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

Abstract

The invention relates to a security marking printed via intaglio process, corresponding engraved intaglio printing plate, and methods and devices for producing, encoding / decoding and authenticating said security marking. The security marking comprising multitone 2D intaglio patterns of intaglio modules containing encoded data according to specific encoding symbols, the decoding of which, together with measured tone values for tiny shifts around their positions at decoding stage on a digital image of said intaglio patterns in correlation with said symbols, provides a robust and precise estimation of a register error between modules of distinct tones, thus allowing to easily detect any fake marking obtained via offset printing, even with a smartphone.

Description

TECHNICAL FIELD

[001] The present invention relates to the technical field oftwo-dimensional multicolor patterns printed by an intaglioprocess intaglio on security documents (e.g. banknotes,passports), stamps or tickets, via intaglio printing withmultitone intaglio inks (see e.g. the patent EP 2 956 514 B1) toprovide protection against their counterfeit or illegalreproduction. The invention further relates to the technicalfield of encoding / decoding and authenticating methods of twodimensionalencoded patterns (see e.g. the patents EP 2 619 714B1 or EP 2 780 865 B1).BACKGROUND OF THE INVENTION

[002] Security markings containing two-dimensional patternsprinted by intaglio processes with thin intaglio lines, possiblyof different colors or different tones of a same color (e.g.half-tone intaglio patterns) are widely used as a protectionagainst their illegal reproduction due to the great difficultyto produce copies without registration errors.

[003] Intaglio printing delivers the most consistent and highquality printing of fine lines. It can be considered as theprinting technology of choice for generating fine designs in thefield of security documents, in particular banknotes and stamps.One of the distinguishing features of the intaglio printingprocess is that intaglio reliefs may be varied from a fewmicrometers to several tens of micrometers by usingcorrespondingly shallow or respectively deep recesses of theintaglio printing plate. Intaglio relief resulting from theintaglio ink layer thickness is emphasized by the embossing ofthe substrate, said embossing being produced by the pressureduring the ink transfer. The tactility resulting from intaglioprinting gives the banknotes their typical and recognizabletouch feeling.

[004] It is known in the art that intaglio inks may be used toproduce multitone designs, i.e. by printing a single intaglioink in a single step process, it is possible to produce printeddesigns comprising several different color shades as result ofthe different printed ink thickness resulting from the differentintaglio plate engravings depths. The patent EP 2 956 514 B1discloses an intaglio ink composition suitable to print multicharacteristicintaglio printed design in a predictable manner.The intaglio ink composition disclosed in EP 2 956 514 B1comprise two components being pigment particles of differentmodal particle diameter.

[005] In case the intaglio printing of a pattern (i.e. "intagliopattern") is realized in only one color, a commonly usedtechnique for copying said pattern is the offset printingtechnique. In this case there is indeed no registration error onthe generally high quality copy so that its visual appearance ismisleading, but of course the specific tactile perceptionprovided by a relief feature of an intaglio printed pattern islost. This is one of reasons why many banknotes in the worldcontain intaglio security features.

[006] In case of an intaglio pattern in multitone or multicolor,the offset printing technique cannot provide a copy withoutregistration errors (due to the multiple offset steps andprinting plates), and the more colorful the worst it is.However, when the offset printing is performed with great care,these registration errors can be hardly perceptible to the humannaked eye or the man in the street and necessitate specificanalytical tools (e.g. magnifying lens, microscope) that are notcommonly available.

[007] Thus, there is a need for security intaglio patterns ofwhich counterfeit or illegal reproduction, even withsophisticated offset printing techniques, could be easilydetectable by commonly available means capable to take a meredigital image of a pattern, like for example a smartphoneequipped with a camera, even if the camera of the smartphone hasa quite low resolving power.SUMMARY OF THE INVENTION

[008] In order to overcome the above mentioned drawbacks andlimitations of the prior art, a security marking involving bothmultitone intaglio printing of patterns and encoding of saidpatterns according to highly secure encoding schemes, forexample such as the ones specified into the above cited patentsEP 2 619 714 B1 or EP 2 780 865 B1, has been developed accordingto the invention.

[009] Specifically, the patent EP 2 780 865 B1 discloses apattern for coding a numerical information item on a surface,comprising a specific arrangement of a plurality of symbolsbelonging to a set of symbols, each symbol in the arrangementbeing intended for the coding of a portion of said numericalinformation item, wherein each symbol consists of at least onedifferential pair of elements positioned in a specific way, eachelement including a parameter, the parameter of the firstelement of each differential pair having a first value and theparameter of the second element of each differential pair havinga second value different from the first value, and said specificarrangement corresponds to a predetermined arrangement of saidsymbols constituting a reference pattern.

[010] Moreover, corresponding methods and devices for producing,encoding or decoding, and authenticating (via commonly availablemeans) said security marking have also been developed accordingto the invention, so that any counterfeited reproduction can beeasily detected by everyone.

[011] With alternative printing processes such as e.g. offset,silkscreen, gravure or inkjet, the use of different colored inksis mandatory to produce designs comprising three or more colors.However, these printing processes are not suitable to providethe requested resolution and / or register for the differentcolors of the security marking of the invention. Thus, theseprinting processes cannot be used to produce multitone securityfeatures comprising multitone security patterns as disclosedherein. Should these alternative printing processes be used inan attempt to reproduce the security marking as disclosed hereinon a value document, the authentication according to theinvention of the copy would fail and the corresponding printedsecurity document would be identified as being a counterfeit.

[012] A security element according to a first aspect of theinvention comprises a substrate and a security marking, thesecurity marking comprising a two-dimensional intaglio patternprinted on a surface of the substrate via intaglio printing withintaglio ink, the two-dimensional intaglio pattern having anassociated color and comprising at least one base intagliopattern comprising a plurality of intaglio modules, each baseintaglio pattern only comprises intaglio modules of which tonesbelong to a corresponding set of two distinct allowed tones,each intaglio module having one tone of said associated color, atone of an intaglio module being selected from a tone palette ofa plurality of distinct reference tones of the associated color,a CIE color difference ΔE* between the tones being greater thanor equal to 2.0, a width ε of an intaglio module being greaterthan 20 μm and less than or equal to 50 μm, wherein the twodimensionalintaglio pattern is produced based on an associatedtwo-dimensional encoded pattern comprising at least onereference pattern, wherein each of the at least one baseintaglio pattern is based on an associated reference pattern ofthe at least one reference pattern, each base intaglio patternand an associated reference pattern encoding a same portion ofinformation, which specifies a unique identification number ofthe associated reference pattern, wherein each reference patternincludes a specific arrangement of a plurality of symbols, eachsymbol in the arrangement allowing to encode data according to astate of said symbol, the state of each symbol consisting in aspecific arrangement of a first two-dimensional element E1 and asecond 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 valuedifferent from a zero value of the parameter of the secondelement E2, wherein each reference pattern only comprises darksymbols and light symbols, the parameter value of the firstelement of a dark symbol corresponding to the darker tone of thepair of allowed tones of an associated base intaglio pattern,and the parameter value of the first element of a light symbolcorresponding to the lighter tone of said associated pair ofallowed tones, wherein each intaglio module of each baseintaglio pattern of the two-dimensional intaglio pattern isprinted either as a separate intaglio module corresponding to afirst element of a symbol of an associated reference pattern ofthe two-dimensional encoded pattern and its location in thereference pattern; or as a portion of an intaglio linecorresponding to a line connecting two first elements having asame parameter value p, respectively belonging to two distinctadjacent symbols of an associated reference pattern of the twodimensionalencoded pattern, and a location of the line in thereference pattern.

[013] An intaglio module may be printed separated from anotherneighboring intaglio module, or may be a portion of (thin)intaglio line that can be connected to another portion ofintaglio line of a neighboring intaglio module so as to form acontinuous path.

[014] The above security marking, may comprise a plurality oftwo-dimensional intaglio patterns, each two-dimensional intagliopattern having a corresponding associated color from a colorpalette, any color of the color palette being distinct from abackground color of the substrate.

[015] Preferably, the security element comprises a plurality oftwo-dimensional intaglio patterns, each two-dimensional intagliopattern having a corresponding associated color being distinctfrom a background color of the substrate

[016] Preferably, a two-dimensional intaglio pattern of theabove security marking is comprised in a cell having a boundaryformed by intaglio lines of which height is greater than anyheight of the intaglio modules of said two-dimensional intagliopattern, a distance between an edge of the two-dimensionalintaglio pattern and the boundary of the cell being greater thanor equal to 40 μm.

[017] More preferably, the CIE color difference ΔE* between thetones is greater than or equal to 2.5.

[018] According to a second aspect, the invention relates to anengraved plate for an intaglio printing machine, comprisinggrooves of variable engraving depth adapted to receive intaglioink to print on a surface of a substrate a two-dimensionalintaglio pattern of a security marking of a security element asspecified above.

[019] Thus, the above engraved plate allows to print on thesurface of the substrate the intaglio pattern having a color ofthe intaglio ink and comprising a plurality of intaglio modulesforming a specific pattern of intaglio lines, each intagliomodule having a tone of said color, a tone of an intaglio moduleof a plurality of distinct reference tones of the color rangingfrom a darker tone to a lighter tone, a CIE color difference ΔE*between the tones being greater than or equal to 2.0, and allowsto print the two-dimensional intaglio pattern on the surface ofthe substrate in which a width ε of an intaglio line is greaterthan 20 μm and less than or equal to 50 μm.

[020] In fact, the engraved plate variable engraving depth isadapted to produce intaglio modules such that an intaglio moduleof the two-dimensional intaglio pattern either corresponds to:- a first element of a symbol of a reference pattern of the twodimensionalencoded pattern; or- constitutes a portion of an intaglio line corresponding to aline connecting two elements having a same positive parametervalue p, respectively belonging to two distinct adjacent symbolsof a reference pattern of the two-dimensional encoded pattern.

[021] According to a third aspect, the invention also relates toa corresponding method of producing a security elementcomprising a security marking comprising a two-dimensionalintaglio pattern according to any one of claims 1 to 4, themethod comprising the steps of:(i) generating a two-dimensional encoded pattern comprisingat least one reference pattern by encoding a portion ofinformation into each of said at least one reference patternforming the two-dimensional encoded pattern, said portion ofinformation specifying for each reference pattern its uniqueidentification number, each reference pattern including aspecific arrangement of a plurality of symbols, each symbol inthe arrangement allowing to encode data according to a state ofsaid symbol, the state of each symbol consisting in a specificarrangement of a first two-dimensional element E1 and a seconddistinct two-dimensional element E2 within the symbol, eachelement E having a corresponding value of a parameter p(E), theparameter of the first element E1 having a positive valuedifferent from a zero value of the parameter of the secondelement E2, wherein each reference pattern only comprises darksymbols and light symbols, the parameter value of the firstelement of a dark symbol corresponding to a darker tone of anassociated pair of allowed tones, selected from a tone paletteof a plurality of distinct reference tones of a color associatedwith the two-dimensional encoded pattern, and the parametervalue of the first element of a light symbol corresponding tothe lighter tone of said associated pair of allowed tones,wherein the two-dimensional intaglio pattern to be produced isbased on the generated two-dimensional encoded pattern andcomprising at least one base intaglio pattern comprising aplurality of intaglio modules, each intaglio module having onetone of said associated color, a tone of an intaglio modulebeing selected from the tone palette,wherein each of the at least one base intaglio pattern isbased on an associated reference pattern of the at least onereference pattern, each base intaglio pattern encoding a sameportion of information than an associated reference pattern,wherein each base intaglio pattern only comprising intagliomodules of which tones belong to the pair of allowed tones ofthe associated reference pattern, andeach intaglio module of each base intaglio pattern of thetwo-dimensional intaglio pattern is to be printedeither as a separate intaglio module corresponding to a firstelement of a symbol of an associated reference pattern of thetwo-dimensional encoded pattern and its location in thereference pattern; oras a portion of an intaglio line corresponding to a lineconnecting two first elements having a same parameter value p,respectively belonging to two distinct adjacent symbols of anassociated reference pattern of the two-dimensional encodedpattern, and a location of the line in the reference pattern;(ii) engraving a plate of an intaglio printing machine withvariable engraving depth adapted to receive an intaglio ink ofsaid color and to reproduce the intaglio modules of the twodimensionalintaglio pattern according to the generated twodimensionalencoded pattern; and(iii) inking the plate with the intaglio ink and using the inkedplate with the intaglio printing machine to print on a surfaceof a substrate the corresponding two-dimensional intagliopattern.

[022] According to a corresponding fourth aspect, the inventionrelates to a method of decoding information encoded into a twodimensionalintaglio pattern of a security marking of a securityelement, the two-dimensional intaglio pattern comprising atleast one base intaglio pattern comprising a plurality ofintaglio modules, the method comprising the steps of:imaging the two-dimensional intaglio pattern;detecting, from the imaged intaglio modules of the imaged twodimensionalintaglio pattern, any reference pattern of at leastone reference pattern forming a two-dimensional encoded patternbased on which a corresponding genuine two-dimensional intagliopattern of a genuine security marking of a genuine securityelement according to any one of claims 1 to 4 was produced;- decoding each detected reference pattern and retrievingcorresponding decoded information of the two-dimensional encodedpattern; and- validating each decoded reference pattern and correspondingbase intaglio pattern.

[023] Preferably, the above method of decoding information maycomprise the steps of:a) imaging the two-dimensional intaglio pattern of the securitymarking with a camera, equipped with a processor and a memory,of which light sensor is adapted to detect a color of intagliomodules forming the genuine intaglio pattern and distinct tonesof the tone palette of said color, to obtain a digital image ofthe two-dimensional intaglio pattern and store the obtaineddigital image in the memory, the memory storing a set ofassociated reference patterns and, for each stored referencepattern, the portion of information specifying a correspondingreference pattern identification number;- each stored reference pattern including a specific arrangementof a plurality of symbols, each symbol in the arrangementallowing to encode data according to a state of said symbol, thestate of each symbol consisting in a specific arrangement of afirst two-dimensional element E1 and a second distinct twodimensionalelement E2 within the symbol, each element E havinga corresponding value of a parameter p(E), the parameter of thefirst element E1 having a positive value different from a zerovalue of the parameter of the second element E2;- each pair of allowed tones of intaglio modules of each baseintaglio pattern being associated with a corresponding baseintaglio pattern and stored in the memory in association withthe reference pattern identification number of the referencepattern associated with said base intaglio pattern;- each stored reference pattern only comprising dark symbols andlight symbols, the parameter value of the first element of adark symbol corresponding to the darker tone of the pair ofallowed tones of an associated base intaglio pattern, and theparameter value of the first element of a light symbolcorresponding to the lighter tone of said associated pair ofallowed tones; b) detecting in the stored digital image, viaimage processing of the pixels of the digital image scanned withthe processor through a movable window of a size of a referencepattern, intaglio modules within the window, and- b1) checking whether each of said detected intaglio modules,for a candidate reference pattern selected from the stored setof reference patterns, is either a separate intaglio modulecorresponding toa first element of a symbol of the candidate referencepattern and its location in the candidate reference pattern; orrepresents a portion of an intaglio line corresponding to aline connecting two first elements having a same parameter valuep, respectively belonging to two distinct adjacent symbols ofthe candidate reference pattern, and a location of the line inthe candidate reference pattern; and- b2) in case the detected intaglio modules correspond torespective elements of the symbols of the candidate referencepattern, decoding the candidate reference pattern to obtain itscandidate reference pattern identification number, therebydecoding the associated base intaglio pattern, validating thedecoded candidate reference pattern and the decoded associatedbase intaglio pattern to obtain corresponding associatedvalidated reference pattern and validated base intaglio pattern,and storing in the memory corresponding location data indicatinga location on the digital image of the validated base intagliopattern; and- b3) in case the detected intaglio modules do not correspond torespective elements of the symbols of the candidate referencepattern, selecting a new candidate reference pattern from thestored set of reference patterns, and performing steps b1) tob2) with said new candidate reference pattern; and- b4) in case the detected intaglio modules do not correspond torespective elements of the symbols of any one of the candidatereference patterns, moving the window with respect to thedigital image to scan another area of the digital image anddetect intaglio modules through the moved window, and performingstep b1) to b3) until all the digital image is scanned throughthe window;c) in case the full digital image has been scanned through thewindow and the detected intaglio modules do not correspond torespective elements of the symbols of any candidate referencepattern, delivering a signal indicating that the decoding of thetwo-dimensional intaglio pattern failed.

[024] According to a fifth aspect of the invention, incorrespondence with the above mentioned fourth aspect, theinvention further relates to a device for decoding informationencoded into a two-dimensional intaglio pattern of a securitymarking of a security element, the two-dimensional intagliopattern comprising at least one base intaglio pattern comprisinga plurality of intaglio modules, the device comprising a camera,equipped with a processor, a memory, a light sensor adapted todetect a color of a genuine intaglio pattern and distinct tonesfrom a tone palette of said color, the genuine two-dimensionalintaglio pattern of a genuine security marking of a genuinesecurity element being according to any one of claims 1 to 4,the processor being adapted to perform image processing of adigital image of the genuine two-dimensional intaglio patterntaken by the camera and stored in the memory and performdecoding operations on encoded information detected on thedigital image, the memory storing at least one reference patternforming a two-dimensional encoded pattern on the basis of whichthe genuine two-dimensional intaglio pattern was produced, thedevice being adapted to perform the operations of:- imaging the two-dimensional intaglio pattern;- detecting, from the imaged intaglio modules of the imagedtwo-dimensional intaglio pattern, any reference pattern of theat least one reference pattern of the two-dimensional encodedpattern on the basis of which the associated genuine twodimensionalintaglio pattern was produced;- decoding each detected reference pattern and retrievingcorresponding decoded information of the two-dimensional encodedpattern; and- validating each decoded reference pattern and correspondingbase intaglio pattern.

[025] In a preferred mode of the above device for decodinginformation, whereineach stored reference pattern includes a specific arrangementof a plurality of symbols, each symbol in the arrangementallowing to encode data according to a state of said symbol, thestate of each symbol consisting in a specific arrangement of afirst two-dimensional element E1 and a second distinct twodimensionalelement E2 within the symbol, each element E havinga corresponding value of a parameter p(E), the parameter of thefirst element E1 having a positive value different from a zerovalue of the parameter of the second element E2;each pair of allowed tones of intaglio modules of each baseintaglio pattern being associated with the base intaglio patternand stored in the memory in association with the referencepattern identification number of the reference patternassociated with said base intaglio pattern;each stored reference pattern only comprises dark symbols andlight symbols, the parameter value of the first element of adark symbol corresponding to the darker tone of the pair ofallowed tones of an associated base intaglio pattern, and theparameter value of the first element of a light symbolcorresponding to the lighter tone of said associated pair ofallowed tones;the device being adapted to perform the operations of:a) imaging the two-dimensional intaglio pattern of the securitymarking with the camera to obtain a digital image of the twodimensionalintaglio pattern and store the obtained digitalimage in the memory;b) detecting in the stored digital image intaglio modules withina movable window of a size of an allowed reference pattern, viaimage processing of the pixels of the digital image scanned withthe processor through the window; andb1) checking whether each of said detected intaglio modules,for a candidate reference pattern selected from the set ofreference patterns, either is a separate intaglio modulecorresponding to a first element of a symbol of the candidatereference pattern and its location in the candidate referencepattern; orrepresents a portion of an intaglio line corresponding to aline connecting two first elements having a same parameter valuep, respectively belonging to two distinct adjacent symbols ofthe candidate reference pattern, and a location of the line inthe candidate reference pattern; andb2) in case the detected intaglio modules correspond torespective elements of the symbols of the candidate referencepattern, decoding the candidate reference pattern to obtain itscandidate reference pattern identification number, therebydecoding the associated base intaglio pattern, validating thedecoded candidate reference pattern and the decoded associatedbase intaglio pattern to obtain corresponding associatedvalidated reference pattern and validated base intaglio pattern,and storing in the memory corresponding location data indicatinga location on the digital image of the validated base intagliopattern; andb3) in case the detected intaglio modules do not correspondto respective elements of the symbols of the candidate referencepattern, selecting a new candidate reference pattern from thestored set of reference patterns, and performing steps b1) tob2) with said new candidate reference pattern; andb4) in case the detected intaglio modules do not correspondto respective elements of the symbols of any one of thecandidate reference patterns, moving the window with respect tothe digital image to scan another area of the digital image anddetect intaglio modules through the moved window, and performingstep b1) to b3) until all the digital image is scanned throughthe window;c) in case the full digital image has been scanned through thewindow and the detected intaglio modules do not correspond torespective elements of the symbols of any candidate referencepattern, delivering a signal indicating that the decoding of thetwo-dimensional intaglio pattern failed.

[026] Said device for decoding information may be a smartphoneequipped with a camera, a processor programmed to decode dataand perform digital image processing operations on an digitalimage taken by the camera according to the steps of the abovementioned method of decoding information, the camera having anda mere RGB (i.e. "Red Green Blue") light sensor adapted todetect intaglio modules of a two-dimensional intaglio patternand their tones according to the invention.

[027] According to a sixth aspect, the invention also relates toa method of authenticating a two-dimensional intaglio pattern ofa security marking of a security element, comprising the stepsof:- performing the operations of decoding information encoded intothe two-dimensional intaglio pattern according to the abovemethod of decoding information, to obtain, from the imaged twodimensionalintaglio pattern, each base intaglio pattern of thetwo-dimensional intaglio pattern and corresponding validatedreference pattern;- shifting along a plurality of distinct directions, withrespect to initial positions corresponding to respectivepositions of the detected intaglio modules of each validatedbase intaglio pattern on the taken image of the two-dimensionalintaglio pattern at decoding, for each validated base intagliopattern, intaglio modules respectively corresponding to a toneof an allowed pair of tones from the tone palette of the colorof the genuine two-dimensional intaglio pattern, to obtaincorresponding shifted images of said intaglio modules;- determining for each validated base intaglio pattern, fromdifferences between measured tone values at initial positions ofthe shifted intaglio modules on the respective shifted imagesand measured tone values at same initial positions of saidintaglio modules on the image of the two-dimensional intagliopattern at decoding, with respect to each corresponding symbolof the associated reference pattern, a pair of shift values,respectively for intaglio modules corresponding to each tone ofsaid pair of allowed tones, providing a maximal tone value forall the intaglio modules, respectively corresponding to eachtone of said pair of allowed tones, of the validated baseintaglio pattern; and- determining that, for that allowed pair of tones, therespective intaglio modules of the two-dimensional intagliopattern are in register only if a norm value of an average ofthe determined pairs of shift values, over all the validatedbase intaglio patterns of the two-dimensional intaglio pattern,is less than or equal to 10 μm, and preferably less than orequal to 5 μm.

[028] Preferably, the above method of authenticating a twodimensionalintaglio pattern according to the invention, maycomprise the steps of:A) performing the operations a), b), b1), b2) b3), b4) and -c)of the method of decoding information encoded in a twodimensionalintaglio pattern according to the above, on the twodimensionalintaglio pattern to be authenticated;B) in case in step A) the decoding of the two-dimensionalintaglio pattern fails, delivering an information indicatingthat the decoding of the two-dimensional intaglio patternfailed;C) in case the step A) provides the information encoded in thetwo-dimensional intaglio pattern indicating, for each storedlocation on the digital image of a validated base intagliopattern, the reference pattern identification number of theassociated validated reference pattern, verifying a multitoneaspect of the validated base intaglio patterns bydetermining, from their respective associated validatedreference pattern identification numbers, their allowed pair ofdistinct tones Ta, Tb;measuring on the digital image a printed tone value of eachintaglio module of the validated base intaglio patterns,determining a dynamic range of the digital image from themeasured tone values;calculating an average printed tone value Ta of the intagliomodules of the validated base intaglio patterns corresponding tothe allowed tone Ta as an average of the measured printed tonevalues of the intaglio modules corresponding to the allowed toneTa, and an average printed tone value Tb of the intaglio modulesof the validated base intaglio patterns corresponding to theallowed tone Tb as an average of the measured printed tonevalues of the intaglio modules corresponding to the allowed toneTb;checking, for each allowed pair of tones Ta, Tb, whether amultitone criterion that a difference between the calculatedaverage printed tone values Ta and Tb is greater than 1% of thedynamic range is met; andin case the multitone criterion is met for each allowed pairof tones, deciding that the two-dimensional intaglio pattern hassaid multitone aspect, orin case the multitone criterion is not satisfied for anallowed pair of tones Ta, Tb, deciding that the two-dimensionalintaglio pattern does not have said multitone aspect and is notgenuine;D) in case the two-dimensional intaglio pattern has themultitone aspect, for each validated base intaglio patternhaving a corresponding pair of allowed tones Ta, Tb, of whichlocation 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 onthe digital image, through a sampling grid having an arrangementof symbols with respective sites of their two-dimensionalelements E1 and E2 corresponding to the symbols of the validatedreference pattern associated with said validated base intagliopattern, printed tone values of the intaglio modules of thevalidated base intaglio pattern from pixels of the digital imagelocated within the respective sites of elements E1 and E2 ofeach symbol of the sampling grid to obtain corresponding initialtone values attributed to the respective elements E1 and E2 ofthe symbols of the validated reference pattern associated withthe validated base intaglio pattern, calculating for each symbolof the sampling grid a difference between the initial tone valueattributed to the element E1 and the initial tone valueattributed to the element E2 to obtain an initial differentialsymbol tone value and storing in the memory the obtained initialdifferential symbol tone values attributed to the elements ofthe symbols of the validated reference pattern, and, for each oneof the two allowed tones Ta, Tb of the validated referencepattern, a corresponding zero-shift sum is obtained by summingthe initial differential symbol tone values for the all thesymbols of the validated reference pattern corresponding,respectively, to the tone Ta and the tone Tb, and the obtainedzero-shift sum for the allowed tone Ta and zero-shift sum forthe allowed tone Tb are stored in the memory;D2) shifting by at least one pixel size, backward and forwardalong each of said plurality of distinct directions with respectto the digital image, the pixels of the digital image locatedwithin the respective sites of elements E1 and E2 of each symbolof the sampling grid of which first element corresponds to theallowed tone Ta to obtain corresponding shifted pixels havingshifted positions with respect to the sampling grid;measuring through the sampling grid, respectively for thebackward shift and the forward shift along each direction,corresponding backward shifted and forward shifted tone valuesattributed to the respective elements E1 and E2 of the symbolsof the sampling grid of which first element corresponds to theallowed tone Ta, from shifted pixels of the digital imagelocated within the respective sites of elements E1 and E2 ofeach symbol of the sampling grid of which first elementcorresponds to the allowed tone Ta, and storing in the memorythe obtained shifted tone values;calculating, for each shift, for each symbol of the samplinggrid of which first element corresponds to the allowed tone Ta,a difference between the shifted tone value attributed to theelement E1 and the shifted tone value attributed to the elementE2 to obtain corresponding shifted differential symbol tonevalues, and storing in the memory the obtained shifteddifferential symbol tone values attributed to the elements ofthe symbols of the validated reference pattern of which firstelement corresponds to the allowed tone Ta;for each shift, summing the calculated shifted differentialsymbol tone values of all the symbols of the sampling gridcorresponding to the allowed tone Ta, to obtain correspondingvalidated reference pattern shift sum for the allowed tone Ta;andselecting a best shift, for the allowed tone Ta,corresponding to the greatest calculated shift sum among thecalculated shift sums and the zero-shift sum, for the validatedreference pattern;calculating a global shift Δa for the two-dimensionalintaglio pattern by determining an average of all the selectedbest shifts, for the allowed tone Ta, over the validatedreference patterns associated with the respective validated baseintaglio patterns of the two-dimensional intaglio pattern havingthe corresponding pair of allowed tones Ta, Tb;D3) shifting by at least one pixel size, backward and forwardalong each of said plurality of distinct directions with respectto the digital image, the pixels of the digital image locatedwithin the respective sites of elements E1 and E2 of each symbolof the sampling grid of which first element corresponds to theallowed tone Tb to obtain corresponding shifted pixels havingshifted positions with respect to the sampling grid;measuring through the sampling grid, respectively for thebackward shift and the forward shift along each direction,corresponding backward shifted and forward shifted tone valuesattributed to the respective elements E1 and E2 of the symbolsof the sampling grid of which first element corresponds to theallowed tone Tb, from shifted pixels of the digital imagelocated within the respective sites of elements E1 and E2 ofeach symbol of the sampling grid of which first elementcorresponds to the allowed tone Tb, and storing in the memorythe obtained shifted tone values;calculating, for each shift, for each symbol of the samplinggrid of which first element corresponds to the allowed tone Tb,a difference between the shifted tone value attributed to theelement E1 and the shifted tone value attributed to the elementE2 to obtain corresponding shifted differential symbol tonevalues, and storing in the memory the obtained shifteddifferential symbol tone values attributed to the elements ofthe symbols of the validated reference pattern of which firstelement corresponds to the allowed tone Tb;for each shift, summing the calculated shifted differentialsymbol tone values of all the symbols of the sampling gridcorresponding to the allowed tone Tb, to obtain correspondingvalidated reference pattern shift sum for the allowed tone Tb;andselecting a best shift, for the allowed tone Tb,corresponding to the greatest calculated shift sum among thecalculated shift sums and the zero-shift sum, for the validatedreference pattern;calculating a global shift Δb for the two-dimensionalintaglio pattern by determining an average of all the selectedbest shifts, for the allowed tone Tb, over the validatedreference patterns associated with the respective validated baseintaglio patterns of the two-dimensional intaglio pattern havingthe corresponding pair of allowed tones Ta, Tb;D4) performing the above operations D) for each remainingpair of allowed tones Ta', Tb' from the pairs of allowed tonesof the stored validated base intaglio patterns of the twodimensionalintaglio pattern, to obtain corresponding pairs ofglobal shifts (Δa', Δb'), and storing in the memory said pairs ofglobal shifts;(E) estimating a register error value r(a, b) for the intagliomodules corresponding to any allowed pair of tones Ta, Tb of allthe validated base intaglio patterns of the two-dimensionalintaglio pattern as r(a, b) = λ / f (Δa 2 + Δb 2)1 / 2, wherein λ is asize in μm (i.e. 10-6 m) of a pixel of the digital image and f isa scale factor of the digital image, and deciding that the twodimensionalintaglio pattern is genuine if each register errorvalue r(a, b) for each allowed pair of tones Ta, Tb of therespective validated base intaglio patterns of the twodimensionalintaglio pattern is less than or equal to 10 μm, andpreferably less than or equal to 5 μm.

[029] According to a seventh aspect, in correspondence with theabove authentication method, the invention further relates to adevice for authenticating a two-dimensional intaglio pattern ofa security marking of a security element, a genuine twodimensionalintaglio pattern of a genuine security marking of agenuine security element being according to any one of claims 1to 4, comprising a camera, equipped with a processor, a memory,a light sensor adapted to detect a color of a genuine intagliopattern and distinct tones of a tone palette of said color, theprocessor being adapted to perform image processing of a digitalimage of the genuine two-dimensional intaglio pattern taken bythe camera and stored in the memory and perform decodingoperations on encoded information detected on the digital image,the memory storing at least one reference pattern forming a twodimensionalencoded pattern based on which the genuine twodimensionalintaglio pattern was produced, the device beingadapted to perform the operations of:- decoding information encoded into the two-dimensional intagliopattern according to the above decoding method, to obtain, fromthe image taken by the camera of the two-dimensional intagliopattern, each validated base intaglio pattern of the twodimensionalintaglio pattern and corresponding validatedreference pattern;- shifting along a plurality of distinct directions, withrespect to initial positions corresponding to respectivepositions of the detected intaglio modules of each validatedbase intaglio pattern on the taken image of the two-dimensionalintaglio pattern, for each validated base intaglio pattern,intaglio modules respectively corresponding to a tone of anallowed pair of tones from the tone palette of the color of thegenuine two-dimensional intaglio pattern, to obtaincorresponding shifted images of said intaglio modules;- determining for each validated base intaglio pattern, fromdifferences between measured tone values at initial positions ofthe shifted intaglio modules on the respective shifted imagesand measured tone values at same initial positions of saidintaglio modules on the image of the two-dimensional intagliopattern at decoding, with respect to each corresponding symbol ofthe associated reference pattern, a pair of shift values,respectively for intaglio modules corresponding to each tone ofsaid pair of allowed tones, providing a maximal tone value forall the intaglio modules, respectively corresponding to eachtone of said pair of allowed tones, of the validated baseintaglio pattern; and- determining that, for that allowed pair of tones, therespective intaglio modules of the two-dimensional intagliopattern are in register only if a norm value of an average ofthe determined pairs of shift values, over all the validatedbase intaglio patterns of the two-dimensional intaglio pattern,is less than or equal to 10 μm, and preferably less than orequal to 5 μm.

[030] Preferably, the above device, may be adapted to:A) perform the operations a), b), b1), b2) b3), b4) and -c) ofthe method of decoding with the processor information encoded ina two-dimensional intaglio pattern according to the above, onthe two-dimensional intaglio pattern to be authenticated, thememory further storing the allowed pairs of tones of a genuinetwo-dimensional intaglio pattern;B) in case in step A) the decoding of the two-dimensionalintaglio pattern fails, delivering an information indicatingthat the decoding of the two-dimensional intaglio patternfailed;C) in case the step A) provides the information encoded in thetwo-dimensional intaglio pattern indicating, for each storedlocation on the digital image of a validated base intagliopattern, the reference pattern identification number of theassociated validated reference pattern, verifying a multitoneaspect of the validated base intaglio patterns bydetermining, from their respective associated validatedreference pattern identification numbers, their allowed pair ofdistinct tones Ta, Tb;measuring on the digital image a printed tone value of eachintaglio module of the validated base intaglio patterns,determining a dynamic range of the digital image from themeasured tone values;calculating an average printed tone value Ta of the intagliomodules of the validated base intaglio patterns corresponding tothe allowed tone Ta as an average of the measured printed tonevalues of the intaglio modules corresponding to the allowed toneTa, and an average printed tone value Tb of the intaglio modulesof the validated base intaglio patterns corresponding to theallowed tone Tb as an average of the measured printed tonevalues of the intaglio modules corresponding to the allowed toneTb;checking, for each allowed pair of tones Ta, Tb, whether amultitone criterion that a difference between the calculatedaverage printed tone values Ta and Tb is greater than 1% of thedynamic range is met; andin case the multitone criterion is met for each allowed pairof tones, deciding that the two-dimensional intaglio pattern hassaid multitone aspect, orin case the multitone criterion is not satisfied for anallowed pair of tones Ta, Tb, deciding that the two-dimensionalintaglio pattern does not have said multitone aspect and is notgenuine;D) in case the two-dimensional intaglio pattern has themultitone aspect, for each validated base intaglio patternhaving a corresponding pair of allowed tones Ta, Tb, of whichlocation 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 onthe digital image, through a sampling grid having an arrangementof symbols with respective sites of their two-dimensionalelements E1 and E2 corresponding to the symbols of the validatedreference pattern associated with said validated base intagliopattern, printed tone values of the intaglio modules of thevalidated base intaglio pattern from pixels of the digital imagelocated within the respective sites of elements E1 and E2 ofeach symbol of the sampling grid to obtain corresponding initialtone values attributed to the respective elements E1 and E2 ofthe symbols of the validated reference pattern associated withthe validated base intaglio pattern, calculating for each symbolof the sampling grid a difference between the initial tone valueattributed to the element E1 and the initial tone valueattributed to the element E2 to obtain an initial differentialsymbol tone value and storing in the memory the obtained initialdifferential symbol tone values attributed to the elements ofthe symbols of the validated reference pattern, and, for eachone of the two allowed tones Ta, Tb of the validated referencepattern, a corresponding zero-shift sum is obtained by summingthe initial differential symbol tone values for the all thesymbols of the validated reference pattern corresponding,respectively, to the tone Ta and the tone Tb, and the obtainedzero-shift sum for the allowed tone Ta and zero-shift sum forthe allowed tone Tb are stored in the memory;D2) shifting by at least one pixel size, backward and forwardalong each of said plurality of distinct directions with respectto the digital image, the pixels of the digital image locatedwithin the respective sites of elements E1 and E2 of each symbolof the sampling grid of which first element corresponds to theallowed tone Ta to obtain corresponding shifted pixels havingshifted positions with respect to the sampling grid;measuring through the sampling grid, respectively for thebackward shift and the forward shift along each direction,corresponding backward shifted and forward shifted tone valuesattributed to the respective elements E1 and E2 of the symbolsof the sampling grid of which first element corresponds to theallowed tone Ta, from shifted pixels of the digital imagelocated within the respective sites of elements E1 and E2 ofeach symbol of the sampling grid of which first elementcorresponds to the allowed tone Ta, and storing in the memorythe obtained shifted tone values;calculating, for each shift, for each symbol of the samplinggrid of which first element corresponds to the allowed tone Ta,a difference between the shifted tone value attributed to theelement E1 and the shifted tone value attributed to the elementE2 to obtain corresponding shifted differential symbol tonevalues, and storing in the memory the obtained shifteddifferential symbol tone values attributed to the elements ofthe symbols of the validated reference pattern of which firstelement corresponds to the allowed tone Ta;for each shift, summing the calculated shifted differentialsymbol tone values of all the symbols of the sampling gridcorresponding to the allowed tone Ta, to obtain correspondingvalidated reference pattern shift sum for the allowed tone Ta;andselecting a best shift, for the allowed tone Ta,corresponding to the greatest calculated shift sum among thecalculated shift sums and the zero-shift sum, for the validatedreference pattern;calculating a global shift Δa for the two-dimensionalintaglio pattern by determining an average of all the selectedbest shifts, for the allowed tone Ta, over the validatedreference patterns associated with the respective validated baseintaglio patterns of the two-dimensional intaglio pattern havingthe corresponding pair of allowed tones Ta, Tb;D3) shifting by at least one pixel size, backward and forwardalong each of said plurality of distinct directions with respectto the digital image, the pixels of the digital image locatedwithin the respective sites of elements E1 and E2 of each symbolof the sampling grid of which first element corresponds to theallowed tone Tb to obtain corresponding shifted pixels havingshifted positions with respect to the sampling grid;measuring through the sampling grid, respectively for thebackward shift and the forward shift along each direction,corresponding backward shifted and forward shifted tone valuesattributed to the respective elements E1 and E2 of the symbolsof the sampling grid of which first element corresponds to theallowed tone Tb, from shifted pixels of the digital imagelocated within the respective sites of elements E1 and E2 ofeach symbol of the sampling grid of which first elementcorresponds to the allowed tone Tb, and storing in the memorythe obtained shifted tone values;calculating, for each shift, for each symbol of the samplinggrid of which first element corresponds to the allowed tone Tb,a difference between the shifted tone value attributed to theelement E1 and the shifted tone value attributed to the elementE2 to obtain corresponding shifted differential symbol tonevalues, and storing in the memory the obtained shifteddifferential symbol tone values attributed to the elements ofthe symbols of the validated reference pattern of which firstelement corresponds to the allowed tone Tb;for each shift, summing the calculated shifted differentialsymbol tone values of all the symbols of the sampling gridcorresponding to the allowed tone Tb, to obtain correspondingvalidated reference pattern shift sum for the allowed tone Tb;andselecting a best shift, for the allowed tone Tb,corresponding to the greatest calculated shift sum among thecalculated shift sums and the zero-shift sum, for the validatedreference pattern;calculating a global shift Δb for the two-dimensionalintaglio pattern by determining an average of all the selectedbest shifts, for the allowed tone Tb, over the validatedreference patterns associated with the respective validated baseintaglio patterns of the two-dimensional intaglio pattern havingthe corresponding pair of allowed tones Ta, Tb;D4) performing the above operations D) for each remainingpair of allowed tones {Ta', Tb'} from the pairs of allowed tonesof the stored validated base intaglio patterns of the twodimensionalintaglio pattern, to obtain corresponding pairs ofglobal shifts (Δa', Δb'), and storing in the memory said pairs ofglobal shifts;E) estimating a register error value r(a, b) for the intagliomodules corresponding to any allowed pair of tones Ta, Tb of allthe validated base intaglio patterns of the two-dimensionalintaglio pattern as r(a, b) = λ / f (Δa 2 + Δb 2)1 / 2, wherein λ is asize in μm of a pixel of the digital image and f is a scalefactor of the digital image, and deciding that the twodimensionalintaglio pattern is genuine if each register errorvalue r(a, b) for each allowed pair of tones Ta, Tb of therespective validated base intaglio patterns of the twodimensionalintaglio pattern is less than or equal to 10 μm, andpreferably less than or equal to 5 μm.

[031] More preferably, the above device for authenticating, is asmartphone wherein the light sensor is a RGB light sensor, thecamera has a resolving power of at least 20 μm, and whereinmeasuring a tone of an intaglio module of the two-dimensionalintaglio pattern illuminated with an illumination light isperformed by summing the respective Red, Green and Blue lightintensity components detected with the RBG light sensor fromreceived light reflected by said intaglio module.The illumination may be provided by a flash of the smartphone,or may be mere daylight (if of sufficient light intensity).

[032] The present invention will be described more fullyhereinafter with reference to the accompanying drawings, inwhich prominent aspects and features of the invention areillustrated.BRIEF DESCRIPTION OF THE DRAWINGSFig.1 show a picture of a part of a specimen banknotecomprising a security marking according to the present inventioncomprising a plurality of two-dimensional intaglio patterns,printed on an industrial intaglio press.Fig.2A-D show pictures of enlarged reproductions ofdetails of two-dimensional intaglio patterns of securitymarkings, according to the present invention, printed with alaser printer on a A4 standard printing paper.Fig.3A-B illustrate an example of a reference patternwith an arrangement of symbols, and states of these symbols,according to the prior art disclosed in EP 2 619 714 B1 and EP 2780 865 B1.Fig.4A-D illustrate an example of an encodedreference pattern as adapted to be in one-to-one correspondencewith an example of a base intaglio pattern obtained by theintaglio printing process of the present invention.Fig.5 is a flowchart illustrating an example ofdecoding of a two-dimensional intaglio pattern with asmartphone.Fig.6A-G Schematically illustrates operations of anexample of the method of authentication of a two-dimensionalintaglio pattern.DETAILED DESCRIPTION EXAMPLES

[033] The present invention is now described in more detailswith reference to non-limiting examples.Example E1 Banknote Specimen

[034] Fig.1 shows a picture of a part of a specimen banknotecomprising a security marking (100) according to an embodimentof the present invention. The specimen banknote was printed onan industrial intaglio press under industrial conditions using acyan intaglio printing ink. The two-dimensional intaglio patternof the security marking (100) is embedded in the intagliodesign, in particular in the right-hand-side jacket lapel. Asillustrated on Fig.1, the two-dimensional intaglio patterncannot be detected with naked eye and is fully integrated in theintaglio design. The first example Ex1 of two-dimensionalintaglio pattern of the security marking shown in Fig.1 wasprinted on an intaglio printing press (Super Orloff SOI II)under banknote industrial printing conditions, on a banknotepaper substrate (fiduciary paper from Louisenthal) using anoxidative drying intaglio ink comprising alkyd-based oxidativevarnishes, mineral fillers, mineral oils, waxes, oxidativedriers, and 4.3 wt% of a blue pigment (Copper Phthalocyanineblue (6CI) (CAS Nr 147-14-8) from Synthesia). The securitymarking was printed using an intaglio plate comprisingengravings having a depth of 8 μm (L area), 16 μm (M area), and22 μm (D area) to produce the full two-dimensional intagliopattern (201) on the jacket lapel as illustrated on Fig.2A.

[035] The security marking (100) of Fig.1 corresponds to theman's portrait and his cloths. The full two-dimensional intagliopattern (201) of Fig.2A is located in the man's jacket lapelshown on Fig.1 and comprises a plurality of cells, each cellincluding its own (small) two-dimensional intaglio pattern(220). The two-dimensional intaglio pattern (201) wasauthenticated, according to the invention, as genuine using aSamsung Galaxy S10 or S21 smartphone (with camera sensor of 12mega pixels) being at a distance of about 10 cm from thebanknote specimen surface.Examples Ex2-Ex4 and Comparative Examples CEx1-CEx2

[036] Figures 2A-D show pictures of enlarged reproductions ofdetails of two-dimensional intaglio patterns of securitymarkings, with two-dimensional intaglio patterns respectivelycorresponding to Examples 2-4 (Ex2-Ex4), according to thepresent invention, printed with a laser printer on a A4 standardprinting paper. The two-dimensional intaglio patterns of thesecurity markings of examples Ex2-Ex4 and the comparativeexamples CEx1-CEx2 (see Fig.2B-D) were printed on a conventionalwhite printer paper substrate (A4 format). The printed twodimensionalintaglio patterns of the security markings have alength of about 26.5 cm and a width of about 14.5 cm,corresponding to an about 3.5 times enlargement as compared tothe security marking printed on the banknote specimen of exampleEx1 of Fig.1 (the rectangular frame in Fig.2A corresponds to theA4 paper substrate edges) using the design vectorial file thatwas used to prepare the intaglio plate used for the example Ex1.Portions of a same tone of the multitone intaglio lines formingthe two-dimensional intaglio pattern (201) correspond to thedifferent intaglio modules (205) (shown on Fig.2D) of the twodimensionalintaglio pattern.

[037] Fig.2A shows the two-dimensional intaglio pattern (201)disclosed herein and the multitone intaglio printed referenceareas ("RA"): L (light tone area), M (middle tone area), D (darktone area). The multitone reference areas were printed with thethree tones of the (small) multitone two-dimensional intagliopattern (220, of Fig.2C) comprised in the security marking. Themultitone reference areas (RA-L, RA-M and RA-D) were used todetermine with a spectrometer the ΔE* of the three tones of themultitone security pattern. The intaglio modules (205L, 205M,205D) respectively correspond to the portions of a same tone L,M and D of the multitone intaglio lines forming the twodimensionalintaglio pattern (220).

[038] Fig.2B shows an enlarged picture of a part of the intagliopattern reproduction of Fig.2A. As illustrated in Fig.2B,intaglio lines (210), surrounding the (small) two-dimensionalintaglio pattern (220), and forming the boundaries of a cellcontaining said two-dimensional intaglio pattern (220) (seFig.2C-D), are clearly visible in this enlarged representation.As illustrated in Fig.2B, in this example, the intaglio lines(210) were printed in thick intaglio layer (dark tone) and weresurrounding the two-dimensional intaglio pattern (220) likewalls of an height greater than any height of the intagliomodules of the two-dimensional intaglio pattern, thus providingprotection to the (small) two-dimensional intaglio pattern(220), and the full two-dimensional intaglio pattern (201),against mechanical abrasion.

[039] Fig.2C shows a picture of the Fig.2B as seen under amagnifying glass with the magnified two-dimensional intagliopattern (220) disclosed herein. Even under the magnifying glass,the multitone characteristics of the two-dimensional intagliopattern are hardly discernable with naked eyes, while thereading and authentication device according to the invention, inparticular the smartphone, is able to distinguish the differenttones and to decode and authenticate the two-dimensionalintaglio pattern (201).

[040] In order to mimic multitone characteristics of intaglioprinted security markings, grey levels (Ex2-Ex3 and CEx1) orcyan levels (Ex4 and CEx2) were assigned to the file layercorresponding to the three file layers corresponding to thelight / middle / dark tones (i.e. L / M / D tones) of the twodimensionalintaglio patterns and to the reference areas (RA-L,RA-M and RA-D), using Adobe Illustrator (AI). The referenceareas (RA-L, RA-M and RA-D) were printed as squares with thefollowing dimensions: 1.3 cm x 1.3 cm.

[041] The two-dimensional intaglio patterns of the securitymarkings corresponding to Ex2-Ex4 and the comparative examplesCEx1-CEx2 were printed using a laser printer (Konica Minoltabizhub C558; Quality printer parameters: Resolution: 600 dpi;Pattern: fine; Image compression: best quality; Edgeenhancement: none) (Fig.2A).

[042] The CIE color difference ΔE* values between the threereference squares (RA-L, RA-M and RA-D), i.e. the ΔE* value L-Mbetween the light (RA-L) and the middle (RA-M) reference squaresand the ΔE* value M-D between the middle (RA-M) and the dark(RA-D) reference squares were measured using a spectrophotometerDC 45 from Datacolor (see below Table 1). The values ΔE* beingcalculated according to a known formula, e.g. as detailed atparagraph 102 of the cited patent EP 2 956 514 B1.

[043] The two-dimensional intaglio patterns of examples Ex2-Ex4and the comparative examples CEx1-CEx2 were analyzed using aSamsung Galaxy S10 or S21 smartphone disposed at a distance ofabout 35 cm from the prints Ex2-Ex4 or CEx1-CEx2 (i.e. at adistance about 3.5 times larger than the detection distance usedfor authenticating the security marking of the banknote specimenof example Ex1, see Fig.1).

[044] As shown in Table 1 below, the two-dimensional intagliopatterns of examples Ex2-Ex4 having ΔE* ≥ 2.0, in particular ΔE*≥ 3.13, have been authenticated by the smartphone (seecorresponding Y, "Yes", in the column "Authentication" of Table1). Contrary to the two-dimensional intaglio patterns ofexamples Ex2-Ex4, the comparative example CEx1 having ΔE* valuesof 0.19 and 1.58, could not be authenticated by the smartphone,as indicated by the negative and zero Y-values (seecorresponding N, "No", in the column "Authentication" of Table1). The comparative example CEx2 having ΔE* values of 1.6 and2.9 was just below the authentication threshold of thesmartphone as indicated by the zero Y-value. The tone values (orcolor values) were measured as the luminance Y-values accordingto the well-known YUV color encoding system. The YUV color formatdescribes a color by using the color components luminance andchrominance. The luminance component (Y) represents thebrightness information of a color, the chrominance components (Uand V) contain the color differences.Table 1wherein1) L means light, M means middle and D means dark,2) L-M corresponds to ΔE* between the two consecutive toneslight and middle; and M-D corresponds to ΔE* between the twoconsecutive tones middle and dark,3) values in % set in Adobe Illustrator(e.g. for the greyexamples, 0% would correspond to white and 100% would correspondto black), and4) Luminance difference Y-values correspond to the averagedvalue over the entire printed pattern in arbitrary units.The CIE tone difference (or CIE color difference) has beenintroduced by the International Commission on Illumination (CIE)in 1976. Given any two colors (or tones of a color) in theCIEL*a*b* color space, i.e. (L1, a1, b1) and (L2, a2, b2), theΔE* formula is defined as:Equationwhere:L1 is the CIE L* value of tone for a zone 1 of the imagea1 is the CIE a* value of tone for a zone 1 of the imageb1 is the CIE b* value of tone for a zone 1 of the imageL2 is the CIE L* value of tone for a zone 2 of the imagea2 is the CIE a* value of tone for a zone 2 of the imageb2 is the CIE b* value of tone for a zone 2 of the imageThe CIELAB space is three-dimensional and covers the entirerange of human color perception: L* for perceptual lightness,and a* and b* for the four unique colors of human vision: red,green, blue, and yellow. The lightness value, L* defines black at0 and white at 100. The a* axis is relative to the green-redopponent colors, with negative values toward green and positivevalues toward red. The b* axis represents the blue-yellowopponents, with negative numbers toward blue and positive towardyellow.

[045] Intaglio inks as disclosed herein may be machine readableinks, such as for example magnetic inks, luminescent inks, IRabsorbing inks and forensic inks, have been widely used in thefield of security documents, in particular for banknotesprinting, to confer the security document an additional covertsecurity feature. The protection of security document againstcounterfeit and illegal reproduction provided by covert securityfeatures relies on the concept that such features typicallyrequire specialized equipment and knowledge for their detection.

[046] The intaglio ink described herein may comprise one or moremachine readable materials selected from the group consisting ofluminescent materials known in the art, magnetic materials knownin the art, IR absorbing materials known in the art, forensicmarkers or taggants known in the art, and mixtures thereof suchas to provide security markings with enhanced counterfeitingresistance, provided that said one or more machine readable donot negatively interfere with the authentication methoddescribed herein. As used herein, the term "machine readablematerial" refers to a material which exhibits at least onedistinctive property which is detectable by a device or amachine, such as for example a magnetic detector (when themachine readable materials have magnetic properties) or an IRcamera(when the machine readable materials have IR-absorbingproperties), or a spectrophotometer (when the machine readablematerials have luminescent properties) so as to confer a way toauthenticate said security markings by the use of a particularequipment for its detection and / or authentication.

[047] Should the two-dimensional intaglio pattern of thesecurity marking described herein be made with the intaglio inkcomprising the one or more machine readable materials describedherein, said security marking may be further authenticated withspecialized equipment, said equipment being different from thedevice, for example the smartphone, for authenticating describedherein. Moreover, these machine readable materials may be usedalso for authenticating the two-dimensional intaglio pattern ofthe invention (as an additional security level).

[048] Luminescent materials may also be detected with the nakedeye provided that they emit in the visible range upon suitableexcitation. Luminescent materials may be inorganic (inorganichost crystals or glasses doped with luminescent ions), organicor organometallic (complexes of luminescent ion(s) with organicligand(s)) substances). Luminescent materials in pigment formhave been widely used in inks (see e.g. US 6 565 770, WO2008 / 033059 A2 and WO 2008 / 092522 A1). Examples of luminescentmaterials include among others sulfides, oxysulfides,phosphates, vanadates, etc. of non-luminescent cations, dopedwith at least one luminescent cation chosen from the groupconsisting of transition-metal and the rare-earth ions; rareearth oxysulfides and rare-earth metal complexes such as thosedescribed in e.g. WO 2009 / 005733 A2,in EP 0 985 007 B1, US6,180,029 B1, US 6,153,123 A, or in US 7 108 742. Examples ofinorganic materials include without limitation La202S:Eu,ZnSi04:Mn, and YV04:Nd. Other examples include those disclosedin WO 2014 / 083145 A1, WO 2012 / 160182 A1, WO 2013 / 068275 A1, WO2013 / 079521 A1, WO 1998 / 036888 A1, US 2006 / 0083694 A1, WO2011 / 002960 A1, WO 2011 / 041657 A1. IR absorbing materialsinclude organic compounds, inorganic materials, glassescomprising substantial amounts of IR-absorbing atoms or ions.Typical examples of IR absorbing compounds include among otherscarbon black, quinone-diimmonium or ammonium salts, polymethines(e.g. cyanines, squaraines, croconaines), phthalocyanine ornaphthalocyanine type (IR-absorbing pi-system), dithiolenes,quaterrylene diimides, metal (such as for example transitionmetal or lanthanide) salts (such as for example fluorides,chlorides, bromides, iodides, nitrates, nitrites, sulfites,sulfates, phosphates, carbonates, borates, benzoates, acetates,chromates, hexaborides, molybdates, manganates, ferrates,organosulfates, organosulfonates, organophosphonates,organophosphates and phosphono-tungstanates), metal oxides (suchas for example indium tin oxide, antimony tin oxide in nanoparticulateform, and doped tin(IV) oxide), metal nitrides.Examples may be found for example in WO 2007 / 060133 A2, WO2007 / 132214 A1, WO 2019 / 219250, EP 3 068 728 B1 and WO2018 / 178021. According to one embodiment, the security markingdescribed herein comprises a first portion consisting of amachine readable security feature IR-absorbing and a secondportion consisting of the security marking comprising one ormore compounds absorbing in another region of theelectromagnetic spectrum (UV or Vis) so as to form a combinedsecurity marking. The first and second portions of the combinedsecurity marking described herein may be adjacent, overlappingeach other or spaced apart. Magnetic materials include magneticsubstances such, as high- or medium-coercivity substances (suchas for example iron oxides, barium ferrites, strontium ferritesor black iron oxides) and core-shell magnetic pigment particles.Examples of suitable magnetic materials include magnetic coreshellpigments particles comprising a magnetic core (preferablymade of nickel, cobalt, iron and iron containing alloys andoxides) surrounded by one or more additional layers made of oneor more materials selected from the group consisting of organicmaterials and group of inorganic materials such as thosedescribed for example in WO 2010 / 115986 A2 and WO 2016 / 005158A1. The organic materials described herein are preferablyselected from the group consisting of polyacrylates,polystyrenes, parylenes, alkoxysilanes and mixtures thereof. Theinorganic materials described herein are preferably selectedfrom the group consisting of metals (preferably selected fromthe group consisting of silver, aluminum and gold), metal oxides(preferably selected from the group consisting of MgO and ZnO,Al2O3, Y2O3, Ln2O3 (wherein Ln is a lanthanide), SiO2, TiO2, ZrO2,CeO2 and mixtures thereof) and metal sulfides (preferablyselected from the group consisting of ZnS; CaS and mixturesthereof).

[049] Fig.3A shows an example of a two-dimensional referencepattern (300) according to the prior art, comprising a specificarrangement of 32 symbols (310,320,330,340), each having someoval shape. Fig.3B shows an example of two-dimensional elementsE1 (black dots) and E2 (non-printed elements, or "white" dots)specifying the states of some symbols of the reference pattern(300). The reference pattern (300) with its symbols and theirstates can encode 32 bits of data.

[050] Fig.4A is an example of an arrangement of symbols of areference pattern (400) of a two-dimensional encoded pattern, asused in an embodiment of the invention. A two-dimensionalencoded pattern may comprise only one reference pattern, butmore often it comprises many reference patterns disposedaccording to a given arrangement, each reference patternencoding a part of the information contained in the twodimensionalencoded pattern. According to the invention, a (twodimensional)reference pattern encodes its corresponding(unique) identification number. In this example, the referencepattern (400) comprises 18 symbols (S1,S2,..., S18), that aredisposed according to a specific arrangement. The symbols mayhave any shape, but in this example, for simplicity, only twoshapes of oval type are possible. These symbols are used forencoding bits of data of the part of the information to beencoded in the reference pattern. The symbols belongs to afinite set of symbols, each symbol in the arrangement allows toencode data according to a state of said symbol. Fig.4B shows anexample of reference pattern wherein the states of the symbolsare specified. In this example, a state of a symbol S consistsin a specific arrangement of only two two-dimensional elements,i.e. E1 and E2, within the symbol S, each element E having acorresponding value of a parameter p(E). The parameter of theelement E1 of each state of a symbol has a positive value p(E1)different from a zero value of a parameter of the other elementE2 of said symbol. Elements E2 having a parameter value p(E2) =0, they are not shown on the symbols S of Fig.4B (although theyoccupy a corresponding site within S), as they correspond to anon-printed or "neutral" element (or "white" element, like awhite dot, when represented on a white surface). Alternatively,an element E2 could be represented, for example, with a crosseddot. A reference pattern as used in the invention may only havetwo types of elements having distinct positive parameter values:here the elements corresponding to black dots and thosecorresponding to grey dots. Of course, the allowed parametervalues for the distinct elements must generally only havedistinct values, but it is always possible to assign distinctpositive values to the distinct elements of type E1, and a zerovalue to the neutral element E2 via a mere one-to-one mapping,this is just a convention (non-limitative). The (twodimensional)elements E1 of a symbol may have any shape, but inthis example, for simplicity, they all have a same dot shape ofsame size. For example, the element E1 of symbol S1 is shown asa black dot ("black element"), while the element E1 of theneighbor symbol S2 is shown as a grey dot ("grey element"), andtheir respective parameter values are distinct (and positives)so that, for example, p(black dot E1) > p(grey dot E1) >p(E2)=0. Consequently, there are only two types of symbols inthe reference pattern, those with a black dot, corresponding to"black symbols" (or "dark symbols"), and those with a grey dot,corresponding to "grey symbols" (or "light symbols"). In theexample shown on Fig.4B, the reference pattern (400) has 18symbols of two elements (E1, E2) and thus can encode 18 bits ofdata.

[051] For a two-dimensional intaglio pattern of a given color,its intaglio modules can have tones from a given tone palettefor that color and form a specific pattern of intaglio lines.According to the invention, a two-dimensional intaglio patternis in one-to-one correspondence with an associated twodimensionalencoded pattern, and they both encode the sameinformation. A two-dimensional intaglio pattern may compriseonly one base intaglio pattern, but more often it comprises manybase intaglio patterns, each base intaglio pattern encoding apart of the information contained in the two-dimensional encodedpattern (and thus, in the two-dimensional intaglio pattern). Theplurality of base intaglio patterns are arranged within the twodimensionalintaglio pattern so as to form the specific patternof intaglio lines corresponding to said two-dimensional intagliopattern. According to the invention, each base intaglio patternof the two-dimensional intaglio pattern is in one-to-onecorrespondence with an associated reference pattern of the twodimensionalencoded pattern, this two-dimensional encodedpattern being associated with said two-dimensional intagliopattern, and the base intaglio pattern and its associatedreference pattern both encode a same part of the informationencoded in the two-dimensional intaglio pattern. This part ofthe information corresponds to a unique identification numberattributed to this reference pattern. Also, due to the aboveone-to-one correspondence, each base intaglio pattern belongs toa finite set of base intaglio patterns (necessary for encodingthe information). Accordingly, encoded information in each baseintaglio pattern of the two-dimensional intaglio patternspecifies a reference pattern identification number of itsassociated reference pattern.

[052] According to the invention, an intaglio module of the twodimensionalintaglio pattern corresponds to either:- a two-dimensional element (E1) of a symbol of the referencepattern of the two-dimensional encoded pattern having a positiveparameter value (p(E1)> 0); or- a portion of an intaglio line corresponding to a lineconnecting two elements (E1) having a same positive parametervalue (p(E1)), respectively belonging to two distinct adjacentsymbols Si and Sj (i=j) of a reference pattern of the twodimensionalencoded pattern associated with the two-dimensionalintaglio pattern.For example, in case the two-dimensional intaglio pattern has ablue color, and a base intaglio pattern of this two-dimensionalintaglio pattern has intaglio modules of two allowed tones "darkblue" and "light blue" (from a tone palette of the blue color),the associated reference pattern of the base intaglio pattern(belonging to the two-dimensional encoded pattern associatedwith the two-dimensional intaglio pattern) will have symbolscomprising either a black dot as element E1 (e.g. correspondingto the dark blue tone) or a grey dot as element E1 (thencorresponding to the light blue tone).

[053] Fig.4C shows details of an intermediate step of theconstruction of a base intaglio pattern (430), shown alone onright side of Fig.4C, starting from the reference pattern (400)of Fig.4A (shown of left side of Fig.4C). In this example, forsimplicity, the respective tones of the intaglio modules of thebase intaglio pattern and of the elements E1 of its associatedreference pattern are the same, i.e. black and grey. In fact,Fig.4C shows the reference pattern (400) in superimposition withthe base intaglio pattern (430) to clearly show the one-to-onecorrespondence between them (and with the respective symbols,and their states, of the reference pattern). For example, theintaglio module (431) of black tone corresponds to the blackelement E1 of symbol S1, the intaglio module (432) of grey tonecorresponds to an intaglio line corresponding to a lineconnecting the two grey elements E1 (having a same positiveparameter value) respectively belonging to the two distinctadjacent symbols S2 and S15 of the reference pattern (400), theintaglio module (433) of black tone corresponds to an intaglioline corresponding to a line connecting the two black elementsE1 respectively belonging to the two distinct adjacent symbolsS11 and S18 of the reference pattern (400) (this line has ashape of an inverted Y), and the intaglio module (434) of greytone is a portion of intaglio line that corresponds to thesingle grey element E1 of symbol S8 of the reference pattern(400). The intaglio modules (431) and (432) form two portions ofa continuous intaglio line. Thus, from the above rules forconnecting the modules, for each allowed reference pattern, itis possible to build an associated base intaglio pattern, andthe correspondence between their identification numbers (inpractice, they have the same identification number, i.e. that ofthe associated reference pattern) unambiguously determines theirone-to-one correspondence.

[054] On top of Fig.4D, a partial view of an example of twodimensionalintaglio pattern (450) is illustrated, showing onlytwo base intaglio patterns (430) and (440), respectivelycorresponding (one-to-one) with two reference patterns (400) and(410) of an associated two-dimensional encoded pattern (420)shown on bottom of Fig.4D. In the example shown on Fig.1, thesecurity marking (100) comprises about 1400 two-dimensionalintaglio patterns in as many cells, each two-dimensionalintaglio pattern comprising 16 reference patterns, eachreference pattern comprising 18 symbols of two elements of twopossible tones. As clearly visible on the two-dimensionalintaglio pattern (450), its intaglio modules form a specificpattern of intaglio lines (like interconnected branches of agraph forming a continuous path). In a variant of the aboveexample (not shown), a two-dimensional intaglio pattern may alsocontain discrete intaglio modules (i.e. not connected with otherintaglio modules so as to form a continuous intaglio line).

[055] A two-dimensional intaglio pattern e.g. like the oneschematically illustrated on Fig.4D, or the one of the securitymarking (100) shown on Fig.1, is obtained by using an engravedplate for an intaglio printing machine (not shown), saidengraved plate comprising grooves of variable depth adapted toreceive an intaglio ink to print on a surface of a substratesaid two-dimensional intaglio pattern. The variations of depthproviding printed intaglio lines of corresponding variableheight that provide the multitone aspect of the lines.

[056] The variable engraving depth and the specific arrangementof engraved grooves of variable engraving depth of the engravedplate allow, once appropriately inked with the intaglio ink, toprint on a surface of a substrate the two-dimensional intagliopattern having the color of the intaglio ink and comprising aplurality of intaglio modules forming a specific pattern ofintaglio lines corresponding to the inked grooves, each intagliomodule having a tone of said color: this tone belongs to a tonepalette of a plurality of distinct reference tones of the color(ranging from a darker tone to a lighter tone) that correspondto the distinct engraving depths of the grooves on the intaglioplate. The choice of intaglio ink and depth variations of thegrooves is such that a CIE color index difference ΔE* betweenany two consecutive tones of the tone palette is greater orequal than 2.0 (that can be detected with a mere smartphone),and allow to print the two-dimensional intaglio pattern on thesurface in which a width ε of an intaglio line is greater than20 μm and less or equal than 50 μm.

[057] The above engraved plate can be used in an intaglioprinting machine for printing a two-dimensional intaglioprinting according to the invention. Thus, an example of theproduction of a two-dimensional intaglio pattern of a securitymarking, to be marked on a surface of a substrate, involves thefollowing operations of:- generating a two-dimensional encoded pattern (420) comprising16 reference patterns, particularly the two reference patterns(400) and (410), by encoding a corresponding portion of receivedinformation (i.e. their respective identification numbers) intosaid two reference patterns (400, 410) belonging to a finiteset of 16 reference patterns forming the two-dimensional encodedpattern, each reference pattern including a specific arrangementof a plurality of symbols belonging to a finite set of symbols,each symbol in the arrangement allowing to encode data accordingto a state of said symbol;- a state of a symbol consisting in a specific arrangement of afirst two-dimensional element E1 and a second distinct twodimensionalelement E2 within the symbol, each element E havinga corresponding value of a parameter p(E), the parameter of thefirst element E1 having a positive value different from a zerovalue of the parameter of the second element E2;- each reference pattern only comprises dark symbols (e.g.reference pattern (400) has the dark symbols S1, S3, S5, S7, S9,S11, S13, S15 and S18) and light symbols (S2, S4, S6, S8, S10,S12, S14, S16 and S17), the parameter value of the first elementof a dark symbol corresponding to a darker tone of an associatedpair of allowed tones (e.g. this parameter value designates afirst element E1 being a black dot), selected from a tonepalette of a plurality of distinct reference tones of a colorassociated with the two-dimensional encoded pattern (i.e. thecolor of the two-dimensional intaglio pattern to be printed),and the parameter value of the first element of a light symbolcorresponding to the lighter tone of said associated pair ofallowed tones (e.g. this parameter value designates a firstelement E1 being a grey dot);- the two-dimensional intaglio pattern (450) to be producedbeing in one-to-one correspondence with the generated twodimensionalencoded pattern (420) and encoding the sameinformation, and comprising a plurality of intaglio modules,each intaglio module having a tone of said associated color, atone of an intaglio module being selected from the tone palette;- the two-dimensional intaglio pattern comprising two baseintaglio patterns (430) and (440), each base intaglio patternencoding a portion of said information and being in one-to-onecorrespondence with an associated reference pattern(respectively, 400 and 410) of the two-dimensional encodedpattern (420);- each base intaglio pattern only comprising intaglio modules ofwhich tones belong to the pair of allowed tones of theassociated reference pattern (in this example of Fig.4, the twoallowed tones are the black and the grey, i.e. the same tones asfor the reference pattern);- an intaglio module of the two-dimensional intaglio pattern(450) associated with the generated two-dimensional encodedpattern (420) corresponding to eithera first element of a symbol of a reference pattern of thetwo-dimensional encoded pattern (e.g. the dark element E1 ofsymbol S1 of the reference pattern 400); ora portion of an intaglio line corresponding to a lineconnecting two elements having a same positive parameter valuep, respectively belonging to two distinct adjacent symbols of areference pattern of the two-dimensional encoded pattern (e.g.portion (432) of intaglio line of base intaglio pattern (430),linking grey elements E1 of adjacent symbols S2 and S15 of thereference pattern (400));(ii) engraving a plate of an intaglio printing machine withvariable engraving depth adapted to receive an intaglio ink ofthe color of the two-dimensional intaglio pattern (450) and toreproduce the intaglio modules according to the generated twodimensionalencoded pattern (420)); and(iii) inking the plate with the intaglio ink and using the inkedplate with the intaglio printing machine to print on a surfaceof a substrate the corresponding two-dimensional intagliopattern (450).

[058] Fig.5 is a flowchart illustrating an example of a methodof decoding a two-dimensional intaglio pattern of the inventionwith a smartphone equipped with a camera, a processor, a memoryand a RGB ("Red Green Blue") light sensor. The processor isadapted (i.e. specifically programmed) to carry out imageprocessing operations and decoding operations on a digital imageof a genuine two-dimensional intaglio pattern to extractinformation encoded in the two-dimensional intaglio pattern. Thegenuine two-dimensional intaglio pattern considered forillustrating the decoding operations is that of Fig.4D. Thisgenuine two-dimensional intaglio pattern (450) merely comprisestwo base intaglio patterns (430, 440), each base intagliopattern (430, 440) including a plurality of intaglio modulesforming a (known) pattern of intaglio lines specific to the twodimensionalintaglio pattern (450). Each base intaglio pattern(430, 440) only comprises intaglio modules of two alloweddistinct tones (dark and grey), corresponding to the allowedpair of tones {dark, grey}, i.e. dark modules (in black) andgrey modules. These dark and grey tones of the referencepatterns in fact generally stand for two distinct tones of agiven color, e.g. respectively dark blue and mid blue tones, ofa two-dimensional intaglio pattern of which color is blue. As anexample, for all the intaglio modules, the CIE color differenceΔE* between these two tones verifies ΔE* ≥ 2.5. The width ε ofthe intaglio lines of the two-dimensional intaglio pattern areof about 30 μm (± 2 μm).

[059] The RGB light sensor of the camera is adapted to detect acolor of intaglio modules forming the genuine intaglio patternand the above mentioned two distinct tones of the tone palette,e.g. {dark blue, mid blue} of a blue color, to obtain a digitalimage of the two-dimensional intaglio pattern. The twodimensionalintaglio pattern (450) is in one-to-onecorrespondence with the associated two-dimensional encodedpattern (420), and each base intaglio pattern (430, 440) is inone-to-one correspondence, respectively, with each of thereference patterns (400, 410) forming said two-dimensionalencoded pattern (420). The identification numbers of the baseintaglio patterns being the same as the identification numbersof the corresponding (associated) reference patterns(respectively encoded in these reference patterns). According tothe invention, in order to decode the information encoded into atwo-dimensional intaglio pattern, it is necessary to firstdetect on a digital image of the two-dimensional intagliopattern taken by the camera, the intaglio modules that form thetwo-dimensional intaglio pattern, and then verify that thedetected intaglio modules are in one-to-one correspondence withthe dark and grey elements of the arrangement of symbols of theassociated two-dimensional encoded pattern. According to theinvention, the mapping rules for establishing a one-to-onecorrespondence between a detected intaglio module and states ofsymbols of the two-dimensional encoded pattern are that anintaglio module of the genuine two-dimensional intaglio patternmay only correspond to either- an element of a symbol of the reference pattern of the twodimensionalencoded pattern having a positive parameter value(here, corresponding to a dark element or a grey element); or- a portion of an intaglio line corresponding to a lineconnecting two elements having a same positive parameter value p(here, connecting two dark elements or two grey elements),respectively belonging to two distinct adjacent symbols of areference pattern of the two-dimensional encoded pattern.The memory of the smartphone storing a finite set of allowedreference patterns forming a known two-dimensional encodedpattern associated with the known genuine two-dimensionalintaglio pattern.

[060] In order to detect on the digital image of the twodimensionalintaglio pattern the intaglio modules that form thetwo-dimensional intaglio pattern, the processor performs animage processing operation on the acquired digital imageconsisting in using a movable window, having a size of a baseintaglio pattern, that is shifted with respect to the digitalimage to successively scan the full digital image (see also the"analysis window" in the cited document EP 2 780 865 B1, ande.g. its claim 12). At each position of the window on thedigital image, the RGB intensities (here, the respectiveintensities of the Red, Green and Blue components are justsummed) of the pixels within the window are analyzed to detect apresence of distinct groups of pixels of two different tones:these detected groups of pixels corresponding to the respectivedigital images of a set of intaglio modules of two differenttones. In practice, a dynamic range of the digital image isdetermined from the tone values measured by the RGB sensor, anda check is performed to control that a difference of measuredtone values of any two intaglio modules is indeed greater thanat least 1% of the determined dynamic range. These intagliomodules (in fact, their corresponding groups of pixels) detectedthrough the window are thus of two distinct tones, those of adarker tone and those of a lighter tone.

[061] Each time a set of intaglio modules is detected throughthe window, the processor checks whether, respectively, thesegroups of pixels can be mapped, in a one-to-one correspondence,on one of the allowed reference patterns stored in the memoryand corresponding to a base intaglio pattern of the genuine twodimensionalintaglio pattern: the groups of pixels beingtentatively mapped according to the above mapping rules on thetwo-dimensional elements E1 of the symbols of a candidatereference pattern, selected from the stored set of referencepatterns forming the two-dimensional encoded pattern. In casesuch a mapping is possible, the corresponding candidatereference pattern is decoded, and its identification number isextracted (also giving the identification number of theassociated candidate base intaglio pattern). Then, the candidatereference pattern is validated, as well as the correspondingbase intaglio pattern, and these validations are stored in thememory in relation to the decoded identification number of thecandidate reference pattern. If the tentative mapping fails,then the candidate reference pattern is rejected and a newcandidate reference pattern is considered for a correspondingnew tentative mapping (and new further check), until all theallowed reference patterns stored in the memory have beentested. In case none of the possible candidate referencepatterns can be validated, the window is moved (shifted) withrespect to the digital image toward a new location, and theabove operations are repeated by the processor until all thedigital image is scanned through the window.

[062] Clearly, if no candidate reference pattern can bevalidated after the scan of the full digital image, the imagedtwo-dimensional intaglio pattern either corresponds to a(strongly) damaged genuine two-dimensional intaglio pattern orto a fake two-dimensional intaglio pattern. The above operationsare summarized on the flowchart of Fig.5 relating to an exampleof embodiment of the decoding according to the invention,wherein:- at step 500, the decoding starts for a given two-dimensionalintaglio pattern (printed on a surface of a substrate, e.g. asecurity document);- at step 510, the two-dimensional intaglio pattern is imagedwith the camera of the smartphone and the acquired digital imageis stored in the memory;- at step 520, a candidate reference pattern from the set ofstored reference patterns is selected;- at step 530, the groups of pixels are respectively tentativelymapped, according to the mapping rules, on the black elements ofthe symbols of the candidate reference pattern and on the greyelements of the symbols of the candidate reference pattern, andif such a mapping is possible (Y) the following step 540 isperformed, or if the mapping is not possible (N) the step 560 isperformed;- at step 540, the corresponding candidate reference pattern isdecoded, and its identification number is extracted (also givingthe identification number of the corresponding candidate baseintaglio pattern);- at step 550, the candidate reference pattern and thecorresponding base intaglio pattern are validated, and thesevalidations are stored in the memory in relation to the decodedidentification number of the candidate reference pattern;- at step 560, the candidate reference pattern is rejected and anew candidate reference pattern is considered for acorresponding a new tentative mapping (and new further check) atstep 520 if all the possible candidate reference patterns fromthe allowed reference patterns stored in the memory have notbeen tested ("Y"), and if on the contrary all the possiblecandidate reference patterns from the allowed reference patternsstored in the memory have been tested ("N"), the step 570 isperformed;- at step 570, if all the digital image is scanned the step 580is performed ("Y"), if not ("N") the window is moved (shifted)with respect to the digital image to a new location, and theabove operations from step 520 are repeated by the processor;- at step 580, the operations stop.

[063] There are many possibilities for checking a one-to-onecorrespondence between the (pixels of the) intaglio moduleswithin the window and the two-dimensional elements of thesymbols of a (candidate) reference pattern, in accordance withthe mapping rules. For example, it is possible to use a samplinggrid corresponding to the arrangement of symbols, including thesites occupied by their corresponding two-dimensional elementsE1, of the (candidate) reference pattern and disposed within thewindow (via image processing with the processor). Then, it ispossible to detect through the grid the pixels relating to eachintaglio module that are comprised within the sites of the twodimensionalelements E1 of the respective symbols of the(candidate) reference pattern. As illustrated on Fig.4A, thesampling grid within the window is in fact the "grid" formed bythe respective boundaries of the symbols corresponding to the(candidate) reference pattern, once the reference pattern hasbeen put to scale with the size of the window: the groups ofpixels relating to an intaglio module can then be directly(tentatively) located within the symbols, and checking whethersaid group of pixels is fully, or only partially, located withina symbol at a location corresponding to that of an element E1 ofthe symbol is easy via image processing. A criterion may be usedin case of a partial location of a group of pixels within asymbol: e.g., in case more than a quarter of said pixels arewithin the boundary of the symbol, but out of a site within thesymbol corresponding to that of an element E1, this group ofpixels (and thus, the corresponding detected intaglio module) isconsidered as not corresponding to the element E1 of the symbol.

[064] According to the invention, a two-dimensional intagliopattern of a security marking can be authenticated, oncedecoded, for example with the smartphone used for decoding (seeabove), by both verifying a multitone aspect of said twodimensionalintaglio pattern, and determining that a registererror between its various intaglio modules that areinterconnected so as to form a pattern of intaglio lines isbelow a given small value (e.g.10 μm and preferably 5 μm), evenif the typical width of an intaglio line of said two-dimensionalintaglio pattern is greater than 20 μm (and less than or equalto 50 μm). A characteristic feature of the authentication of atwo-dimensional intaglio pattern according to the invention isthe synergy existing between the operations of decoding of thetwo-dimensional intaglio pattern, verifying its multitone aspectand determining whether a register error between the variousparts of its specific pattern of intaglio lines is below a giventhreshold. Indeed, the decoding of the intaglio modules of abase intaglio pattern, together with tiny shifts performedaround their positions at decoding stage on a digital image ofsaid base intaglio pattern, with respect to a sampling "grid"formed by the arrangement of symbols of the associated referencepattern, in order to maximize (on average) a difference of tonevalues between elements E1 and E2 of the symbols of the grid,provides a robust and precise estimation of a register errorbetween intaglio modules of distinct tones, thus allowing toeasily detect any fake marking obtained in particular viamultitone offset printing, even with a mere smartphone.

[065] Fig.6A illustrates an example of a base intaglio pattern(600) of a two-dimensional intaglio pattern (not shown) thatcomprises a plurality of base intaglio patterns, and itsassociate reference pattern (610) of a two-dimensional encodedpattern (not shown) associated with the two-dimensional intagliopattern according to the invention. As seen above, at decodingstage of the base intaglio pattern (600), a mapping is(approximately) established between detected groups of pixelscorresponding to intaglio modules (typically, there are at least3 pixels per module) and states of the symbols belonging to thereference pattern. However, this mapping is not necessarily"perfect", as it is illustrated on Fig.6C wherein the detectedgroups of pixels on the digital image of a base intagliopattern, when superimposed to the arrangement of symbols (withthe sites of their elements E1 and E2 shown with dotted lines)of the sampling grid (615) corresponding to the referencepattern (610) shown on Fig.6B (corresponding to the referencepattern of Fig.4A), once put to scale, may partly overlap theindicated boundaries of the symbols. Clearly, certain groups ofpixels are not perfectly superimposed to the sites of the twodimensionalelements (dark or grey) of the reference pattern.For example, the pixels relating to the dark intaglio modulecorresponding to the dark element E1 of symbol S7 on Fig.6C arepartly shifted out of the symbol S7 (nevertheless, the rightstate of symbol S7 can be retrieved from the pixels locatedwithin the site of element E1). This means that a measureddifference of tone, for a symbol (e.g. S7) of the sampling grid,between pixels of the digital image in the window correspondingto the element E1 of the symbol and the neutral element E2, i.e.between the pixels respectively located within the sitescorresponding to the elements E1 and E2 of the symbol, will notreach its standard value (some light intensity being lost due tothe pixels shifted out of the symbol). The standard value beingreached when the group of pixels corresponding to E1 is well("perfectly") positioned within the symbol. Nevertheless, amapping can still be obtained as there are still some pixels atthe site of element E1 in the symbol (and thus a difference ofparameter values between E1 and E2 of the symbol still exists),and the base intaglio pattern can be decoded.

[066] Fig.6C illustrates the case of a base intaglio patternthat has been decoded although its detected intaglio modules arenot perfectly superimposed to the symbols of the associatedreference pattern. The corresponding positions of the intagliomodules are in fact the positions of the groups of pixelsdetected via image processing on the digital image,corresponding to the intaglio modules. They are located withinthe symbols. As explained above, it is indeed possible that abase intaglio pattern can be decoded while its intaglio modulesare not in register: for example, some pixels may be beyond aborder of a site of an element of a symbol, although a majorityof pixels of each group of pixels corresponding to therespective intaglio modules are correctly located within thesites of the elements of corresponding symbols of the associatedreference pattern (the states are correctly identified), andthus decoding of the base intaglio pattern is still possible.Below, these decoding positions are considered as constitutinginitial positions of the groups of pixels on the digital image,or initial positions of the detected intaglio modules in short.

[067] In order to verify a multitone aspect of a decoded baseintaglio pattern, i.e. of a validated base intaglio pattern asexplained above, the following operations are performed with thesmartphone, wherein locations on the digital image of thevalidated base intaglio patterns and their correspondingassociated validated reference pattern identification numbersare stored in the memory of the smartphone:- determining, from the associated validated reference patternidentification number (obtained at decoding), its allowed pairof distinct tones {Ta, Tb} (stored in the memory);- measuring on the digital image a printed tone value of eachintaglio module of the validated base intaglio pattern; e.g. bymeasuring with the RGB light sensor of the smartphone the RGBcomponents of the corresponding pixels, detected through theabove mentioned sampling grid corresponding to the associatedvalidated reference pattern, at the sites of the elements E1 ofthe symbols;- determining a dynamic range of the digital image of thevalidated base intaglio pattern, for the allowed pair of tones,from the measured tone values (here, the dynamic range is thedifference between the highest measured tone value and lowestmeasured tone value);- checking whether a difference of measured mean tone values ofthe intaglio modules verifies the condition that it is greaterthan at least 1% of the dynamic range and,- in case the condition is satisfied, deciding that thevalidated base intaglio pattern has said multitone aspect,- in case the condition is not satisfied, deciding that thevalidated base intaglio pattern has not said multitone aspectandis not genuine.More precisely, the above difference is calculated as follows:-first, an average printed tone value <Ta> of the intagliomodules of the validated base intaglio patterns corresponding tothe allowed tone Ta is calculated as an average of the measuredprinted tone values of the intaglio modules corresponding to theallowed tone Ta. Also, an average printed tone value <Tb> of theintaglio modules of the validated base intaglio patternscorresponding to the allowed tone Tb is calculated as an averageof the measured printed tone values of the intaglio modulescorresponding to the allowed tone Tb. Then:- a difference between the calculated average printed tonevalues <Ta> and <Tb> is calculated, and- it is checked, for each allowed pair of tones {Ta, Tb},whether a multitone criterion that a difference between thecalculated average printed tone values <Ta> and <Tb> is greaterthan 1% of the dynamic range is met.Thus, the validated base intaglio patterns that have not amultitone aspect are rejected and the two-dimensional intagliopattern is considered as being not genuine.

[068] Once a multitone aspect of the validated base intagliopatterns has been established, the determination of a registererror can start based on these validated base intaglio patterns(of which locations on the digital image are stored in thememory of the smartphone). For example, considering the(validated) base intaglio pattern as illustrated on Fig.6C, thathas been decoded and has clearly a multitone aspect, thisvalidated base intaglio pattern having a corresponding pair ofallowed tones {Ta, Tb} (here, dark and grey), is observedthrough the sampling grid (615) of corresponding associatedvalidated reference pattern (see Fig.6B) disposed within thewindow (and put to scale within the window) at location in thedigital image of said validated base intaglio pattern (stored inthe memory), and the processor of the smartphone performs theoperations of:(1) measuring within the window, through the arrangement ofsymbols with respective sites of their two-dimensional elementsE1 and E2 corresponding to said validated reference pattern, theprinted tone values of the intaglio modules of the validatedbase intaglio pattern from the pixels of the digital image thatare located within the respective sites of the elements E1 andE2 of each symbol of the sampling grid. The measure of a tonevalue resulting from a sum of the respective R, G and B (Red,Green and Blue) intensity components of the detected pixels.This measuring provides initial (i.e. from positions of thepixels corresponding to the ones at the decoding stage) tonevalues attributed to the respective elements E1 and E2 of thesymbols of the validated reference pattern associated with thevalidated base intaglio pattern. Then, for each symbol of thesampling grid, a difference between the initial tone valueattributed to the element E1 and the initial tone valueattributed to the element E2 is calculated with the processor toobtain an initial differential symbol tone value for each symbolof the sampling grid (i.e. of the corresponding referencepattern), and the obtained initial differential symbol tonevalues attributed to the elements of the symbols of thevalidated reference pattern (corresponding to the pair of tones{Ta, Tb}) are stored in the memory of the smartphone. One mayconsider that these stored initial differential symbol tonevalues in fact correspond to a zero-shift. Then, for each one ofthe two allowed tones {Ta, Tb} of the validated referencepattern, a corresponding zero-shift sum is obtained by summingthe initial differential symbol tone values for the all thesymbols of the sampling grid (i.e. of the validated referencepattern) corresponding, respectively, to the tone Ta and thetone Tb, and the obtained zero-shift sum for the allowed tone Taand zero-shift sum for the allowed tone Tb are stored in thememory.(2) shifting by at least one pixel size, via image processing ofthe digital image with the processor, backward and forward alongeach of a plurality of distinct directions (see Fig.6D-E, forthe vertical direction, where forward corresponds to top as onFig.6D, and backward corresponds to bottom as on Fig.6E) withrespect to the digital image (in this example, there are onlytwo directions: the vertical and the horizontal ones), thepixels of the digital image located within the respective sitesof elements E1 and E2 of each symbol of the sampling grid ofwhich first element E1 corresponds to the allowed tone Ta toobtain corresponding shifted pixels having shifted positionswith respect to the elements of the symbols of the sampling grid(see Fig.6C); in fact, all the pixels within the window areshifted in block along the direction, via image processing:consequently, the pixels that were initially within a site of anelement E1 or E2 are shifted in block, and the (new) shiftedpixels of the digital image that are within the site of E1 or E2(respectively) are used for measuring the new tone value at thissite. As a remark, it would be equivalent to say that thewindow, together with the sampling grid, has been shifted in theopposite sense along the direction with respect to the digitalimage.- measuring through the sampling grid, respectively for thebackward shift and the forward shift along each direction (i.e.there are two shifts per direction), corresponding backwardshifted and forward shifted tone values attributed to therespective elements E1 and E2 of the symbols of the samplinggrid of which first element E1 corresponds to the allowed toneTa, from the shifted pixels of the digital image that arelocated within the respective sites of elements E1 and E2 ofeach symbol of the sampling grid (of which first elementcorresponds to the allowed tone Ta), and storing in the memorythe obtained shifted tone values. For example, with saidsampling grid of symbols, once the pixels of an intaglio modulehave been shifted from their initial positions within a symbol,only a part of said pixels possibly remains located within thesite of element E1 of the symbol, these remaining pixels areused for determining the new tone value (shifted tone value) forthis element of the symbol.- calculating, for each shift, and for each symbol of thesampling grid of which first element corresponds to the allowedtone Ta, a difference between the shifted tone value attributedto the element E1 and the shifted tone value attributed to theelement E2 to obtain corresponding shifted differential symboltone values (i.e. δ = tone value attributed to E1 - tone valueattributed to E2), and storing in the memory the obtainedshifted differential symbol tone values attributed to theelements of the symbols of the validated reference pattern (ofwhich first element corresponds to the allowed tone Ta);- for each shift, summing the calculated shifted differentialsymbol tone values of all the symbols of the sampling gridcorresponding to the allowed tone Ta, to obtain correspondingvalidated reference pattern shift sum for the allowed tone Ta;- selecting a best shift, for the allowed tone Ta, correspondingto the greatest calculated shift sum among the calculated shiftsums (in this example, there are four shift sums) and the zeroshiftsum, for the validated reference pattern. It is thuspossible that the best shift, for the allowed tone Ta, in factcorresponds to the zero-shift if the zero-shift sum for theallowed tone Ta has the greatest value. This allows in fact toselect the shift giving a best matching of the pixels of theintaglio modules with the elements E1 of corresponding symbols.And then,- once the above operations are repeated for each one of thevalidated base intaglio patterns forming the two-dimensionalintaglio pattern (and having the corresponding pair of allowedtones {Ta, Tb}), calculating a global shift Δa for the twodimensionalintaglio pattern by determining an average of allthe selected best shifts, for the allowed tone Ta, over thevalidated reference patterns associated with the respectivevalidated base intaglio patterns of the two-dimensional intagliopattern having the corresponding pair of allowed tones {Ta, Tb}.Thus, only the best shifts for the validated reference patternsare added, and then divided by the number of said validatedreference patterns, to obtain the global shift for the twodimensionalintaglio pattern.(3) shifting by at least one pixel size, backward and forwardalong each of said plurality of distinct directions with respectto the digital image, the pixels of the digital image locatedwithin the respective sites of elements E1 and E2 of each symbolof the sampling grid of which first element corresponds to theallowed tone Tb to obtain corresponding shifted pixels havingshifted positions with respect to the sampling grid;- measuring through the sampling grid, respectively for thebackward shift and the forward shift along each direction,corresponding backward shifted and forward shifted tone valuesattributed to the respective elements E1 and E2 of the symbolsof the sampling grid of which first element corresponds to theallowed tone Tb, from shifted pixels of the digital imagelocated within the respective sites of elements E1 and E2 ofeach symbol of the sampling grid of which first elementcorresponds to the allowed tone Tb, and storing in the memorythe obtained shifted tone values;- calculating, for each shift, for each symbol of the samplinggrid of which first element corresponds to the allowed tone Tb,a difference between the shifted tone value attributed to theelement E1 and the shifted tone value attributed to the elementE2 to obtain corresponding shifted differential symbol tonevalues, and storing in the memory the obtained shifteddifferential symbol tone values attributed to the elements ofthe symbols of the validated reference pattern of which firstelement corresponds to the allowed tone Tb;- for each shift, summing the calculated shifted differentialsymbol tone values of all the symbols of the sampling gridcorresponding to the allowed tone Tb, to obtain correspondingvalidated reference pattern shift sum for the allowed tone Tb;- selecting a best shift, for the allowed tone Tb, correspondingto the greatest calculated shift sum among the calculated shiftsums and the zero-shift sum, for the validated referencepattern; and then,- once the above operations are repeated for each one of thevalidated base intaglio patterns forming the two-dimensionalintaglio pattern (and having the corresponding pair of allowedtones {Ta, Tb}), calculating a global shift Δb for the twodimensionalintaglio pattern by determining an average of allthe selected best shifts, for the allowed tone Tb, over thevalidated reference patterns associated with the respectivevalidated base intaglio patterns of the two-dimensional intagliopattern having the corresponding pair of allowed tones {Ta, Tb}.As a result of operations (1) to (3) performed on all thevalidated base intaglio patterns of the two-dimensional intagliopattern (for the pair of allowed tones {Ta, Tb}), we obtain aset of points, in the space of the shifts along each saidplurality of directions, of coordinates (Δa,Δb)1, (Δa,Δb)2,....(4) performing the above operations (1-3) for each remainingpair of allowed tones {Ta', Tb'} from the pairs of allowed tonesof the stored validated base intaglio patterns of the twodimensionalintaglio pattern, to obtain corresponding pairs ofglobal shifts (Δa', Δb'), and storing in the memory said pairs ofglobal shifts. As a result of operations (1) to (4) performed onall the validated base intaglio patterns of the two-dimensionalintaglio pattern (for the pair of allowed tones {Ta', Tb'}), weobtain a set of points, in the space of the shifts along eachsaid plurality of directions, of coordinates(Δa',Δb')1, (Δa',Δb')2,...(5) estimating a register error value r(a, b) for the intagliomodules corresponding to any allowed pair of tones {Ta, Tb} ofall the validated base intaglio patterns of the two-dimensionalintaglio pattern as: r(a, b) = λ / f (Δa2 + Δb2)1 / 2, wherein λ is asize in μm of a pixel of the digital image and f is a scalefactor of the digital image. Thus, for the points (Δa,Δb)1,(Δa,Δb)2,... in the space of the shifts corresponding to the pairof allowed tones {Ta, Tb}, there are corresponding registererrors r1(a, b), r2(a, b),.... Then, the processor decides that thetwo-dimensional intaglio pattern is genuine only if eachregister error value r1(a, b), r2(a, b),..., for each allowed pairof tones {Ta, Tb}, of the respective validated base intagliopatterns of the two-dimensional intaglio pattern is ≤ 10 μm(preferably less than or equal to 5 μm).

[069] The above operations of shifting the positions on thedigital image of the intaglio modules of a validated baseintaglio pattern, from their initial positions at decoding stageof said base intaglio pattern to their shifted positions, inorder to measure symbol by symbol resulting differences betweenmeasured printed tone values, are illustrated on Fig.6D-E. Forsimplicity, only two direction of shift are considered: thevertical direction and the horizontal direction on the digitalimage. Starting from initial positions of the intaglio moduleson the digital image (i.e. initial positions of pixels on thedigital image corresponding to the detected intaglio modules atdecoding stage) of the validated base intaglio pattern ofFig.6C, a forward shift along the vertical direction of theintaglio modules corresponding to a darker tone is representedon Fig.6D, and a backward shift along said direction of theintaglio modules corresponding to a darker tone is representedon Fig.6E. Clearly from these two figures, the shift sum forsaid vertical direction (and for the intaglio modules of darkertone) having the greatest value corresponds to the backwardshift represented on Fig.6E, wherein a majority of "dark"intaglio modules better fit in position with the positions ofthe elements E1 (of black color) in the symbols of the validatedreference pattern associated with the validated base intagliopattern of Fig.6C. The above forward and backward shifts (of thedarker intaglio modules) are repeated but this time for thehorizontal direction, as represented on respectivelycorresponding Fig.6F and Fig.6G. From these two figures, theshift sums for said horizontal direction (and for the intagliomodules of darker tone), when considering all the "dark"symbols, the respective forward and backward shifts will provideshift sums of which values are smaller than the above shift sumrelating to Fig.6E. Consequently, the best shift, for the darkintaglio modules, from the backward and forward shifts over thetwo directions, is the backward shift of Fig.6E, and it is theshift corresponding to this shift sum that will be retained forcalculating the global shift Δa for all the intaglio modules(corresponding to the darker tone) of all the validated baseintaglio patterns of the two-dimensional intaglio pattern (forthe allowed tone Ta), as we consider here only one base intagliopattern (i.e. that of Fig.6C) for simplicity.

[070] As it is clear from the Fig.6D-G wherein neither all thedark (black) intaglio modules nor all the light (grey) intagliomodules are in register, the result of the above detailedcalculations of the global shift Δa (for dark tone Ta) togetherwith a global shift Δb obtained from equivalent shiftingoperations performed on the light intaglio modules (i.e. forlight tone Tb), will provide a register error value r that isgreater than the threshold value of 10 μm (for this thresholdvalue of 10 μm, or lower, no alignment defect of the intagliomodules could be visible to the naked eye on the figures).Consequently, the two-dimensional intaglio pattern of Fig.6Cwill be considered as being not genuine, although it could bedecoded.

[071] The above method for estimating a register error betweenmodules of different tones of a color works particularly wellfor security markings containing a great number of twodimensionalintaglio patterns, like for example on Fig.1. Thisis due to the statistical basis of the method: the more symbolsin the security marking, the more precision is reached fordetecting a register error. For example, with the securitymarking of Fig.1, a smartphone (e.g. a Samsung S10, with 12 megapixels) having stored about 800 validated base intaglio patternscorresponding to reference patterns having each 18 symbols, itis possible to detect a register error as low as 5 μm althoughthe smartphone has a resolving power of about 24 μm (for an imagetaken at about 10 cm from the marking). Consequently, a fakesecurity marking obtained by offset printing can easily bedetected, whereas it cannot be detected via naked eye, as it isnot possible to realize a multitone offset printing copy ofmultitone intaglio lines of width between 20μm and 50μm whereinthe different portions of intaglio lines of different tones arein perfect register. The resolving power of the camera is itsability to differentiate two lines (or points) on an image of anobject, the greater the resolving power, the smaller the minimumdistance between two lines or points that can still bedistinguished. The larger the numerical aperture of theobjective lens of the camera, the higher the resolving power.

[072] The above disclosed subject matter is to be consideredillustrative, and not restrictive, and serves to provide abetter understanding of the invention defined by the independentclaims.

Claims

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, 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 ε of an intaglio module being greater than 20 μm and less than or equal to 50 μm, characterized in that 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 or 2, 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 twodimensional 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 any one of claims 1 to 3, 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 twodimensional intaglio pattern of a security marking of a security element according to any one of claims 1 to 4.

6. A method of producing a security element comprising a security marking comprising a two-dimensional intaglio pattern according to any one of claims 1 to 4, 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 twodimensional intaglio pattern according to the generated twodimensional 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 twodimensional 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, characterized by 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 any one of claims 1 to 4 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 device for decoding information encoded into a twodimensional 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 any one of claims 1 to 4, 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 twodimensional 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 twodimensional 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.

10. The device of claim 9, 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 twodimensional 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 twodimensional 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.

11. Method for authenticating a two-dimensional intaglio pattern of a security marking of a security element, characterized by 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 twodimensional 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.

12. The method according to claim 11, comprising the steps of: A) performing the operations a), b), b1), b2) b3), b4) and -c) of the method of decoding information encoded in a twodimensional intaglio pattern according to claim 8, on the twodimensional 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 Δa 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 Δb 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 twodimensional intaglio pattern, to obtain corresponding pairs of global shifts (Δa', Δb'), 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 (Δa 2 + Δb 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 twodimensional 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 twodimensional intaglio pattern is less than or equal to 10 μm.

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 any one of claims 1 to 4, 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, characterized by being adapted to perform the operations of: decoding information encoded into the two-dimensional intaglio pattern according to the method of claim 7, 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 the 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 according to claim 8, 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 Δa 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 Δb 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 twodimensional intaglio pattern, to obtain corresponding pairs of global shifts (Δa', Δb'), 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 (Δa 2 + Δb 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 twodimensional 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 twodimensional intaglio pattern is less than or equal to 10 μm.

15. The device of claim 13 or 14, 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.