Security element, corresponding engraved intaglio printing plate, and methods and devices for producing, decoding and authenticating security marking of said security element

1 . A security element comprising a substrate and a security marking, the security marking comprising a two-dimensional intaglio pattern printed on a surface of the substrate via intaglio printing with intaglio ink, the two-dimensional intaglio pattern having an associated color and comprising at least one base intaglio pattern comprising a plurality of intaglio modules, each base intaglio pattern only comprises intaglio modules of which tones belong to a corresponding set of two distinct allowed tones forming a pair of allowed tones, each intaglio module having one tone of said associated color, a tone of an intaglio module being selected from a tone palette of a plurality of distinct reference tones of the associated color, a CIE color difference ΔE* between the tones being greater than or equal to 2.0, a width & of an intaglio module being greater than 20 μm and less than or equal to 50 μm,

wherein the two-dimensional intaglio pattern is produced based on an associated two-dimensional encoded pattern comprising at least one reference pattern, wherein each of the at least one base intaglio pattern is based on an associated reference pattern of the at least one reference pattern, each base intaglio pattern and an associated reference pattern encoding a same portion of information, which specifies a unique identification number of the associated reference pattern,

wherein each reference pattern includes a specific arrangement of a plurality of symbols, each symbol in the arrangement allowing to encode data according to a state of said symbol, the state of each symbol consisting in a specific arrangement of a first two-dimensional element E1 and a second distinct two-dimensional element E2 within the symbol, each element E having a corresponding value of a parameter p(E), the parameter of the first element E1 having a positive value different from a zero value of the parameter of the second element E2,

wherein each reference pattern only comprises dark symbols and light symbols, the parameter value of the first element of a dark symbol corresponding to the darker tone of the pair of allowed tones of an associated base intaglio pattern, and the parameter value of the first element of a light symbol corresponding to the lighter tone of said associated pair of allowed tones,

wherein each intaglio module of each base intaglio pattern of the two-dimensional intaglio pattern is printed

either as a separate intaglio module corresponding to a first element of a symbol of an associated reference pattern of the two-dimensional encoded pattern and its location in the reference pattern; or

as a portion of an intaglio line corresponding to a line connecting two first elements having a same parameter value p, respectively belonging to two distinct adjacent symbols of an associated reference pattern of the two-dimensional encoded pattern, and a location of the line in the reference pattern.

2 . The security element of claim 1 , comprising a plurality of two-dimensional intaglio patterns, each two-dimensional intaglio pattern having a corresponding associated color being distinct from a background color of the substrate.

3 . The security element according to claim 1 , wherein at least one two-dimensional intaglio pattern is comprised in a cell having a boundary formed by intaglio lines of which height is greater than any height of the intaglio modules of said two-dimensional intaglio pattern, a distance between an edge of the two-dimensional intaglio pattern and the boundary of the cell being greater than or equal to 40 μm.

4 . The security element according to claim 1 , wherein a CIE color difference ΔE* between the tones is greater than or equal to 2.5.

5 . An engraved plate for an intaglio printing machine, comprising grooves of variable engraving depth adapted to receive intaglio ink to print on a surface of a substrate a two-dimensional intaglio pattern of a security marking of a security element according to claim 1 .

6 . A method of producing a security element comprising a security marking comprising a two-dimensional intaglio pattern according to claim 1 , the method comprising the steps of:

(i) generating a two-dimensional encoded pattern comprising at least one reference pattern by encoding a portion of information into each of said at least one reference pattern forming the two-dimensional encoded pattern, said portion of information specifying for each reference pattern its unique identification number, each reference pattern including a specific arrangement of a plurality of symbols, each symbol in the arrangement allowing to encode data according to a state of said symbol, the state of each symbol consisting in a specific arrangement of a first two-dimensional element E1 and a second distinct two-dimensional element E2 within the symbol, each element E having a corresponding value of a parameter p(E), the parameter of the first element E1 having a positive value different from a zero value of the parameter of the second element E2, wherein each reference pattern only comprises dark symbols and light symbols, the parameter value of the first element of a dark symbol corresponding to a darker tone of an associated pair of allowed tones, selected from a tone palette of a plurality of distinct reference tones of a color associated with the two-dimensional encoded pattern, and the parameter value of the first element of a light symbol corresponding to the lighter tone of said associated pair of allowed tones,

wherein the two-dimensional intaglio pattern to be produced is based on the generated two-dimensional encoded pattern and comprising at least one base intaglio pattern comprising a plurality of intaglio modules, each intaglio module having one tone of said associated color, a tone of an intaglio module being selected from the tone palette; wherein each of the at least one base intaglio pattern is based on an associated reference pattern of the at least one reference pattern, each base intaglio pattern encoding a same portion of information than an associated reference pattern,

wherein each base intaglio pattern only comprising intaglio modules of which tones belong to the pair of allowed tones of the associated reference pattern, and

each intaglio module of each base intaglio pattern of the two-dimensional intaglio pattern is to be printed

either as a separate intaglio module corresponding to

a first element of a symbol of an associated reference pattern of the two-dimensional encoded pattern and its location in the reference pattern; or

as a portion of an intaglio line corresponding to a line connecting two first elements having a same parameter value p, respectively belonging to two distinct adjacent symbols of an associated reference pattern of the two-dimensional encoded pattern, and a location of the line in the reference pattern;

(ii) engraving a plate of an intaglio printing machine with variable engraving depth adapted to receive an intaglio ink of said color and to reproduce the intaglio modules of the two-dimensional intaglio pattern according to the generated two-dimensional encoded pattern; and

(iii) inking the plate with the intaglio ink and using the inked plate with the intaglio printing machine to print on a surface of a substrate the corresponding two-dimensional intaglio pattern.

7 . A method of decoding information encoded into a two-dimensional intaglio pattern of a security marking of a security element, the two-dimensional intaglio pattern comprising at least one base intaglio pattern comprising a plurality of intaglio modules, comprising the steps of:

imaging the two-dimensional intaglio pattern;

detecting, from the imaged intaglio modules of the imaged two-dimensional intaglio pattern, any reference pattern of at least one reference pattern forming a two-dimensional encoded pattern based on which a corresponding genuine two-dimensional intaglio pattern of a genuine security marking of a genuine security element according to claim 1 was produced;

decoding each detected reference pattern and retrieving corresponding decoded information of the two-dimensional encoded pattern; and

validating each decoded reference pattern and corresponding base intaglio pattern.

8 . The method of claim 7 , comprising the steps of:

a) imaging the two-dimensional intaglio pattern of the security marking with a camera, equipped with a processor and a memory, of which light sensor is adapted to detect a color of intaglio modules forming the genuine intaglio pattern and distinct tones of the tone palette of said color, to obtain a digital image of the two-dimensional intaglio pattern and store the obtained digital image in the memory, the memory storing a set of associated reference patterns and, for each stored reference pattern, the portion of information specifying a corresponding reference pattern identification number;

each stored reference pattern including a specific arrangement of a plurality of symbols, each symbol in the arrangement allowing to encode data according to a state of said symbol, the state of each symbol consisting in a specific arrangement of a first two-dimensional element E1 and a second distinct two-dimensional element E2 within the symbol, each element E having a corresponding value of a parameter p(E), the parameter of the first element E1 having a positive value different from a zero value of the parameter of the second element E2;

each pair of allowed tones of intaglio modules of each base intaglio pattern being associated with a corresponding base intaglio pattern and stored in the memory in association with the reference pattern identification number of the reference pattern associated with said base intaglio pattern;

each stored reference pattern only comprising dark symbols and light symbols, the parameter value of the first element of a dark symbol corresponding to the darker tone of the pair of allowed tones of an associated base intaglio pattern, and the parameter value of the first element of a light symbol corresponding to the lighter tone of said associated pair of allowed tones;

b) detecting in the stored digital image, via image processing of the pixels of the digital image scanned with the processor through a movable window of a size of a reference pattern, intaglio modules within the window, and

b1) checking whether each of said detected intaglio modules, for a candidate reference pattern selected from the stored set of reference patterns,

either is a separate intaglio module corresponding to a first element of a symbol of the candidate reference pattern and its location in the candidate reference pattern; or

represents a portion of an intaglio line corresponding to a line connecting two first elements having a same parameter value p, respectively belonging to two distinct adjacent symbols of the candidate reference pattern, and a location of the line in the candidate reference pattern; and

b2) in case the detected intaglio modules correspond to respective elements of the symbols of the candidate reference pattern, decoding the candidate reference pattern to obtain its candidate reference pattern identification number, thereby decoding the associated base intaglio pattern, validating the decoded candidate reference pattern and the decoded associated base intaglio pattern to obtain corresponding associated validated reference pattern and validated base intaglio pattern, and storing in the memory corresponding location data indicating a location on the digital image of the validated base intaglio pattern; and

b3) in case the detected intaglio modules do not correspond to respective elements of the symbols of the candidate reference pattern, selecting a new candidate reference pattern from the stored set of reference patterns, and performing steps b1) to b2) with said new candidate reference pattern; and

b4) in case the detected intaglio modules do not correspond to respective elements of the symbols of any one of the candidate reference patterns, moving the window with respect to the digital image to scan another area of the digital image and detect intaglio modules through the moved window, and performing step b1) to b3) until all the digital image is scanned through the window;

c) in case the full digital image has been scanned through the window and the detected intaglio modules do not correspond to respective elements of the symbols of any candidate reference pattern, delivering a signal indicating that the decoding of the two-dimensional intaglio pattern failed.

9 . A method for authenticating a two-dimensional intaglio pattern of a security marking of a security element, comprising the steps of:

performing the operations of decoding information encoded into the two-dimensional intaglio pattern according to the method of claim 7 , to obtain, from the imaged two-dimensional intaglio pattern, each base intaglio pattern of the two-dimensional intaglio pattern and corresponding validated reference pattern;

shifting along a plurality of distinct directions, with respect to initial positions corresponding to respective positions of the detected intaglio modules of each validated base intaglio pattern on the taken image of the two-dimensional intaglio pattern at decoding, for each validated base intaglio pattern, intaglio modules respectively corresponding to a tone of an allowed pair of tones from the tone palette of the color of the genuine two-dimensional intaglio pattern, to obtain corresponding shifted images of said intaglio modules;

determining for each validated base intaglio pattern, from differences between measured tone values at initial positions of the shifted intaglio modules on the respective shifted images and measured tone values at same initial positions of said intaglio modules on the image of the two-dimensional intaglio pattern at decoding, with respect to each corresponding symbol of the associated reference pattern, a pair of shift values, respectively for intaglio modules corresponding to each tone of said pair of allowed tones, providing a maximal tone value for all the intaglio modules, respectively corresponding to each tone of said pair of allowed tones, of the validated base intaglio pattern; and

determining that, for that allowed pair of tones, the respective intaglio modules of the two-dimensional intaglio pattern are in register only if a norm value of an average of the determined pairs of shift values, over all the validated base intaglio patterns of the two-dimensional intaglio pattern, is less than or equal to 10 μm.

10 . The method according to claim 9 , comprising the steps of:

A) performing operations a), b), b1), b2) b3), b4) and -c) of the method of decoding information encoded in a two-dimensional intaglio pattern comprising the steps of:

a) imaging the two-dimensional intaglio pattern of the security marking with a camera, equipped with a processor and a memory, of which light sensor is adapted to detect a color of intaglio modules forming the genuine intaglio pattern and distinct tones of the tone palette of said color, to obtain a digital image of the two-dimensional intaglio pattern and store the obtained digital image in the memory, the memory storing a set of associated reference patterns and, for each stored reference pattern, the portion of information specifying a corresponding reference pattern identification number;

each stored reference pattern including a specific arrangement of a plurality of symbols, each symbol in the arrangement allowing to encode data according to a state of said symbol, the state of each symbol consisting in a specific arrangement of a first two-dimensional element E1 and a second distinct two-dimensional element E2 within the symbol, each element E having a corresponding value of a parameter p(E), the parameter of the first element E1 having a positive value different from a zero value of the parameter of the second element E2;

each pair of allowed tones of intaglio modules of each base intaglio pattern being associated with a corresponding base intaglio pattern and stored in the memory in association with the reference pattern identification number of the reference pattern associated with said base intaglio pattern;

each stored reference pattern only comprising dark symbols and light symbols, the parameter value of the first element of a dark symbol corresponding to the darker tone of the pair of allowed tones of an associated base intaglio pattern, and the parameter value of the first element of a light symbol corresponding to the lighter tone of said associated pair of allowed tones;

b) detecting in the stored digital image, via image processing of the pixels of the digital image scanned with the processor through a movable window of a size of a reference pattern, intaglio modules within the window, and

b1) checking whether each of said detected intaglio modules, for a candidate reference pattern selected from the stored set of reference patterns,

either is a separate intaglio module corresponding to a first element of a symbol of the candidate reference pattern and its location in the candidate reference pattern; or

represents a portion of an intaglio line corresponding to a line connecting two first elements having a same parameter value p, respectively belonging to two distinct adjacent symbols of the candidate reference pattern, and a location of the line in the candidate reference pattern; and

b2) in case the detected intaglio modules correspond to respective elements of the symbols of the candidate reference pattern, decoding the candidate reference pattern to obtain its candidate reference pattern identification number, thereby decoding the associated base intaglio pattern, validating the decoded candidate reference pattern and the decoded associated base intaglio pattern to obtain corresponding associated validated reference pattern and validated base intaglio pattern, and storing in the memory corresponding location data indicating a location on the digital image of the validated base intaglio pattern; and

b3) in case the detected intaglio modules do not correspond to respective elements of the symbols of the candidate reference pattern, selecting a new candidate reference pattern from the stored set of reference patterns, and performing steps b1) to b2) with said new candidate reference pattern; and

b4) in case the detected intaglio modules do not correspond to respective elements of the symbols of any one of the candidate reference patterns, moving the window with respect to the digital image to scan another area of the digital image and detect intaglio modules through the moved window, and performing step b1) to b3) until all the digital image is scanned through the window;

c) in case the full digital image has been scanned through the window and the detected intaglio modules do not correspond to respective elements of the symbols of any candidate reference pattern, delivering a signal indicating that the decoding of the two-dimensional intaglio pattern failed, on the two-dimensional intaglio pattern to be authenticated;

B) in case in step A) the decoding of the two-dimensional intaglio pattern fails, delivering an information indicating that the decoding of the two-dimensional intaglio pattern failed;

C) in case the step A) provides the information encoded in the two-dimensional intaglio pattern indicating, for each stored location on the digital image of a validated base intaglio pattern, the reference pattern identification number of the associated validated reference pattern, verifying a multitone aspect of the validated base intaglio patterns by

determining, from their respective associated validated reference pattern identification numbers, their allowed pair of distinct tones Ta, Tb;

measuring on the digital image a printed tone value of each intaglio module of the validated base intaglio patterns, determining a dynamic range of the digital image from the measured tone values;

calculating an average printed tone value Ta of the intaglio modules of the validated base intaglio patterns corresponding to the allowed tone Ta as an average of the measured printed tone values of the intaglio modules corresponding to the allowed tone Ta, and an average printed tone value Tb of the intaglio modules of the validated base intaglio patterns corresponding to the allowed tone Tb as an average of the measured printed tone values of the intaglio modules corresponding to the allowed tone Tb;

checking, for each allowed pair of tones Ta, Tb, whether a multitone criterion that a difference between the calculated average printed tone values Ta and Tb is greater than 1% of the dynamic range is met; and

in case the multitone criterion is met for each allowed pair of tones, deciding that the two-dimensional intaglio pattern has said multitone aspect, or in case the multitone criterion is not satisfied for an allowed pair of tones Ta, Tb, deciding that the two-dimensional intaglio pattern does not have said multitone aspect and is not genuine;

D) in case the two-dimensional intaglio pattern has the multitone aspect, for each validated base intaglio pattern having a corresponding pair of allowed tones Ta, Tb, of which location in the digital image is stored in the memory, performing with the processor the operations of:

D1) measuring within the window disposed at said location on the digital image, through a sampling grid having an arrangement of symbols with respective sites of their two-dimensional elements E1 and E2 corresponding to the symbols of the validated reference pattern associated with said validated base intaglio pattern, printed tone values of the intaglio modules of the validated base intaglio pattern from pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid to obtain corresponding initial tone values attributed to the respective elements E1 and E2 of the symbols of the validated reference pattern associated with the validated base intaglio pattern, calculating for each symbol of the sampling grid a difference between the initial tone value attributed to the element E1 and the initial tone value attributed to the element E2 to obtain an initial differential symbol tone value and storing in the memory the obtained initial differential symbol tone values attributed to the elements of the symbols of the validated reference pattern, and, for each one of the two allowed tones Ta, Tb of the validated reference pattern, a corresponding zero-shift sum is obtained by summing the initial differential symbol tone values for the all the symbols of the validated reference pattern corresponding, respectively, to the tone Ta and the tone Tb, and the obtained zero-shift sum for the allowed tone Ta and zero-shift sum for the allowed tone Tb are stored in the memory;

D2) shifting by at least one pixel size, backward and forward along each of said plurality of distinct directions with respect to the digital image, the pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid of which first element corresponds to the allowed tone Ta to obtain corresponding shifted pixels having shifted positions with respect to the sampling grid;

measuring through the sampling grid, respectively for the backward shift and the forward shift along each direction, corresponding backward shifted and forward shifted tone values attributed to the respective elements E1 and E2 of the symbols of the sampling grid of which first element corresponds to the allowed tone Ta, from shifted pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid of which first element corresponds to the allowed tone Ta, and storing in the memory the obtained shifted tone values;

calculating, for each shift, for each symbol of the sampling grid of which first element corresponds to the allowed tone Ta, a difference between the shifted tone value attributed to the element E1 and the shifted tone value attributed to the element E2 to obtain corresponding shifted differential symbol tone values, and storing in the memory the obtained shifted differential symbol tone values attributed to the elements of the symbols of the validated reference pattern of which first element corresponds to the allowed tone Ta;

for each shift, summing the calculated shifted differential symbol tone values of all the symbols of the sampling grid corresponding to the allowed tone Ta, to obtain corresponding validated reference pattern shift sum for the allowed tone Ta; and

selecting a best shift, for the allowed tone Ta, corresponding to the greatest calculated shift sum among the calculated shift sums and the zero-shift sum, for the validated reference pattern;

calculating a global shift Δ a for the two-dimensional intaglio pattern by determining an average of all the selected best shifts, for the allowed tone Ta, over the validated reference patterns associated with the respective validated base intaglio patterns of the two-dimensional intaglio pattern having the corresponding pair of allowed tones Ta, Tb;

D3) shifting by at least one pixel size, backward and forward along each of said plurality of distinct directions with respect to the digital image, the pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid of which first element corresponds to the allowed tone Tb to obtain corresponding shifted pixels having shifted positions with respect to the sampling grid;

measuring through the sampling grid, respectively for the backward shift and the forward shift along each direction, corresponding backward shifted and forward shifted tone values attributed to the respective elements E1 and E2 of the symbols of the sampling grid of which first element corresponds to the allowed tone Tb, from shifted pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid of which first element corresponds to the allowed tone Tb, and storing in the memory the obtained shifted tone values;

calculating, for each shift, for each symbol of the sampling grid of which first element corresponds to the allowed tone Tb, a difference between the shifted tone value attributed to the element E1 and the shifted tone value attributed to the element E2 to obtain corresponding shifted differential symbol tone values, and storing in the memory the obtained shifted differential symbol tone values attributed to the elements of the symbols of the validated reference pattern of which first element corresponds to the allowed tone Tb;

for each shift, summing the calculated shifted differential symbol tone values of all the symbols of the sampling grid corresponding to the allowed tone Tb, to obtain corresponding validated reference pattern shift sum for the allowed tone Tb; and

selecting a best shift, for the allowed tone Tb, corresponding to the greatest calculated shift sum among the calculated shift sums and the zero-shift sum, for the validated reference pattern;

calculating a global shift Δ b for the two-dimensional intaglio pattern by determining an average of all the selected best shifts, for the allowed tone Tb, over the validated reference patterns associated with the respective validated base intaglio patterns of the two-dimensional intaglio pattern having the corresponding pair of allowed tones Ta, Tb;

D4) performing the above operations D) for each remaining pair of allowed tones Ta′, Tb′ from the pairs of allowed tones of the stored validated base intaglio patterns of the two-dimensional intaglio pattern, to obtain corresponding pairs of global shifts (Δ a ′, Δ b ′), and storing in the memory said pairs of global shifts;

E) estimating a register error value r(a, b) for the intaglio modules corresponding to any allowed pair of tones Ta, Tb of all the validated base intaglio patterns of the two-dimensional intaglio pattern as r(a, b)=λ/f (A a 2 +Δ b 2 ) 1/2 , wherein λ is a size in μm of a pixel of the digital image and f is a scale factor of the digital image, and deciding that the two-dimensional intaglio pattern is genuine if each register error value r(a, b) for each allowed pair of tones Ta, Tb of the respective validated base intaglio patterns of the two-dimensional intaglio pattern is less than or equal to 10 μm.

11 . A device for decoding information encoded into a two-dimensional intaglio pattern of a security marking of a security element, the two-dimensional intaglio pattern comprising at least one base intaglio pattern comprising a plurality of intaglio modules, the device comprising a camera, equipped with a processor, a memory, a light sensor adapted to detect a color of a genuine intaglio pattern and distinct tones from a tone palette of said color, the genuine two-dimensional intaglio pattern of a genuine security marking of a genuine security element being according to claim 1 , the processor being adapted to perform image processing of a digital image of the genuine two-dimensional intaglio pattern taken by the camera and stored in the memory and perform decoding operations on encoded information detected on the digital image, the memory storing at least one reference pattern forming a two-dimensional encoded pattern on the basis of which the genuine two-dimensional intaglio pattern was produced, the device being adapted to perform the operations of:

imaging the two-dimensional intaglio pattern;

detecting, from the imaged intaglio modules of the imaged two-dimensional intaglio pattern, any reference pattern of the at least one reference pattern of the two-dimensional encoded pattern on the basis of which the associated genuine two-dimensional intaglio pattern was produced;

decoding each detected reference pattern and retrieving corresponding decoded information of the two-dimensional encoded pattern; and

validating each decoded reference pattern and corresponding base intaglio pattern.

12 . The device of claim 11 , wherein

each stored reference pattern includes a specific arrangement of a plurality of symbols, each symbol in the arrangement allowing to encode data according to a state of said symbol, the state of each symbol consisting in a specific arrangement of a first two-dimensional element E1 and a second distinct two-dimensional element E2 within the symbol, each element E having a corresponding value of a parameter p(E), the parameter of the first element E1 having a positive value different from a zero value of the parameter of the second element E2;

each pair of allowed tones of intaglio modules of each base intaglio pattern being associated with the base intaglio pattern and stored in the memory in association with the reference pattern identification number of the reference pattern associated with said base intaglio pattern;

each stored reference pattern only comprises dark symbols and light symbols, the parameter value of the first element of a dark symbol corresponding to the darker tone of the pair of allowed tones of an associated base intaglio pattern, and the parameter value of the first element of a light symbol corresponding to the lighter tone of said associated pair of allowed tones;

the device being adapted to perform the operations of:

a) imaging the two-dimensional intaglio pattern of the security marking with the camera to obtain a digital image of the two-dimensional intaglio pattern and store the obtained digital image in the memory;

b) detecting in the stored digital image intaglio modules within a movable window of a size of an allowed reference pattern, via image processing of the pixels of the digital image scanned with the processor through the window; and

b1) checking whether each of said detected intaglio modules, for a candidate reference pattern selected from the set of reference patterns,

either is a separate intaglio module corresponding to a first element of a symbol of the candidate reference pattern and its location in the candidate reference pattern; or

represents a portion of an intaglio line corresponding to a line connecting two first elements having a same parameter value p, respectively belonging to two distinct adjacent symbols of the candidate reference pattern, and a location of the line in the candidate reference pattern; and

b2) in case the detected intaglio modules correspond to respective elements of the symbols of the candidate reference pattern, decoding the candidate reference pattern to obtain its candidate reference pattern identification number, thereby decoding the associated base intaglio pattern, validating the decoded candidate reference pattern and the decoded associated base intaglio pattern to obtain corresponding associated validated reference pattern and validated base intaglio pattern, and storing in the memory corresponding location data indicating a location on the digital image of the validated base intaglio pattern; and

b3) in case the detected intaglio modules do not correspond to respective elements of the symbols of the candidate reference pattern, selecting a new candidate reference pattern from the stored set of reference patterns, and performing steps b1) to b2) with said new candidate reference pattern; and

b4) in case the detected intaglio modules do not correspond to respective elements of the symbols of any one of the candidate reference patterns, moving the window with respect to the digital image to scan another area of the digital image and detect intaglio modules through the moved window, and performing step b1) to b3) until all the digital image is scanned through the window;

c) in case the full digital image has been scanned through the window and the detected intaglio modules do not correspond to respective elements of the symbols of any candidate reference pattern, delivering a signal indicating that the decoding of the two-dimensional intaglio pattern failed.

13 . A device for authenticating a two-dimensional intaglio pattern of a security marking of a security element, a genuine two-dimensional intaglio pattern of a genuine security marking of a genuine security element being according to claim 1 , comprising a camera, equipped with a processor, a memory, a light sensor adapted to detect a color of a genuine intaglio pattern and distinct tones of a tone palette of said color, the processor being adapted to perform image processing of a digital image of the genuine two-dimensional intaglio pattern taken by the camera and stored in the memory and perform decoding operations on encoded information detected on the digital image, the memory storing at least one reference pattern forming a two-dimensional encoded pattern based on which the genuine two-dimensional intaglio pattern was produced,

the device being adapted to perform the operations of:

decoding information encoded into the two-dimensional intaglio pattern, the two-dimensional intaglio pattern comprising at least one base intaglio pattern comprising a plurality of intaglio modules, according to a method, comprising the steps of:

imaging the two-dimensional intaglio pattern;

detecting, from the imaged intaglio modules of the imaged two-dimensional intaglio pattern, any reference pattern of at least one reference pattern forming a two-dimensional encoded pattern based on which a corresponding genuine two-dimensional intaglio pattern of a genuine security marking of a genuine security element according to claim 1 was produced;

decoding each detected reference pattern and retrieving corresponding decoded information of the two-dimensional encoded pattern; and

validating each decoded reference pattern and corresponding base intaglio pattern, to obtain, from the image taken by the camera of the two-dimensional intaglio pattern, each validated base intaglio pattern of the two-dimensional intaglio pattern and corresponding validated reference pattern;

shifting along a plurality of distinct directions, with respect to initial positions corresponding to respective positions of the detected intaglio modules of each validated base intaglio pattern on the taken image of the two-dimensional intaglio pattern, for each validated base intaglio pattern, intaglio modules respectively corresponding to a tone of an allowed pair of tones from the tone palette of the color of the genuine two-dimensional intaglio pattern, to obtain corresponding shifted images of said intaglio modules;

determining for each validated base intaglio pattern, from differences between measured tone values at initial positions of the shifted intaglio modules on the respective shifted images and measured tone values at same initial positions of said intaglio modules on the image of the two-dimensional intaglio pattern at decoding, with respect to each corresponding symbol of the associated reference pattern, a pair of shift values, respectively for intaglio modules corresponding to each tone of said pair of allowed tones, providing a maximal tone value for all the intaglio modules, respectively corresponding to each tone of said pair of allowed tones, of the validated base intaglio pattern; and

determining that, for that allowed pair of tones, the respective intaglio modules of the two-dimensional intaglio pattern are in register only if a norm value of an average of the determined pairs of shift values, over all the validated base intaglio patterns of the two-dimensional intaglio pattern, is less than or equal to 10 μm.

14 . The device of claim 13 , adapted to:

A) perform operations a), b), b1), b2) b3), b4) and -c) of the method of decoding with the processor information encoded in a two-dimensional intaglio pattern comprising the steps of:

a) imaging the two-dimensional intaglio pattern of the security marking with a camera, equipped with a processor and a memory, of which light sensor is adapted to detect a color of intaglio modules forming the genuine intaglio pattern and distinct tones of the tone palette of said color, to obtain a digital image of the two-dimensional intaglio pattern and store the obtained digital image in the memory, the memory storing a set of associated reference patterns and, for each stored reference pattern, the portion of information specifying a corresponding reference pattern identification number;

each stored reference pattern including a specific arrangement of a plurality of symbols, each symbol in the arrangement allowing to encode data according to a state of said symbol, the state of each symbol consisting in a specific arrangement of a first two-dimensional element E1 and a second distinct two-dimensional element E2 within the symbol, each element E having a corresponding value of a parameter p(E), the parameter of the first element E1 having a positive value different from a zero value of the parameter of the second element E2;

each pair of allowed tones of intaglio modules of each base intaglio pattern being associated with a corresponding base intaglio pattern and stored in the memory in association with the reference pattern identification number of the reference pattern associated with said base intaglio pattern;

each stored reference pattern only comprising dark symbols and light symbols, the parameter value of the first element of a dark symbol corresponding to the darker tone of the pair of allowed tones of an associated base intaglio pattern, and the parameter value of the first element of a light symbol corresponding to the lighter tone of said associated pair of allowed tones;

b) detecting in the stored digital image, via image processing of the pixels of the digital image scanned with the processor through a movable window of a size of a reference pattern, intaglio modules within the window, and

b1) checking whether each of said detected intaglio modules, for a candidate reference pattern selected from the stored set of reference patterns,

either is a separate intaglio module corresponding to a first element of a symbol of the candidate reference pattern and its location in the candidate reference pattern; or

represents a portion of an intaglio line corresponding to a line connecting two first elements having a same parameter value p, respectively belonging to two distinct adjacent symbols of the candidate reference pattern, and a location of the line in the candidate reference pattern; and

b2) in case the detected intaglio modules correspond to respective elements of the symbols of the candidate reference pattern, decoding the candidate reference pattern to obtain its candidate reference pattern identification number, thereby decoding the associated base intaglio pattern, validating the decoded candidate reference pattern and the decoded associated base intaglio pattern to obtain corresponding associated validated reference pattern and validated base intaglio pattern, and storing in the memory corresponding location data indicating a location on the digital image of the validated base intaglio pattern; and

b3) in case the detected intaglio modules do not correspond to respective elements of the symbols of the candidate reference pattern, selecting a new candidate reference pattern from the stored set of reference patterns, and performing steps b1) to b2) with said new candidate reference pattern; and

b4) in case the detected intaglio modules do not correspond to respective elements of the symbols of any one of the candidate reference patterns, moving the window with respect to the digital image to scan another area of the digital image and detect intaglio modules through the moved window, and performing step b1) to b3) until all the digital image is scanned through the window;

c) in case the full digital image has been scanned through the window and the detected intaglio modules do not correspond to respective elements of the symbols of any candidate reference pattern, delivering a signal indicating that the decoding of the two-dimensional intaglio pattern failed, on the two-dimensional intaglio pattern to be authenticated;

B) in case in step A) the decoding of the two-dimensional intaglio pattern fails, delivering an information indicating that the decoding of the two-dimensional intaglio pattern failed;

C) in case the step A) provides the information encoded in the two-dimensional intaglio pattern indicating, for each stored location on the digital image of a validated base intaglio pattern, the reference pattern identification number of the associated validated reference pattern, verifying a multitone aspect of the validated base intaglio patterns by

determining, from their respective associated validated reference pattern identification numbers, their allowed pair of distinct tones Ta, Tb;

measuring on the digital image a printed tone value of each intaglio module of the validated base intaglio patterns, determining a dynamic range of the digital image from the measured tone values;

calculating an average printed tone value Ta of the intaglio modules of the validated base intaglio patterns corresponding to the allowed tone Ta as an average of the measured printed tone values of the intaglio modules corresponding to the allowed tone Ta, and an average printed tone value Tb of the intaglio modules of the validated base intaglio patterns corresponding to the allowed tone Tb as an average of the measured printed tone values of the intaglio modules corresponding to the allowed tone Tb;

checking, for each allowed pair of tones Ta, Tb, whether a multitone criterion that a difference between the calculated average printed tone values Ta and Tb is greater than 1% of the dynamic range is met; and

in case the multitone criterion is met for each allowed pair of tones, deciding that the two-dimensional intaglio pattern has said multitone aspect, or

in case the multitone criterion is not satisfied for an allowed pair of tones Ta, Tb, deciding that the two-dimensional intaglio pattern does not have said multitone aspect and is not genuine;

D) in case the two-dimensional intaglio pattern has the multitone aspect, for each validated base intaglio pattern having a corresponding pair of allowed tones Ta, Tb, of which location in the digital image is stored in the memory, performing with the processor the operations of:

D1) measuring within the window disposed at said location on the digital image, through a sampling grid having an arrangement of symbols with respective sites of their two-dimensional elements E1 and E2 corresponding to the symbols of the validated reference pattern associated with said validated base intaglio pattern, printed tone values of the intaglio modules of the validated base intaglio pattern from pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid to obtain corresponding initial tone values attributed to the respective elements E1 and E2 of the symbols of the validated reference pattern associated with the validated base intaglio pattern, calculating for each symbol of the sampling grid a difference between the initial tone value attributed to the element E1 and the initial tone value attributed to the element E2 to obtain an initial differential symbol tone value, and storing in the memory the obtained initial differential symbol tone values attributed to the elements of the symbols of the validated reference pattern, and, for each one of the two allowed tones Ta, Tb of the validated reference pattern, a corresponding zero-shift sum is obtained by summing the initial differential symbol tone values for the all the symbols of the validated reference pattern corresponding, respectively, to the tone Ta and the tone Tb, and the obtained zero-shift sum for the allowed tone Ta and zero-shift sum for the allowed tone Tb are stored in the memory;

D2) shifting by at least one pixel size, backward and forward along each of said plurality of distinct directions with respect to the digital image, the pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid of which first element corresponds to the allowed tone Ta to obtain corresponding shifted pixels having shifted positions with respect to the sampling grid;

measuring through the sampling grid, respectively for the backward shift and the forward shift along each direction, corresponding backward shifted and forward shifted tone values attributed to the respective elements E1 and E2 of the symbols of the sampling grid of which first element corresponds to the allowed tone Ta, from shifted pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid of which first element corresponds to the allowed tone Ta, and storing in the memory the obtained shifted tone values;

calculating, for each shift, for each symbol of the sampling grid of which first element corresponds to the allowed tone Ta, a difference between the shifted tone value attributed to the element E1 and the shifted tone value attributed to the element E2 to obtain corresponding shifted differential symbol tone values, and storing in the memory the obtained shifted differential symbol tone values attributed to the elements of the symbols of the validated reference pattern of which first element corresponds to the allowed tone Ta;

for each shift, summing the calculated shifted differential symbol tone values of all the symbols of the sampling grid corresponding to the allowed tone Ta, to obtain corresponding validated reference pattern shift sum for the allowed tone Ta; and

selecting a best shift, for the allowed tone Ta, corresponding to the greatest calculated shift sum among the calculated shift sums and the calculated zero-shift sum, for the validated reference pattern;

calculating a global shift Δ a for the two-dimensional intaglio pattern by determining an average of all the selected best shifts, for the allowed tone Ta, over the validated reference patterns associated with the respective validated base intaglio patterns of the two-dimensional intaglio pattern having the corresponding pair of allowed tones Ta, Tb;

D3) shifting by at least one pixel size, backward and forward along each of said plurality of distinct directions with respect to the digital image, the pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid of which first element corresponds to the allowed tone Tb to obtain corresponding shifted pixels having shifted positions with respect to the sampling grid;

measuring through the sampling grid, respectively for the backward shift and the forward shift along each direction, corresponding backward shifted and forward shifted tone values attributed to the respective elements E1 and E2 of the symbols of the sampling grid of which first element corresponds to the allowed tone Tb, from shifted pixels of the digital image located within the respective sites of elements E1 and E2 of each symbol of the sampling grid of which first element corresponds to the allowed tone Tb, and storing in the memory the obtained shifted tone values;

calculating, for each shift, for each symbol of the sampling grid of which first element corresponds to the allowed tone Tb, a difference between the shifted tone value attributed to the element E1 and the shifted tone value attributed to the element E2 to obtain corresponding shifted differential symbol tone values, and storing in the memory the obtained shifted differential symbol tone values attributed to the elements of the symbols of the validated reference pattern of which first element corresponds to the allowed tone Tb;

for each shift, summing the calculated shifted differential symbol tone values of all the symbols of the sampling grid corresponding to the allowed tone Tb, to obtain corresponding validated reference pattern shift sum for the allowed tone Tb; and

selecting a best shift, for the allowed tone Tb, corresponding to the greatest calculated shift sum among the calculated shift sums and the zero-shift sum, for the validated reference pattern;

calculating a global shift Δ b for the two-dimensional intaglio pattern by determining an average of all the selected best shifts, for the allowed tone Tb, over the validated reference patterns associated with the respective validated base intaglio patterns of the two-dimensional intaglio pattern having the corresponding pair of allowed tones Ta, Tb;

D4) performing the above operations D) for each remaining pair of allowed tones Ta′, Tb′ from the pairs of allowed tones of the stored validated base intaglio patterns of the two-dimensional intaglio pattern, to obtain corresponding pairs of global shifts (Δ a ′, Δ b ′), and storing in the memory said pairs of global shifts;

E) estimating a register error value r(a, b) for the intaglio modules corresponding to any allowed pair of tones Ta, Tb of all the validated base intaglio patterns of the two-dimensional intaglio pattern as r(a, b)=λ/f (Δ a 2 +Δ b 2 ) 1/2 , wherein λ is a size in μm of a pixel of the digital image and f is a scale factor of the digital image, and deciding that the two-dimensional intaglio pattern is genuine if each register error value r(a, b) for each allowed pair of tones Ta, Tb of the respective validated base intaglio patterns of the two-dimensional intaglio pattern is less than or equal to 10 μm.

15 . The device of claim 13 , being a smartphone wherein the light sensor is a RGB light sensor, the camera has a resolving power of at least 20 μm, and wherein measuring a tone of an intaglio module of the two-dimensional intaglio pattern illuminated with an illumination light is performed by summing the respective Red, Green and Blue light intensity components detected with the RBG light sensor from received light reflected by said intaglio module.