) for the first level, 512 non-leaf nodes a(2,1).....a(2,512) for the second level, 256 non-leaf nodes a(3,1).....a(3,256) for the third level, etc., up to the penultimate level of nodes (at this point, level 10) with non-leaf nodes a(10,1) ya(10,2), and the top node corresponding to the root node R (level 11 of the tree). The leaf node values ​​are a(1 ,j) = x¡ = H(Dj), j=1,...,1024, the second level node values ​​are a(2,1) = H(a(1,1)+a(1,2)),...., a(2,512) = H(a(1,1023)+a(1,1024)), etc., and the reference root digital signature R is R = H(a(10,1)+a(10,2)).Each digital verification key k¡ is therefore a sequence of 10 node values. The digital security mark 210 of the digital passport A¡ includes the digital passport data D¡ and the corresponding digital verification key k¡ (i.e., the verification information V¡ = (D¡,k¡)). The operation of verifying that the digital passport data D¡ and the digital verification key kj in the digital security mark 210 of a biometric digital passport A¡ correspond, in fact, to passport data of a genuine biometric digital passport belonging to the batch of μ biometric digital passports having the batch value R only needs to calculate the passport digital signature x¡ = H(Dj) and verify that x¡ and the digital verification key k¡ allow the recovery of the corresponding reference root digital signature R available through the composition ten times (since at this point, the tree has ten levels below the root level) of a slice function of a concatenation of the node value a(1 ,j) and the node values ​​in k¡ (according to the node order in the binary tree and the tree concatenation order with the conventional concatenation scheme).Consequently, a biometric digital passport secured according to the invention provides both a tamper-proof link between the personal data and the biometric data of its holder, and a unique, tamper-proof link between the holder's physical person and the holder's identity. Figure 2B illustrates a control process for the secured biometric digital passport Ai of Figure 2A, with its passport data mark 230 corresponding to a certain John Doe, with its biometric data 230e corresponding to John Doe's fingerprint, and with additional digital passport data corresponding to a digital identity photograph 255 of John Doe that is accessible via the information database link 250 included in the information access mark 240. The passport data further comprises the unique serial number 235 assigned by the passport-issuing authority. The passport digital security mark 210 Ai contains the verification information (Di,ki), with passport digital data Di corresponding to the printed passport data 230a-230d, the biometric data 230e and the unique serial number 235, and the digital verification key ki corresponding to the sequence of 10 node values ​​{a(1,2),a(2,2),...,a(10,2)} which are necessary to retrieve the root value R of the node value a(1,1) of the digital passport Ai (with a(1,1) = xi = H(Di)). The reference root digital signature R can have a timestamp and be stored on a 260-block chain. In this example, the biometric data 230e of the respective holders of the biometric passports in the batch are also stored on the 260-block chain in association with their respective unique serial numbers (to make this data immutable). The stored biometric data of John Doe can be retrieved by sending a request to the 260-block chain indicating the unique serial number 235 mentioned in his passport. The authorities in charge of checking the identity of people (e.g., the police, customs, etc.)They can access the blockchain 260 through a communication link and, in this illustrative modality, they also have local storage capabilities to store the (published) root digital signatures of all delivered batches of biometric digital passports. In the example shown in Figure 2B, the information database 250 is local (i.e., directly accessible by the authorities, without having to use a public communication network). Furthermore, these authorities are equipped with fingerprint scanners 270 to capture individuals' fingerprints and calculate corresponding machine-readable data representations that characterize the captured fingerprints, i.e., the biometric data 230e. During an identity check of John Doe, say by a police officer or customs agent, the agent receives John Doe's secured biometric digital passport Ai, reads and decodes the verification information Di,ki) stored in the digital security mark 210 in the digital passport by means of an appropriate reader, which may be, for example, a suitable programmed computer 290, the computer being connected to the local storage capabilities 250. Having read the digital passport data Di and the digital verification key ki and sent them to the computer 290, a specialized application (with programmed slicing function H and node value concatenation) running on the computer 290 calculates the digital passport signature xi (as xi=H(Di)) and a candidate batch value Rc as: H(H(H(H(H(H(H(H(H(H(H(a(1,1 )+a(1,2))+a(2,2))+..)+..)+..)+..)+..)+..)+.,)+a(9,2))+a(10,2)), that is, the composition of ten times a slice function of a concatenation of the node value a(1,1) and the node values ​​in ki= {a(1,2),a(2,2),...,a(10,2)}. The computer can then search, for example, in the local information database 250 for a reference root digital signature R ινΐΛ / a / zuz ι / uu hoj that matches the candidate value Rc. If there is no match, the passport is a forgery, and John Doe (i.e., the individual being examined who claims his name is John Doe) can be arrested. If Rc matches a stored reference root digital signature, the passport is considered genuine, and the agent can perform further security checks. - the agent retrieves the digital identity photograph 255 stored in the information database 250, by sending a request through computer 290 containing serial number 235 printed on Ai, receives the same back and displays the received identity photograph 255 on a screen of computer 290: the agent can then visually compare the displayed face (i.e., that of a certain John Doe) with that of the individual being checked and estimate whether the two faces are similar or not; and - the agent retrieves the biometric data 230e in the A1 passport by reading this data on the digital security mark 210 with the computer 290, and scans the individual's fingerprint using a fingerprint scanner 270 connected to the computer 290 and obtains the corresponding individual's biometric data: the agent then checks using a program running on the computer 290 whether the retrieved biometric data 230e is similar (within a given margin of error) to the obtained biometric data of the individual. If the two faces and biometric data are evaluated as similar, everything is correct and the verified individual is, in fact, the real John Doe, the holder of the genuine Ai biometric passport. If any of the aforementioned additional security checks fail, it is clear that the individual in front of the agent is not the true holder of the genuine Ai biometric passport. Therefore, with a biometric digital passport secured according to the invention, a simple offline check can quickly detect any fraud. In fact, it is even possible to reduce a digital biometric passport document to a simple digital file with only a 2D barcode digital representation (like the QR code example above) that includes the verification information V = (D,k): where V comprises the holder's biographical data and (unique) biometric data, such as the holder's fingerprint (within the digital passport data D) and the verification key. Indeed, according to the invention, even this reduced, secure digital passport fully leverages the aforementioned tamper-proof link created between the personal biographical data and the passport holder's biometric data, and the already unique, tamper-proof link between the holder's physical person and identity. Another illustrative embodiment of the invention relates to aircraft components, as shown in Figure 3. Due to the very high price of certain critical components whose failure could affect aircraft safety, such as some parts of the engines (e.g., turbine blades, pumps, etc.) or the landing gear, or batteries, counterfeiters are interested in producing copies of these components, but of course without meeting the required technical safety standards due to their generally lower quality. Even if an aircraft component is usually marked with a corresponding unique serial number for identification, that type of marking can be easily counterfeited. These counterfeit aircraft parts are generally defective and can cause serious damage or even aircraft accidents. This is a growing safety problem today.Furthermore, even if the components are genuine, they may not be suitable for certain versions of the same aircraft type, and there is a serious risk that an inappropriate component could be inadvertently used to repair a given aircraft, for example. Therefore, it is important to secure at least the critical genuine components permitted for a given aircraft. In general, each component has a corresponding technical data sheet (possibly digital) that indicates, for example, the component's technical name, unique serial number, manufacturer, date of manufacture, and certification information. Furthermore, for a given aircraft, a corresponding record contains all the (digital) technical data sheets for its respective components. However, counterfeit components may have a corresponding counterfeit digital technical data sheet, making it difficult to detect fraud (except through technical testing, for example). For instance, how can one be certain that a digital technical data sheet correctly corresponds to a component installed in a specific aircraft (and vice versa)? According to an illustrative embodiment of the invention, parts permitted for use in the manufacture or repair of a given aircraft, or mounted on the aircraft, are deemed to belong to a batch of components (or objects) for that same aircraft. In the specific illustrative modality shown in Figure 3, each component in an aircraft batch, i.e., each aircraft component permitted to be mounted or repaired on a given aircraft, has a corresponding digital aircraft component identification document (AC-ID) that contains the same digital component data as in a conventional technical data sheet (e.g., the aircraft ID code, the aircraft manufacturer's name, the component's technical name, the component's unique serial number, the component manufacturer's name, and the component's manufacturing date) along with additional corresponding digital data, including the aircraft ID code, the aircraft manufacturer's name, the date the component was assembled on the aircraft, the name of the technician in charge of performing the conformity check along with the date of the conformity check, and the corresponding (unique) digital signature of the verifier.Furthermore, each AC-ID aircraft component digital identification document is secured by an embedded machine-readable digital security mark. For clarity, the AC-ID:Ai25 digital data is represented in Figure 3 as equivalent textual and alphanumeric information (i.e., human-readable), and the 310 digital security mark is shown as an equivalent conventional QR code two-dimensional pattern. Preferably, whenever a component or set of components is replaced on the aircraft, corresponding secure digital AC-ID documents are created and a corresponding updated version of the aircraft batch is also created, with the corresponding additional digital data mentioned above (related to the new assembly operations). Therefore, all (critical) components mounted on a specific aircraft (at this point, having the aircraft ID reference HB-SNO) belong to a corresponding batch of assembled components (at this point, which has a total of μ components) and are documented in a corresponding batch of associated μ digital files, i.e., the ACID digital identification document. A 310 digital security mark (at this point in the form of a QR code) is included in each aircraft component digital identification document, for example AC-IDA125, which is associated with the corresponding aircraft component, at this point A125, mounted on aircraft HB-SNO. Figure 3 specifically shows component A125 of the aircraft batch, which is a turbine blade adapted for the type of reactor mounted on aircraft HB-SNO and marked with a unique manufacturing serial number (at this point, 12781).(usually engraved by the manufacturer). The digital data for component D125 in the digital security mark 310 of the aircraft component digital identification document AC-ID:Ai25 associated with component A125 comprises the digital data corresponding to that in the technical data sheet: the aircraft ID code 330a (hereinafter, HB-SNO), the aircraft manufacturer's name 330b (hereinafter, AeroABC), the component's technical name 330c (hereinafter, turbine blade - 1st ring), the component serial number 330d (hereinafter, 12781), the component manufacturer's name 330e (hereinafter, PCX), the component's manufacturing date 330f (hereinafter, November 13, 2017), the component's assembly date on the reactor 330g (hereinafter, February 24, 2018), and the name of the technician responsible for performing the conformity check 330h. (at this point,the verifier is Martin White) along with the date of the conformity check 330i (at this point, March 20, 2018), and the (unique) digital signature of the verifier 330j (at this point, 2w9s02u). A component digital signature X125 of the component digital data D125 of the digital file AC-ID:Ai25 of component A125 is calculated by means of a one-way slicing function H as X125 = H(Di2s). Similarly, all component digital signatures x¡ of the component digital data D¡ of component A¡ are calculated by means of the one-way slicing function H as x¡ = H(D¡) (at this point, i = 1,...,μ). According to the invention, a tree associated with the batch of components Ai Αμ (at this point, a binary tree) and, therefore, with the corresponding batch of digital files AΟ-ΙΟ:Αι,...,ΑΟΙΟ:Αμ, is constructed having μ leaf nodes a(1,1),...,a(1 ,μ) which correspond respectively to the μ component digital signatures χι,...,χμ of respective component digital data Οι,...,Ομ of the component digital identification documents ACID:Ai.....AC-ID:AM of components Ai.....Αμ.At this point, the order of nodes in the binary tree is the conventional order; that is, the nodes a(ij) are arranged according to the values ​​of the indices (i,j): index i indicates the level in the tree, starting from the leaf node level (1=1) to the penultimate level of nodes below the root node, and index j ranges from 1 to μ for the leaf node level (level 1), from 1 to μ / 2 for the next level of nodes (non-leaf) (level 2), and so on, and from 1 to 2 for the penultimate level of nodes. The tree comprises node levels from the leaf nodes to the root node, with each non-leaf node in the tree corresponding to a digital signature by means of the unidirectional function H, which is a concatenation of the respective digital signatures of its child nodes according to the tree's concatenation order. A reference root digital signature R for the batch of μ aircraft components Aι,...Aμ is calculated by means of a one-way function of a (conventional) concatenation of tree node values ​​(as explained below). The reference root digital signature R is then stored in a queryable database (preferably a blockchain) accessible by technicians responsible for monitoring or changing the assembled components.The tree, therefore, comprises node levels from the leaf nodes to the root node of the tree, each non-leaf node of the tree corresponding to a digital signature by means of the one-way function H of a concatenation of the respective digital signatures of its (two) child nodes according to the tree concatenation order (at this point conventional), the root node corresponding to the reference root digital signature R, that is, the digital signature by means of the one-way function H of a concatenation of the digital signatures of the nodes of the penultimate level of nodes in the tree (according to the order of nodes in the tree and the tree concatenation order). For a given component A¡ in the batch, a digital verification key k¡, corresponding to the digital signature of component x¡ (i.e., leaf node a(1,i)) of the digital data of component D¡, is calculated as the sequence of the respective digital signatures, from the leaf node level to the penultimate node level of the tree, of each other leaf node that has the same parent node in the tree as the leaf node a(1,i) corresponding to the digital signature x¡, and successively at each subsequent level in the tree, of each non-leaf node that has the same parent node in the tree as the same parent node considered at the previous level. For each component A¡ mounted on the aircraft HB-SNO, the associated digital data of component D¡ and the corresponding digital verification key k¡ are embedded in the digital security mark 310 included in the corresponding aircraft component digital identification document AC-ID:A¡. For example, in the case of a component check operation on aircraft HB-SNO, a technician can send a request to the searchable database containing component serial number 12781, read from the digital file AC-ID:Ai25 of the component A125 to be checked, or its digital verification key ki25 as read from digital security mark 310 in the corresponding document AC-ID:Ai25, using an appropriate reader, such as a computer programmed to decode the contents of the digital security mark, and will receive back the corresponding batch value R. In a preferred variant that allows for complete offline checking, the technician's computer has a memory that stores all the root digital signatures related to the aircraft to be checked. In this latter version, the technician can then check,If the component is genuine, this is done by reading the digital component data D125 in the digital security mark 310 of AC-ID:Ai25, verifying that the unique serial number 330d (at this point, 12781) extracted from D125 matches the serial number physically marked on the mounted aircraft component A125, calculating the corresponding digital component signature X125 (e.g., by running an application programmed on a computer's CPU that calculates the signature X125 = H(Di2s), from the digital data D125 read), calculating a candidate batch value Rca through the one-way function H programmed on the computer's CPU as the slicing of a concatenation of the leaf node value a(1,125)=xi25 and the node values ​​given in the corresponding digital verification key ki25, and verifying that the candidate batch value Rccocoincides with one of the reference root digital signatures stored in the computer memory (i.e.,the reference value R, which corresponds to aircraft HB-SNO). In case of a complete match (i.e., the serial numbers match and Rc=R), component A125 is considered genuine and belongs to the (updated) aircraft batch of permitted components for aircraft HB-SNO. If Rcno matches a stored reference R root digital signature, or if the serial numbers do not match, component A125 is possibly counterfeit, or is a genuine component not permitted for aircraft HB-SNO (e.g., A125 does not belong to the correct batch for this aircraft), and must be replaced. Similarly, the invention would allow the detection of fraud (or errors) in secured AC-ID batches of replacement parts stored in a warehouse by verifying the authenticity of the markings on the stored parts and checking that the component serial number of the digital security mark matches the marking on the corresponding component.In the case of a highly critical component, a tamper-evident material-based security mark may be additionally applied to the component, while the digital data relating to the corresponding reference unique physical characteristic, i.e., the CDD characteristic digital data (e.g., as captured by a suitable sensor when the material-based security mark is applied) of this mark is preferably made part of the component D digital data in the digital security mark of the aircraft component digital identification document for this component, and corresponding reference UPS unique digital signature data (e.g., by taking a snippet of the CDD characteristic digital data, i.e., UPS = H(CDD)) is calculated and may also be part of the component D digital data.This additional layer of security enhances the security provided by the unique serial number marked on the component by its manufacturer. Preferably, the reference UPC and UPS values ​​are stored on the blockchain (to make them immutable) and are accessible to the technician. Furthermore, these reference values ​​can also be stored in the technician's computer memory to allow offline authentication of the material-based security mark on the highly critical component. The additional offline authentication process for this material-based security mark may involve measuring the unique physical characteristic of the component using a suitable sensor connected to a computer, and obtaining candidate digital characteristic data (CDDca) from the measured characteristic (e.g., via a specific application programmed into the computer's CPU). The technician (or the computer's CPU, if properly programmed) then compares the obtained CDDCca with a copy of the reference CDDca stored in the computer's memory. In the case of a reasonable match (CDDC = CDDca), i.e., within some predefined error tolerance criterion, the material-based security mark, and therefore the component, is considered genuine. As mentioned previously, a copy of the CDD reference feature digital data, instead of being stored in the technician's computer memory, is part of the D digital data included in the digital security mark on the AC-ID:A aircraft component digital identification document for component A and can be obtained by reading directly from the digital security mark. The technician can then read the candidate CDDCs in the digital security mark and verify that the UPS signature stored in the computer memory matches the candidate signature UPSc calculated from the read candidate CDDCs by calculating UPSC = H(CDDC). If UPSC = UPS, the material-based security mark, and therefore the component, is confirmed as genuine. In one variant of the modality, the verification of the authenticity of a component by a technician can be carried out as an alternative through an online process in a similar manner as already explained with the first detailed modality of the invention, and will not be repeated at this point. According to the invention, it is also possible to verify the authenticity of a copy of an aircraft component digital identification document, AC-ID:Ai25 for example, against the original secured digital file. In fact, if a technician in charge of inspection (or repair) operations has access to the AC-ID:Ai25 digital file on their computer (which could be, for example, a suitably programmed smartphone), they can verify that the component digital data corresponds to that of the original document by performing the following operations: - Read the digital component data D125 and the digital verification key ki2s in the digital security mark 310 of the digital component identification document AC-ID:Ai2s; - acquire a reference batch value R from the batch corresponding to document AC-ID:Ai2s; this reference value may already be in the computer's memory or may be acquired via a communication link from a database that stores the reference batch values ​​of digital component identification documents in the case where the computer is equipped with a communication unit, by sending a request containing, for example, the (unique) component serial number or only the ki2s key read from the digital security mark 310, and receiving back the corresponding reference batch value R; - calculate (with the programmed one-way function H) a digital signature of component X125 from the digital data of component D125 read, with xi2s = H(Di2s); - calculate a candidate batch value Rc (by means of the programmed one-way slicing function H and digital signature of a concatenation of digital signatures) as the digital signature by means of the slicing function H of a concatenation of the leaf node value X125 and the node values ​​indicated in the digital verification key ki2s (according to the node order in the tree and the tree concatenation order); and - verify that the candidate lot value R coincides with the reference lot value R. According to the detailed description above, the invention is clearly compatible with offline and local verification operations to verify the authenticity of an assured digital file, or the data conformity of a copy of an assured digital file, with respect to the data associated with the original assured digital file. However, the invention is also compatible with an online verification process, for example, by receiving (via a communication link) a reference batch value (or root digital signature) from an external source (e.g., a server or blockchain), or by performing some or all of the computational steps involving the one-way function or concatenation of digital signatures via external computing means (e.g., operating on a server), or even by verifying that a candidate root digital signature matches a reference root digital signature (and only receiving the result). The subject matter disclosed above is to be considered illustrative, not restrictive, and serves to provide a better understanding of the invention defined by the dependent claims.

Claims

1. A method of securing an original digital file belonging to a batch of a plurality of original digital files against forgery or manipulation, each original digital file including its own digital data, characterized in that it comprises the steps of: for each original digital file in the batch, calculating by means of a one-way function an associated digital file signature of its digital data;forming a tree based on the plurality of digital file signatures calculated for the original digital files in the batch and comprising nodes arranged according to a given node order in the tree, said tree comprising node levels starting from the leaf nodes, which correspond to the plurality of digital file signatures respectively associated with the plurality of original digital files in the batch, to the root node of the tree, each non-leaf node of the tree corresponding to a digital signature by means of the one-way function of a concatenation of the respective digital signatures of its child nodes according to a tree concatenation order, the root node corresponding to a reference root digital signature, i.e., a digital signature by means of the one-way function of a concatenation of the digital signatures of the nodes of a penultimate level of nodes in the tree according to said tree concatenation order;to associate with the given original digital file a corresponding digital verification key that is a sequence of the respective digital signatures, from the leaf node level to the penultimate node level, of each other leaf node that has the same parent node in the tree as the leaf node that corresponds to the digital file signature of the given original digital file, and successively at each subsequent level in the tree, of each non-leaf node that has the same parent node in the tree as the same previous parent node considered at the previous level; to make available to a user the root digital signature of reference to the tree; and to include in the original digital file a corresponding machine-readable digital security mark comprising a representation of its digital data and its corresponding digital verification key, thereby obtaining a marked original digital file whose digital data is secured against falsification or manipulation.

2. The method according to claim 1, further characterized in that the reference root digital signature of the root node of the tree is either published on a user-accessible medium or stored in a user-accessible, searchable root database, or on a blockchain, or in a database secured by a blockchain, accessible to the user.

3. The method according to claim 2, further characterized in that the marked original digital file further comprises root node access data included therein and sufficient information to allow the user to access the reference root digital signature of the root node of the tree corresponding to the batch of original digital files, said information being a link to an operable access interface for receiving from the user a root request containing digital data, or digital file signature, obtained from a digital security mark of a marked original digital file, and sending back a reference root digital signature of the corresponding tree, the access interface ID: A allowing, respectively, one of the following: - the medium on which the reference root digital signature is published; - the searchable root database in which the reference root digital signature is stored;and - the blockchain, or respectively the database secured by a blockchain, in which the reference root digital signature with timestamps is stored.; 4. The method according to any one of claims 1 to 3, further characterized in that a virtual digital file is counted as belonging to the batch of original digital files, said virtual digital file including its own virtual digital data, and an associated virtual digital file signature obtained by means of the one-way function of the virtual digital data, said virtual digital file not being the actual file but used only to generate the associated virtual digital file signature from its virtual digital data; and the reference root digital signature associated with said batch of original digital files being calculated from a tree having all the digital file signatures of the original digital files in the batch, including the virtual digital file signature, as leaf nodes.

5. The method according to any one of claims 1 to 4, further characterized in that additional digital data corresponding to the digital data associated with the original marked digital file are stored in a searchable information database accessible by the user through an information database interface operable to receive from the user a request for information containing digital data, or a digital file signature, obtained from a digital security mark of an original marked digital file, and send back corresponding additional digital data.

6. The method in accordance with any one of claims 1 to 5, further characterized in that the digital data of the original marked digital file includes reference feature digital data of a corresponding unique physical feature of an associated object or individual.

7. The method according to claim 6, further characterized in that the unique physical characteristic of the associated object or individual is, respectively, that of a material-based security mark applied to the associated object or identification of a biometric characteristic of the associated individual.

8. The method according to any one of claims 1 to 7, further characterized in that the digital data of the respective original digital files of the batch are distributed among given fields common to all digital files of the batch, and specific digital data related to these fields are not included in the digital data, but are grouped into a separate field block associated with the batch, and wherein: i) the digital file signature of an original digital file is calculated using the one-way function of a concatenation of the corresponding digital data and the field data block; and ii) the reference root digital signature is made available to the user together with the associated field data block.

9. A method for verifying the authenticity of a secured digital file in accordance with the method of any one of claims 1 to 7, or the conformity of a copy of such secured digital file with respect to the original, characterized in that it comprises the steps of, after processing a test file which is said digital file or said copy of the digital file by means of a processing unit connected to a memory: storing the test file in the memory; reading a representation of digital data and a digital verification key in a digital security mark of the stored test file, and extracting corresponding digital test data and digital verification key from said read representation;having stored in memory a reference root digital signature of a root node of a tree of the batch of original digital files, and having programmed in the processing unit the one-way function to calculate a digital signature of digital data and of a concatenation of digital signatures according to the order of nodes in the tree and the order of tree concatenation; verifying whether the extracted test digital data and the associated test digital verification key correspond, in fact, to the stored reference root digital signature by performing the steps of: calculating with the one-way function a test digital signature of the extracted test digital data, said test digital signature corresponding to a test leaf node in a test tree that corresponds to the digital security mark of the test file;extract from the sequence of digital signatures in the test digital verification key, a digital signature from each other leaf node of the test tree that has the same parent node as the test leaf node and calculate a digital signature from a concatenation of the test digital signature and the digital signature extracted from each other leaf node, thus obtaining a digital signature from the same parent node of the test leaf node;successively at each subsequent level in the test tree and up to the penultimate level of nodes, extract from the sequence of digital signatures in the test digital verification key, a digital signature from each other non-leaf node of the test tree that has the same parent node as the same previous parent node considered in the previous stage and calculate a digital signature from a concatenation of the digital signature of said other non-leaf node and the digital signature obtained from said same previous parent node, thus obtaining a digital signature from said same parent node of said same previous parent node; calculate a digital signature from a concatenation of the digital signatures obtained from the non-leaf nodes that correspond to the penultimate level of nodes of the test tree, thus obtaining a candidate root digital signature of the root node of the test tree;and check if the candidate root digital signature obtained matches the stored reference root digital signature, so that, in the case where said root digital signatures match, the digital data of the test file is that of a genuine digital file.; 10. The method according to claim 9, further characterized in that the original marked digital file is secured according to the method of claim 8, additionally storing the associated field data block in the memory of the processing unit, and wherein: the step of calculating a digital test signature corresponding to a test leaf node in a test tree corresponding to the digital security mark in the test file comprises calculating with the one-way function a digital signature of a concatenation of the extracted digital test data and the stored field data block.

11. The method according to any one of claims 9 and 10, further characterized in that the digital file is secured by storing the reference root digital signature in a user-accessible, searchable root database in accordance with the method of claim 2, and the processing unit is further connected to a communication unit operable for sending and receiving data via a communication link, comprising the preliminary steps of: sending a request to said root database via the communication link using the communication unit, and receiving the reference root digital signature back; and storing the received root digital signature in memory.

12. The method according to any one of claims 9 and 10, further characterized in that the digital file is secured according to the method of claim 3, the processing unit is further connected to a communication unit operable for sending and receiving data via a communication link, comprising the preliminary steps of: reading the root node access data included in the test file; sending with the communication unit via the communication link a root request to said access interface containing digital data, or a digital signature of said digital data, obtained from the digital security mark in the test file, and receiving back a corresponding reference root digital signature of the associated batch; and storing the received reference root digital signature in memory.

13. The method according to any one of claims 9 to 12, further characterized in that the digital file is secured according to the method of claim 5 and the image former is additionally equipped with operable communication means for sending to the information database interface an information request containing digital data, or a digital file signature, obtained from the digital security mark of the proof file, and receiving corresponding additional digital data back.

14. The method according to any one of claims 9 to 13, further characterized in that the digital file is secured according to the method of any one of claims 6 and 7, and the image former is additionally equipped with a sensor operable for detecting a unique physical feature of an associated object or individual, respectively, and the processing unit is programmed to extract corresponding digital feature data from a detection signal received from the sensor, the image former having stored in memory reference digital feature data CDD corresponding to said unique physical feature of the associated object or individual, respectively, comprising the additional steps of,After visualizing a subject that is said object or associated individual: detect with the sensor a unique physical characteristic of the subject and extract corresponding candidate digital feature data (CDDc); compare the obtained candidate digital feature data (CDDc) with the stored reference digital feature data (CDD); and in the case where the candidate digital feature data (CDDc) are similar to the stored reference digital feature data (CDD), within a given tolerance criterion, the subject is considered to correspond respectively to a genuine object or individual validly associated with a genuine digital file.

15. A digital file belonging to a batch of a plurality of original digital files and secured in accordance with the method of any one of claims 1 to 8, characterized in that each original digital file in the batch has its own digital data and corresponding digital verification key, said batch having a corresponding root digital signature of reference, comprising: a machine-readable security mark including a representation of its digital data and its verification key.

16. The digital file according to claim 15, further characterized in that the digital data includes reference feature digital data (CDD) of a corresponding unique physical feature of an associated object or individual.

17. The digital file according to claim 16, further characterized in that the unique physical feature of the associated object is that of a material-based security mark applied to the associated object.

18. A system for verifying the authenticity of a digital file, or the conformity of a copy of such a digital file, with respect to a marked original digital file belonging to a batch of original digital files secured in accordance with the method of any one of claims 1 to 7, characterized in that it comprises an image former having an image former unit, a processing unit with a memory and an image processing unit, the memory storing a root digital signature referencing a tree corresponding to the batch of original digital files, and the processing unit being programmed with the unidirectional function to calculate a digital signature of digital data and a concatenation of digital signatures according to the node order of the tree and the tree concatenation order,said system being operable for: having stored in memory a test file which is said digital file or said copy of the digital file; reading a representation of digital data and of a digital verification key in a digital security mark of the stored test file, and extracting respectively corresponding digital test data and digital verification key from said representation read; verifying whether the extracted digital test data and the digital verification key correspond, in fact, to the stored reference root digital signature by performing in the processing unit the programmed operations of: calculating with the unidirectional function a digital test signature of the extracted digital test data,corresponding to a test leaf node in a test tree that corresponds to the digital security mark of the test file; extract from the sequence of digital signatures in the test digital verification key, a digital signature from each other leaf node of the test tree that has the same parent node as the test leaf node and calculate a digital signature of a concatenation of the test digital signature and the digital signature extracted from each other leaf node, thus obtaining a digital signature from the same parent node of the test leaf node; successively at each subsequent level in the test tree and up to the penultimate level of nodes, extract from the sequence of digital signatures in the test digital verification key,a digital signature of each other non-leaf node of the test tree that has the same parent node as the same parent node considered in the previous stage and calculate a digital signature of a concatenation of the digital signature of each other respective non-leaf node and the digital signature obtained from that same parent node, thus obtaining a digital signature of that same parent node; calculate a digital signature of a concatenation of the digital signatures obtained from the non-leaf nodes that correspond to the penultimate level of nodes in the test tree, thus obtaining a candidate root digital signature of the root node of the test tree; and check if the candidate root digital signature obtained matches the stored reference root digital signature, whereby, in the case where these root digital signatures match,The system is configured to provide an indication that the digital data in the test file is from a genuine digital file.

19. The system according to claim 18, further characterized in that the original marked digital file is secured in accordance with the method of claim 8, additionally storing the associated field data block in the memory of the processing unit, and wherein: the programmed operations of calculating a digital test signature corresponding to a test leaf node in a test tree corresponding to the digital security mark of the test file comprise calculating with the unidirectional function a digital signature of a concatenation of the extracted digital test data and the stored field data block.

20. The system according to any one of claims 18 and 19, further characterized in that the marked original digital file belongs to a batch of original digital files secured according to the method of any one of claims 6 and 7, the system being further equipped with a sensor connected to the processing unit and operable to detect a unique physical characteristic of an associated object or individual, the processing unit being programmed to extract corresponding digital characteristic data from a detection signal received from the sensor, the system having stored in memory reference digital characteristic data CDD corresponding to said unique physical characteristic of the associated object or individual, the system being further operable to: detect with the sensor a unique physical characteristic of a subject that is said associated object or individual,and extract corresponding candidate CDDc digital characteristic data; compare the obtained candidate CDDc digital characteristic data with the stored reference CDD digital characteristic data; and 10 in the case where the candidate CDDc digital characteristic data are similar to the stored reference CDD digital characteristic data, within a given tolerance criterion, provide an indication that the subject is considered to be genuine.

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