DE102020131420A1 - Color separation to ensure document personalization - Google Patents

Abstract

The invention relates to a document (100) with a colored print (104), a first part of the print (110) having colored pigments (114) and the visual impression of an achromatic color showing through in the first part of the print (110). the colored pigments (114) is approximated, and wherein a second part of the print (112) also has achromatic pigments (116) of the achromatic color in addition to colored pigments (114).

Description

The present invention relates to a document with a colored print, a method for producing a document, an electronic printing system and a reading device.

Various valuable and security documents as well as devices and methods for checking the authenticity of valuable or security documents are known from the prior art.

From the DE 38 407 29 A1 For example, a multilayer recording medium, in particular an identity card, is known in which an outer colored layer has a first color and an inner colored layer has a second color.

Color prints are based on subtractive color mixing and serve as a preliminary stage to the printing process. Color separation refers to the calculation of individual printing colors in prepress. In four-color printing, the three transparent printing colors cyan (C), magenta (M) and yellow (Y) as well as black (K) are used. The black (K) is necessary because 100% color application of cyan, magenta and yellow does not produce real black, but only a dark brown. These four basic colors, which result in the so-called CMYK color model, are mixed and form many other colors using subtractive color synthesis. For example, the print data for a document is usually in RGB (red green blue) and must be converted to CMYK for printing. The process of converting RGB to CMYK is called color separation. It either takes place in the printer or is carried out in an image processing program before printing (see also 4 colors 1 image, basic knowledge for image processing with Photoshop and QuarkXPress. 2nd edition, Edition Page by Mattias Nyman, published by Springer; in particular page 12;
Printing in color. Effect, representation and management of digital colors. Bertram Storch, published by Addison-Wesley
Digital color management, colors in publishing practice. 2nd Edition. Jan-Peter Homann, published by Springer; especially page 82)

In the prior art, every color printing system is based on the standardized 4C color model, since every color printer requires these four colors for printing. This means that the "colored" data is converted to the CMYK color model before printing. Information such as personalization data (portrait, signature or text) is converted from the original color space, e.g. RGB, to the CMYK color space. This color separation is usually performed automatically by the printer driver. In the prior art, IR-transparent cyan, magenta, yellow and an IR-visible black or UV-visible black are used for security printing. However, existing solutions are only suitable to a limited extent for security and special printing of documents, where other types of ink are used.

Out of EP 2 953 059 B1 dot code arranged in a display matrix is known. The point code provides a graphical indicator recognizable by an image processing technique.

In contrast, the invention is based on the object of creating an improved document, a method for producing a document, an electronic printing system and a reading device of the type mentioned at the outset.

The object on which the invention is based is achieved with the features of the independent patent claim. Advantageous embodiments of the invention are specified in the dependent patent claims.

According to a preferred embodiment of the invention, a document with a colored print is created, with a first part of the print having colored pigments and in the first part of the print the visual impression of an achromatic color being approximated by the colored pigments. A second part of the print also has achromatic pigments of the achromatic color in addition to chromatic pigments. A "colored imprint" is also understood here to mean an imprint that is imprinted by color printing, but only includes gray values analogous to the reproduction of a black-and-white image on a color television. However, the visual perception of a "colored imprint" does not always have to result in a color impression.

According to the invention, a document is understood to mean, among other things, paper-based and/or plastic-based documents, such as ID documents, in particular passports, ID cards, visas, driver’s licenses, vehicle registration documents, vehicle registration documents, company ID cards, health cards or other ID documents, as well as chip cards, means of payment, in particular bank cards and credit cards, bills of lading or other proof of entitlement.

According to a preferred embodiment of the invention, an optical security feature is provided on the document by the imprint. This has the advantage that the document is forgery-proof.

According to a further preferred embodiment of the invention, personalization information is provided on the document by the imprint. This advantageously protects the document personalization information.

According to a preferred embodiment of the invention, the achromatic color in the document has a property that can be detected by sensors. According to a further preferred embodiment of the invention, the sensor-detectable property integrated in the document is an optical, magnetic, dielectric and/or resistive property. For example, the unbunded paint includes particles whose optical, magnetic, dielectric, and/or resistive properties can be sensed by one or more sensors.

According to a preferred embodiment of the invention, the achromatic color integrated in the document is a color that contains particles that produce black visible in IR or black visible in UV in the print. The machine-readable information in the document is a code for use in a cryptographic process, a cryptographic key or part of a cryptographic key, an identifier, an attribute, a seed value for a cryptographic process and/or a cryptographic certificate or part of a cryptographic certificate.

According to a preferred embodiment of the invention, machine-readable information is provided in the document by dividing the imprint into the first and second parts. An advantage can be to make a falsification more visible and/or to hide the information.

According to a further preferred embodiment of the invention, a barcode, a QR code or a digital watermark with the machine-readable information is given in the document by dividing the imprint into the first and the second part.

According to a preferred embodiment of the invention, the division in the document takes place pixel by pixel. This advantageously creates a high print quality, which advantageously quickly reveals the falsification of a document.

According to a further preferred embodiment of the invention, the division in the document is uncorrelated with a visually perceptible geometric structure of the imprint. This means that there is no connection between the features and so advantageously the forgery of a document is made more difficult.

According to a further preferred embodiment of the invention, the document contains an integrated electronic circuit and a memory for storing data requiring protection and means for executing a cryptographic protocol. There is also an interface to a reading device.

According to a further preferred embodiment of the invention, a reading device with a sensor for detecting an imprint is provided. The program instructions are executed with a processor.

According to one embodiment of the invention, the document is a value or security document, in particular a paper-based and/or plastic-based document, such as an identity document, in particular a machine-readable travel document (MRTD), in particular a passport, identity card, visa, driver's license , vehicle registration document, vehicle title, company ID card, health card or another ID document, a chip card, means of payment, in particular a bank card, credit card or electronic means of payment, a bill of lading or other proof of entitlement.

An electronic printing system is configured to carry out the method.

According to a preferred embodiment, the printer is an inkjet printer that has three differently colored inks with colored pigments and one ink with achromatic pigments. Alternatively, a laser printer or thermal sublimation printer can be used.

According to a further preferred embodiment, the pixels of the second image matrix are selected as a function of information that is modulated onto the printout by replacing the pixels of the first image matrix.

Embodiments of the invention are particularly advantageous since the image pixels themselves carry the additional information and it is not necessary to overlay the image with a point code which does not carry any image information.

• 1 ein Blockdiagramm einer Ausführungsform eines erfindungsgemäßen Dokuments,
• 2 ein Blockdiagramm einer weiteren Ausführungsform eines erfindungsgemäßen Dokuments und ein Lesegerät, und
• 3 ein Blockdiagramm zum Verfahren zur Herstellung eines erfindungsgemäßen Dokuments.

In the following, embodiments of the invention are explained in more detail with reference to the drawings. Show it:

• 1 a block diagram of an embodiment of a document according to the invention,
• 2 a block diagram of a further embodiment of a document according to the invention and a reading device, and
• 3 a block diagram of the method for producing a document according to the invention.

the 1 shows a document 100 with a color separation using the example of a portrait with a pattern personalization. Color separation means that individual, ie separated, printing colors are calculated in prepress. In other words, the color information is separated. The print motif is broken down into its individual print colors. This decomposition of the printing colors is in the 1 shown. Colors are broken down into their four basic components cyan, magenta, yellow and black according to the principle of process colors. Color separation is necessary because color information is usually not available in the form required for printing. The print, here preferably as a four-color print, usually takes place in the CMYK color model, which means a color mixture of the individual colors cyan, magenta, yellow and black.

the 1a shows a document blank 101 of a value or security document. The document blank 101 is in the form of a card here, but it can also have a different shape. The document blank 101 is to be printed here with a colored imprint 104 of a passport photo and with textual information in black. The corresponding image data 102 of the passport photo can, for example, originally be in the RGB color space and are transformed by a first color separation into a number of color channels, here for example cyan, magenta, yellow of the CMY color space, resulting in a first image matrix. In an analogous manner, the image data 102 can be transformed by a second color separation into the same number of color channels and an additional achromatic channel, with the embodiment considered here being cyan, magenta, yellow and the achromatic channel black, such as in the 1b shown.

A part 112 is then selected from the resulting second image matrix with the achromatic channel, which is D-shaped in the example considered here, as in FIG 1a shown. This part 112 is then used to display pixels of the CMY image matrix 1a to replace. The resulting image matrix is then saved as a raster graphics file and the document blank 101 is printed by a printer that can process such a raster graphics file, so that the printer does not have to carry out its own color separation for the printout, as this is already stored as part of the raster graphics file. This results in the imprint 104 on the document blank 101; furthermore, the textual information is also printed by the printer.

Depending on the embodiment, the division of the colored print 104 into the two parts with or without achromatic pigments may or may not be visible to the naked eye.

According to one embodiment of the invention, the achromatic color includes IR-visible black. If the document 100 is illuminated with an IR radiation source 103, the result is that only the achromatic portion of the print appears, as in FIG 1a shown. In the embodiment shown here, only the D-shaped area and the textual information appear from the colored imprint 104 .

Instead of a geometric area, such as the D-shaped area of the 1a and 1d , the pixels can also be selected differently, for example in order to incorporate cryptographic information, such as a cryptographic key, into the imprint.

Like in the 1c shown, instead of the D-shaped region, a selection of pixels from the CMYK image matrix can be made, for example, which encodes such information. A part 112 formed in this way can also be visually perceptible in the resulting print 104 depending on the embodiment or not.

According to embodiments of the invention, a reading device is required for the detection of the distribution of the pixels of the part 112 in the colored print 104 . The reading device can have an IR radiation source 103 for the optical detection of the part 112, e.g. when the achromatic color is IR-visible black.

Since black is represented differently in the CMY and CMYK parts of the print, illumination in the visible spectral range can also be sufficient to detect the differences, for example with a CCD sensor or with the naked eye.

According to further embodiments of the invention, the achromatic color has particles, in particular color particles, with other optical properties, such as UV-visible black, or other physically detectable properties, such as magnetic, dielectric and/or resistive properties, which can be detected by the reader can be detected.

The document 100 in 1 10 thus shows a colored imprint 104. A first part of the imprint 110 has colored pigments 114 and in the first part of the imprint 110 the visual impression of an achromatic color is approximated by the colored pigments 114. A second part of the overprint 112 has, in addition to colored pigments 114, also achromatic pigments 116 of the achromatic color. This means that the imprint 104 is preferably divided into different areas that can be printed with or without the use of achromatic pigments 116 . The imprint 104 provides an optical security feature and personalization information.

Achromatic color means that the color has neither a specific hue nor saturation, i.e. it is black, gray and white. The achromatic color has a property that can be detected by sensors. The property that can be detected by sensors is an optical, magnetic, dielectric and/or resistive property. This has the advantage that the achromatic color can be detected using a number of physical measurement methods.

The division of the imprint 104 into the first part 110 and the second part 112 can result in machine-readable information, cf. in particular 1C . The machine-readable information is a code for use in a cryptographic process, a cryptographic key or part of a cryptographic key, an identifier, an attribute, a seed value for a cryptographic process and/or a cryptographic certificate or a part of a cryptographic certificate.

The pixel-by-pixel division of the imprint 104 into the first part 110 and the second part 112 results in a barcode, a QR code or a digital watermark with the machine-readable information. By combining the information stored in the chromatic and achromatic separation, a key for unlocking the RFID chip can be obtained with the machine-readable area (MRZ). The previous key from the MRZ is preferably combined with the stored information in order to advantageously generate a new, secure key from it. In other words, the stored information is generated by distinguishing between the first part of the print 110, in which black is used, and the second part of the print 112, in which no black is used. A type of fading is preferably carried out between the first part of the print 110 and the second part of the print 112 . This can preferably be done continuously or discontinuously. If continuous, less black is used progressively between the first part of the print 110 and the second part of the print 112 as each pixel is printed. In a discontinuous variant, areas of the image are alternately printed with and without black between the first part of the print 110 and the second part of the print 112 . These areas can preferably be selected to be as small as desired, for example the size of a pixel. The fading advantageously brings about a soft transition of a contrast difference between the first part of the imprint 110 and the second part of the imprint 112. This advantageously makes it more difficult for the forger to forge the document, since the recognition and reproduction of the fading is difficult.

1 thus shows the subtractive color theory in which the chromatic and achromatic separation of the CMYK channels is used in a targeted manner in order to store machine-readable information in images. This has the advantage that, in the event of a falsification, the image to be falsified is additionally secured by the different channels or a falsification becomes more visible.

In subtractive color theory, light of different colors is masked out by color filters or absorbed by pigments. The remaining light forms a mixed color. In other words, the color separation takes place with hidden background information, which advantageously serves to secure the document personalization. The basic colors of subtractive color theory are yellow, magenta and cyan. The implementation of the color separation is carried out with a printing system that is able to print the colors cyan, magenta and yellow in such a way that the black produced is as identical as possible. This has the advantage that the additional information cannot be seen simply by looking at it without tools.

In the present invention, the color separation uses IR-transparent cyan, magenta, yellow and an IR-visible black or, in other embodiments, a UV-visible black. This means that the theory of subtractive color separation is used in such a way that the black channel is used as a mask for a document 100 and the other colors are separated in such a way that they give the same impression even without black. When using an IR or UV-visible black color, only this information is visible, e.g. a text/image, a number or a barcode. The CMYK color separation has the advantage that document falsification is made more difficult or prevented. In addition, depending on the design of the inks, the underlying information can be machine-readable in the IR or UV range.

The invention is particularly advantageous since, in the event of a total forgery, the counterfeiter must separate the image accordingly using the software and, in addition, the printing software and the corresponding inks must be available in order to be able to generate this print.

the 2 shows a further embodiment of the document 100 according to the invention and a reading device 132 using a block diagram. The document 100 includes an integrated electronic circuit 106 and a memory 126 for storing data requiring protection 127 and means for executing a cryptographic protocol 122. There is also an interface 130 to a reader 132, with access by the reader 132 to the data requiring protection 127 via the interface 132 requires the execution of the cryptographic protocol 132 using the machine-readable information.

The electronic circuit 106 includes one or more electronic memories 126 for storing data 127 in particular. The electronic circuit 106 can access the data 127 . In particular, the electronic circuit 106 is used to access the data 127 in order to drive a display 108 for displaying the data 127 so that a user of the document 100 can perceive the display of the data 127 visually. The data 127 can be image data 102, for example. Image data 102 is understood here as data 127 that is suitable for reproduction on the display, i.e., for example, a digital photograph, textual information, a one-dimensional or two-dimensional barcode or the like. The electronic circuit 106 is designed in such a way that during a refresh period all of the display elements of the display 108 are driven once in order to display the data 127 on the display 108 in this way. This can be done with a refresh rate of 25 Hz or 50 Hz, for example.

A cryptographic key is detected by a reader 132 so that the reader 132 can execute a cryptographic protocol 138 together with the document 100 . The cryptographic key can be a symmetric or an asymmetric key. The prerequisite for read access by the reader 132 to the further data 127 is the previous successful execution of the cryptographic protocol 138. The cryptographic protocol 138 can be a so-called challenge-response method, for example. The additional data 127 are protected against unauthorized access by the cryptographic protocol 138 . This is particularly advantageous if the third data is sensitive data, such as biometric data, in particular fingerprint data or iris scan data of the bearer of document 100.

The reading device 132 has means for executing the cryptographic protocol 138 using the derived information 118. This information 118 is, for example, a cryptographic key. Due to the detection of the cryptographic key, the reading device 132 can execute the cryptographic protocol 138 with the document 100 . After the cryptographic protocol 138 has been successfully implemented, the reading device 132 can access a protected memory area of the document 100 in order to read out data 127 from there.

The electronic circuit 106 of the document 100 is designed here as a chip, in particular as an RFID chip. The electronic circuit 106 is connected to an interface 130 of the document 100 for communication with the reading device 132 . The interface 130 can have, for example, one or more antenna turns that run in an edge area of the document 100 . The document 100 has a first colored imprint 110 with colored pigments 114 and a second colored imprint 112 which, in addition to colored pigments 114, also has achromatic pigments 116 of the achromatic color.

The electronic circuit 106 has an electronic memory 126 for storing data requiring protection 127. The data requiring protection 127 can be, for example, biometric data of the bearer of the document 100, such as fingerprint data, iris scan data or the like. The data 127 requiring protection are stored in a specially protected memory area of the memory 126 .

Furthermore, the data 127 and the information 118 are stored in the same memory 126 or in another electronic memory of the document 100 . The colors of the data 127, in particular the colors of the image data 102, are as in FIG 1 described separated data. The electronic circuit 106 also has a processor 128 for executing program instructions, by means of which the steps of a cryptographic protocol 122 relating to the document 100 are implemented. The processor 128 is also used to execute program instructions that implement a control program 120 .

The electronic circuit 106 of the document 100 is designed here as a chip, in particular as an RFID chip. The electronic circuit 106 contains a driver circuit for driving the display 108. For this purpose, the driver circuit contains a driver logic. The driver circuit and the driver logic are in the 2 not to be seen.

The electronic circuit 106 also has a processor 128 for executing program instructions that implement a control program 120 and for executing Pro gram instructions that serve to decode 124 the data 127. This decoding 124 is also referred to as “rendering”. For example, the data 127 is encoded in a standardized data format, ie for example in the JPEG, GIF or TIFF format. The data 106 can be decoded by the program instructions 131 .

The reader 132 has an optical sensor 146 for detecting an imprint 104 from a document 100. The optical sensor 146 can be a CCD sensor or a scanner. The reading device 132 also has an interface 140 which corresponds to the interface 130 of the document 100 . For example, the interface 140 is designed for an RFID communication with the document 100 or its circuit 106 . The interfaces 130, 140 can be in the form of contacts, contactless or as a so-called dual-mode interface. Interfaces 130, 140 may be based on an electrical, capacitive, inductive, magnetic, optical, or other physical coupling method. For contactless coupling via an antenna, this can be designed, for example, as a coil, dipole or in the form of capacitive surfaces.

The document 100 has at least one processor 128 for executing a control program 120 and program instructions, by means of which the steps of the cryptographic protocol 138 relating to the reading device 132 are implemented. The reader 132 can also be used to run an application program 144 . The application program 144 can also be executed by another computer system which is connected to the reading device 132 .

By executing the program instructions, the reading device 132 detects the positions of achromatic pigments 116 received from the document 100 in an imprint 104 detected by the optical sensor 146 . By executing the program instructions, information 118 is derived from the detected positions.

The document 100 then transfers the data 127 of the memory 126 requested by the reader 132 from the interface 130 to the interface 140. These data 127 can be transferred from the control program 120 to the application program 144 for further processing. For example, the data 127 is output on a screen mask.

Before the data 127 is transferred from the memory 126 to the reader 132, further checks may be necessary, e.g. B. according to an EAC method. Alternatively or additionally, a cryptographic protocol 122 based on an asymmetric key can also be used.

The electronic circuit 106 contains one or more electronic memories 126 for storing data 127 in particular. The electronic circuit 106 can access the data 127 . In particular, the electronic circuit 106 is used to access the data 127 in order to control the display 108 for displaying the data 127 so that a user of the document 100 can perceive the display of the data 127 visually.

3 10 shows a block diagram as a method for producing a document 100 according to the invention. First, a document blank and an image are provided, 302. The color of the image is subjected to a first separation into a number of color channels, resulting in a first image matrix, 304. After the first color separation, a second color separation of the image into the number of color channels is performed. The difference here is that an additional achromatic channel is made available, resulting in a second image matrix, 306.

Next, a selection of pixels of the second image matrix is made, 308. Subsequently, the pixels of the first image matrix are replaced by the respective selected pixels of the second image matrix. The spatial arrangement of the pixels in the first image matrix corresponds to the selected pixels of the second image matrix. The pixels of the second image matrix are selected depending on information that is modulated onto the printout by replacing the pixels of the first image matrix, 310. The resulting image matrix is saved as a raster graphics file, 312. Finally, the raster graphics file is printed on the document blank using a Printer printed, 314. The printer is an inkjet printer that has three different colored inks with colored pigments and one ink with achromatic pigments. A raster graphic is also referred to as a pixel graphic because it consists of individual picture elements, i.e. pixels, which can have different colors. Raster graphics is a form of describing an image in the form of computer-readable data. Preferably, the new image is saved in CMYK color mode, e.g., Tiff format, and the separation is preferably saved in profile.

The image is preferably transmitted over a network to a computer system and the computer system receives or generates the information. The computer system carries out the steps of the first and second color separation, the selection and replacing the picture elements and storing the resulting image matrix. Then the raster graphics file is transmitted from the computer system over the network to the printer. An electronic printing system is configured to carry out the method. When printing out the raster graphics file, the embedded color profile is preferably used.

It will be understood by those skilled in the art that only exemplary embodiments are described. The individual features described in the different embodiments can be used in any combination to carry out the invention.

Reference List

100

document

101

document blank

102

image data

103

IR radiation source

104

imprint

106

Electronic switch

108

advertisement

110

First part of the imprint

112

Second part of the imprint

114

colored pigments

116

Achromatic pigments

118

information

120

control program

122

Cryptographic Protocol

124

decoding

126

Storage

128

processor

130

interface

132

reader

134

processor

136

control program

138

Cryptographic Protocol

140

interface

142

user interface

144

application program

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 3840729 A1 [0003]
• EP 2953059 B1 [0006]

Claims (19)

Document (100) with a colored print (104), wherein a first part of the print (110) has colored pigments (114) and in the first part of the print (110) the visual impression of an achromatic color is approximated by the colored pigments (114). and wherein a second part of the imprint (112) also has achromatic pigments (116) of the achromatic color in addition to colored pigments (114). Document (100) after claim 1 , wherein an optical security feature is given by the imprint (104). Document (100) after claim 1 or 2 , wherein personalization information is given by the imprint (104). Document (100) after claim 1 , 2 or 3 , where the achromatic color has a property that can be detected by sensors. Document (100) after claim 4 , wherein the property that can be detected by sensors is an optical, magnetic, dielectric and/or resistive property. A document (100) according to any one of the preceding claims, wherein the achromatic color is IR-visible black or UV-visible black. Document (100) according to one of the preceding claims, machine-readable information being provided by the division of the imprint (104) into the first (110) and the second part (112). Document (100) after claim 7 , wherein a barcode, a QR code or a digital watermark with the machine-readable information is given by the division of the imprint (104) into the first (110) and the second part (112). Document (100) after claim 7 or 8th , where the division is done pixel by pixel. Document (100) after claim 7 , 8th or 9 , the division being uncorrelated with a visually perceptible geometric structure of the print (104). Document (100) according to any one of the preceding Claims 7 - 10 , wherein the machine-readable information is a code for use in a cryptographic method, a cryptographic key or part of a cryptographic key, an identifier, an attribute, a seed value for a cryptographic method and/or a cryptographic certificate or a part of a cryptographic certificate. Document (100) according to any one of the preceding Claims 7 - 11 , wherein the document (100) includes an integrated electronic circuit (106) and a memory (126) for storing data requiring protection (127) and means for executing a cryptographic protocol (122), and having an interface (130) to a reading device (132), access by the reading device (132) to the data requiring protection (127) via the interface (130) requiring the execution of the cryptographic protocol (122) using the machine-readable information. Document (100) according to one of the preceding claims, wherein the document (100) is a value or security document, in particular a paper-based and/or plastic-based document, such as an identity document, in particular a machine-readable travel document (MRTD), in particular a passport, ID card, visa, driver's license, vehicle registration document, vehicle registration document, company ID card, health card or another ID document, a chip card, means of payment, in particular a bank card, credit card or electronic means of payment, a bill of lading or other proof of entitlement. Method for producing a document (100) with the following steps: - Provision of a document blank (101) and an image, - performing a first color separation of the image into a number of color channels, resulting in a first image matrix, - Carrying out a second color separation of the image into the number of color channels and an additional achromatic channel, resulting in a second image matrix, - selection of pixels of the second image matrix, - replacing pixels of the first image matrix, which correspond to the selected pixels of the second image matrix in their spatial arrangement in the first image matrix, with the respective selected pixels of the second image matrix, - Saving the resulting image matrix as a raster graphics file, - Printout of the raster graphic file on the document blank by a printer. procedure after Claim 14 , whereby the printer is a laser printer or thermal sublimation printer or inkjet printer that uses three different colored inks with colored pigments th and has an ink with achromatic pigments. procedure after Claim 14 or 15 , the pixels of the second image matrix being selected as a function of information which is modulated onto the printout by replacing the pixels of the first image matrix. procedure after Claim 16 , wherein the image is communicated to a computer system over a network, and the computer system receives or generates the information, and wherein the computer system performs the steps of first and second color separation, selecting and replacing the picture elements, and storing the resulting image matrix, and then the Raster graphics file is sent from the computer system over the network to the printer. Electronic printing system configured to perform a method according to any one of the preceding Claims 15 , 16 or 17 to execute. Reading device (132) with a sensor (146) for detecting an imprint (104) on a document (100), and with a processor (134) for executing program instructions, the positions of achromatic pigments (116) in an imprint (104) detected by the sensor (146), information (118) being derived from the detected positions by executing the program instructions, with means for executing a cryptographic protocol (138) and an interface (140) for communication with the document (100), access to data (127) requiring protection of the document (100) presupposes that the cryptographic protocol (138) has been successfully implemented using the derived information (118).

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