Printed physical function pattern that is difficult to replicate

Abstract

To prevent counterfeiting, or specifically, provide a method and a device for printing a physical unclonable function pattern.SOLUTION: A method includes applying a clear coat layer to a substrate, drying the clear coat layer to form random microstructures in the clear coat layer, dispensing a printing fluid to print a graphical pattern on the clear coat layer, and generating a physical unclonable function (PUF) pattern by drying the printing fluid that fills the random microstructures formed in the clear coat layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates generally to anti-counterfeiting, and more particularly to a method and apparatus for printing a physical, unclonable function pattern. [Background technology]

[0002] A physical unclonable function (PUF) is a unique random physical pattern that can be used to prevent counterfeiting. PUFs can be very difficult and expensive to replicate and can be manufactured by a probabilistic process that results in a large number of robust tags. PUF patterns can be designed to be readable with appropriate analytical tools. PUF patterns can be recorded and stored for verification. PUFs can be used to prevent counterfeiting, prevent substitution of parts in an assembly, ensure proper management, act as an uncopyable digital signature, etc.

[0003] Current methods for generating PUFs can be expensive to manufacture and may not be economically viable for widespread use. Some examples of these expensive methods may include PUFs integrated with unique markers that contain special optical signatures (e.g., absorption or emission at specific wavelengths or combinations of wavelengths), or the creation of random distributions of nanostructures (e.g., nanoparticles, quantum dots, or self-assembling molecules). Furthermore, detection of these types of PUFs may require specialized analytical equipment that is not readily available to vendors and end users. Summary of the Invention

[0004] According to aspects illustrated herein, a method and a non-transitory computer-readable medium are provided for printing a physically unclonable function (PUF). One disclosed feature of an embodiment is a method that includes applying a clear coat layer to a substrate, drying the clear coat layer to form random microstructures in the clear coat layer, dispensing a printing fluid to print a graphic pattern in the clear coat layer, and generating the physically unclonable function (PUF) pattern by drying the printing fluid that fills the random microstructures formed in the clear coat layer.

[0005] Another disclosed feature of an embodiment is a non-transitory computer-readable medium having stored thereon a plurality of instructions, the plurality of instructions including instructions that, when executed by a processor, cause the processor to perform the following operations: applying a clear coat layer to a substrate; drying the clear coat layer to form random microstructures in the clear coat layer; dispensing a printing fluid to print a graphic pattern in the clear coat layer; and generating a physically unclonable function (PUF) pattern by drying the printing fluid that fills the random microstructures formed in the clear coat layer.

[0006] Another disclosed feature is a method for authenticating a printed PUF, the method including capturing with a camera of a mobile device an image of a printed physical unclonable function (PUF) on a substrate, where the PUF is formed on the substrate using a printing fluid dispensed into a clear coat that is dried to form random microstructures filled with the printing fluid, comparing the image with an image stored on a server, and authenticating the substrate when the image of the PUF matches the image stored on the server. [Brief explanation of the drawings]

[0007] The teachings of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

[0008] [Figure 1]1 illustrates a block diagram of an exemplary PUF of the present disclosure.

[0009] [Figure 2] 1 shows a close-up of a PUF of the present disclosure.

[0010] [Figure 3] 1 illustrates a block diagram of an exemplary apparatus for generating a PUF of the present disclosure.

[0011] [Figure 4] 1 shows a block diagram of an exemplary system for authenticating a PUF of the present disclosure.

[0012] [Figure 5] 1 shows a flowchart of a method for printing a PUF of the present disclosure.

[0013] [Figure 6] 1 shows a flowchart of a method for authenticating a printed PUF of the present disclosure.

[0014] [Figure 7] 1 depicts a high-level block diagram of an exemplary computer suitable for use in performing the functions described herein.

[0015] To facilitate understanding, wherever possible, the same reference numbers have been used to designate identical elements that are common to the figures. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present disclosure broadly discloses methods and apparatus for printing physical unclonable function (PUF) patterns. As discussed above, current methods for generating PUFs are expensive to manufacture and may not be economically viable for widespread use. Some examples of these expensive methods may include PUFs integrated with unique markers containing special optical signatures (e.g., absorption or emission at specific wavelengths or combinations of wavelengths), or the creation of random distributions of nanostructures (e.g., nanoparticles, quantum dots, or self-assembling molecules). Furthermore, detection of these types of PUFs may require specialized analytical equipment that is not readily available to vendors and end users.

[0017] Several low-cost methods using printing are available. For example, an ink-edge anti-counterfeiting method has been developed that uses blowing and pressing to create random structures on the ink edge after printing. However, printing with ink alone results in low resolution that is easier to replicate, reducing the security of the PUF.

[0018]

[0006] Embodiments of the present disclosure provide a printable PUF that is low cost while providing high resolution that is difficult to replicate. In one embodiment, a primary clear coat may be applied to a substrate. The clear coat may be allowed to dry to form random cracks in the coat on the substrate. An ink may be dispensed onto the dried primary clear coat. The ink may fill the random cracks formed in the coat to produce a PUF.

[0019] The PUFs of the present disclosure can also be authenticated using widely available equipment. For example, a camera on a cell phone or mobile device can be used to capture an image of the PUF for authentication. Therefore, no special equipment is required to authenticate the printed PUFs of the present disclosure.

[0020] 1 illustrates an exemplary document 100 that includes a physically unclonable function (PUF) 108 generated via the printing process of the present disclosure. In one embodiment, the PUF 108 is shown printed on the document 100, but the PUF 108 may be printed on any surface, including the surface of packaging, a part, a device, or a housing, etc.

[0021] In one embodiment, document 100 may include substrate 102. Substrate 102 may be paper, cardboard, plastic, or any other type of surface capable of receiving the printing fluid used to print clear coat layer 104 and pattern 106. In one embodiment, clear coat layer 104 may include a colloidal mixture of fluid and solid particulates. For example, the fluid may be a solvent or water. The solid particulates may include silica or other types of polymers. In one embodiment, clear coat layer 104 may also use nanoparticle colloidal mixtures, sol-gel solutions, polymer solutions, polymer blends, etc.

[0022] In one embodiment, the clearcoat layer 104 can be a commercially available colloidal mixture of water and silica containing 30 to 50 weight percent silica. In one embodiment, the clearcoat layer 104 can include 34 weight percent silica. In one embodiment, the clearcoat layer 104 can include 50 weight percent silica.

[0023] In one embodiment, printing fluid is dispensed onto the clear coat layer 104 to form PUFs 1081-1088. n The PUFs 108 may form a pattern 106 (hereinafter referred to individually or collectively as PUFs 108). Note that although the pattern 106 of the PUFs 108 is shown as a series of dots, any pattern 106 may be printed. For example, the pattern 106 may be a barcode, a quick response (QR) code, or a pattern of a different shape (e.g., square, rectangular, oval, irregular shape, etc.).

[0024] In one embodiment, any type of clear coat layer 104 may be used that is capable of adhering to the substrate 102. In one embodiment, any type of printing fluid may be used that is compatible with the clear coat layer 104. In other words, the printing fluid should be able to adhere to the clear coat layer 104 and dry to produce the pattern 106.

[0025] In one embodiment, the PUF 108 can be printed to have a width or diameter 110 as small as 50 micrometers. Thus, the present method can be used to print relatively small PUFs 108 that can use commercially available compounds. Furthermore, the PUF 108 can be analyzed or authenticated using widely available equipment (e.g., a cell phone camera). Thus, the method of the present disclosure provides a PUF 108 that is difficult to counterfeit while maintaining a relatively low cost for printing the PUF 108.

[0026] 2 shows a close-up view of an exemplary PUF 108. In one embodiment, the clear coat layer 104 is dispensed and dried to form random microstructures 1101-110. 11 (hereinafter referred to individually as random microstructures 110 or collectively as microstructures 110). The random microstructures 110 may be cracks formed in the clear coat layer 104 due to drying.

[0027] In one embodiment, the resolution of the cracks or random microstructures 110 may be controlled by the thickness of the clear coat layer 104. For example, if the resolution is too high (e.g., fewer large-sized microstructures 110), the random microstructures 110 may be too easily copied via a printing process alone. If the resolution is too low (e.g., many smaller-sized microstructures 110), the random microstructures 110 may be too small to analyze using images captured from a standard red, green, blue (RGB) camera on a mobile device (e.g., a cell phone camera).

[0028] In one embodiment, the thickness of the clear coat layer 104 may be controlled by the amount of clear coat layer 104 dispensed via the print head of the printer. In one embodiment, the thickness of the clear coat layer 104 may be controlled by the spin speed of the spin coating process. For example, the clear coat layer 104 may be spun onto the substrate 102 at between 100 revolutions per minute (RPM) and 5,000 RPM. In one embodiment, the clear coat layer 104 may be spun onto the substrate 102 at between 500 RPM and 3,000 RPM.

[0029] It should be noted that although a spin coating process is described above, other coating methods may be used. For example, the clear coat layer 104 may be applied using a drawdown coating method, aerosol spraying, etc.

[0030] In one embodiment, the resolution of the random microstructures 110 can be such that the random microstructures 110 have lengths 112 and widths 114 of several hundred micrometers. In one embodiment, the random microstructures 110 can have lengths 112 of 20-70 micrometers and widths 114 of 3-9 micrometers. In one embodiment, the thickness of the clear coat layer 104 to achieve the desired resolution of the random microstructures 110 can be approximately 5 nanometers (nm) to 100 nm for the nanoparticle colloid coating described above. The thickness can vary depending on the type of other material. For example, the thickness of the clear coat layer 104 can be 5 nm to 10 micrometers for a polymer coating.

[0031] In one embodiment, printing fluid may be dispensed onto clear coat layer 104, as described above. The printing fluid may fill random microstructures 110 formed in clear coat layer 104. The image formed by the combination of the printing fluid in random microstructures 110 and the printing fluid at specific locations on clear coat layer 104 may form PUF 108. In other words, in the example shown in FIG. 2 , PUF 1081 may include printing dots and random microstructures 1101 and 1102 that are filled with printing fluid.

[0032] PUF 1082 may differ from PUF 1081 because the arrangement and size of random microstructures 1103 and 1104 may differ from the arrangement and size of random microstructures 1101 and 1102 of PUF 1081. PUF 1083 and PUF 1084 may also differ from PUF 1081 because the arrangement and size of random microstructures 1105-1108 and 1109-1109, respectively. 11 In other words, each PUF 108 in the pattern 106 may be unique or different due to the random microstructures 110 generated at different locations along the surface of the clear coat layer 104.

[0033] 3 shows an exemplary apparatus 300 that may be used to print the PUF 108 of the present disclosure. In one embodiment, apparatus 300 may be a printer including multiple print heads 304 and 306. Print heads 304 and 306 may be used to dispense different fluids. For example, storage container 308 may store clear coat that is dispensed via print head 304. Storage container 310 may store printing fluid that is dispensed via print head 306.

[0034] In one embodiment, processor 302 may be communicatively coupled to print heads 304 and 306 to control the dispensing of clear coat and printing fluid. In one embodiment, substrate 102 may pass under print heads 304 and 306, or print heads 304 and 306 may move over substrate 102.

[0035] In one embodiment, the processor 302 can control the print head 304 to dispense the clear coat to form the clear coat layer 104 on the substrate 102. As described above, the clear coat can be dispensed to form a desired thickness of the clear coat layer 104. The print head 304 can make multiple passes over the substrate 102 to dispense the clear coat.

[0036] The substrate 102 with the clear coat layer 104 can be transferred to a drying apparatus, oven, or air dried to dry the clear coat layer 104. Drying the clear coat layer 104 can create random microstructures 110 within the clear coat layer 104.

[0037] After the clear coat layer 104 is dried, the substrate 102 may be placed under the print heads 304 and 306. The processor 302 may then control the print heads 306 to dispense printing fluid to print the pattern 106 onto the clear coat layer 104. The printing fluid may fill the random microstructures 110 around where the printing fluid is dispensed. The printing fluid may then be dried to form the PUF 108.

[0038] It should be noted that device 300 is simplified for ease of explanation and may include additional components not shown. For example, device 300 may include motors for moving print heads 304 and 306, memory for storing instructions, a power supply, other electrical components, etc.

[0039] 3 illustrates printing the PUF 108 with a single printer or apparatus 300, it should be noted that different devices can be used to print the PUF 108. For example, the clear coat layer 104 can be spin-coated onto the substrate 102 via a spin coater. Additionally, an oven can be used to dry the dispensed clear coat layer 104 and printing fluid.

[0040] Examples 1-3 below provide examples with process parameters for printing the PUF 108 of the present disclosure.

[0041] Example 1:

[0042] A 34 weight percent colloidal silica solution suspended in deionized water was used to form the clear coat layer 104. The colloidal silica solution was applied to a cardboard substrate by spin coating at 500 RPM for 1 minute. The clear coat layer 104 was dried in a 60 degree Celsius (°C) oven for 1 hour. A pattern was printed onto the dried clear coat layer 104 using water-based graphic ink. The ink was dispensed using a Dimatix DMP2800 printer to produce a 600 x 600 dots per inch (dpi) pattern using the following conditions: drop mass = 4.5-4.8 nanograms (ng), drop velocity = 6-7 meters / second, frequency = 5 kilohertz, print head temperature = ambient to 40°C, and voltage = 16-20 volts. The ink was dried in an oven at or above 120°C for 10 minutes.

[0043] Example 2:

[0044] A 34 weight percent colloidal silica solution suspended in deionized water was used to form the clear coat layer 104. The colloidal silica solution was applied to a cardboard substrate by spin coating at 3000 RPM for 1 minute. The clear coat layer 104 was dried in an oven at 60 degrees Celsius (°C) for 1 hour. A pattern was printed onto the dried clear coat layer 104 using water-based graphic ink. The ink was dispensed using a Dimatix DMP2800 printer to produce a 600 x 600 dots per inch (dpi) pattern using the following conditions: drop mass = 4.5-4.8 nanograms (ng), drop velocity = 6-7 meters / second, frequency = 5 kilohertz, print head temperature = ambient to 40°C, and voltage = 16-20 volts. The ink was dried in an oven at or above 120°C for 10 minutes.

[0045] Example 3:

[0046] A 50 weight percent colloidal silica solution suspended in deionized water was used to form the clear coat layer 104. The colloidal silica solution was applied to a cardboard substrate by spin coating at 500 RPM for 1 minute. The clear coat layer 104 was dried in a 60 degree Celsius (°C) oven for 1 hour. A pattern was printed onto the dried clear coat layer 104 using water-based graphic ink. The ink was dispensed using a Dimatix DMP2800 printer to produce a 600 x 600 dots per inch (dpi) pattern using the following conditions: drop mass = 4.5-4.8 nanograms (ng), drop velocity = 6-7 meters / second, frequency = 5 kilohertz, print head temperature = ambient to 40°C, and voltage = 16-20 volts. The ink was dried in a 120°C oven for 10 minutes.

[0047] The above examples were found to produce PUFs 108 with the desired resolution. Random microstructures 110 in the clear coat layer 104 were measured to have widths of 3-9 micrometers and lengths of 20-70 micrometers.

[0048] 4 shows a block diagram of a system 400 used to authenticate the PUF 108 of the present disclosure. As described above, the printed PUF 108 may allow an available device to be used to authenticate the PUF 108. In other words, no expensive specialized equipment is required to authenticate the PUF 108 of the present disclosure.

[0049] In one embodiment, the system 400 may include a mobile device 420 that includes a camera 422. The mobile device 420 may be a cellphone, mobile telephone, smartphone, tablet, laptop, etc. In other words, any device that has an image capture device may be used to capture an image.

[0050] An end user may receive a document printed on substrate 402 and desire to authenticate the document. The end user may capture an image 424 of the PUF 408 printed on substrate 402. The PUF 408 may be printed or formed similarly to how PUF 108 may be printed or formed, as described above.

[0051] 4, the PUF 408 is shown as a barcode. As mentioned above, the PUF 408 can be any pattern (e.g., a barcode, a QR code, a pattern of shapes, etc.). The PUF 408 can include various random microstructures 410 located at different positions and on different lines of the barcode. The PUF 408 can be formed by dispensing a printing fluid or ink onto the dried clearcoat layer 404 to fill the random microstructures 410 with the printing fluid.

[0052] In one embodiment, an image of the PUF 408 may be captured after it is printed during manufacturing. The image may be saved as a stored image 430 in a database (DB) 428 on a network 432. The network 432 may be an internet protocol (IP) network.

[0053] To authenticate a document printed on substrate 402, mobile device 420 can send captured image 424 to application server (AS) 426 via network 432. AS 426 can access stored images 430 in DB 428. AS 426 may compare captured image 424 with stored images 430. If a match is found, AS 426 can send a notification to mobile device 420 that the document is authenticated. If a match is not found, AS 426 can send a notification to mobile device 420 that the document is not authentic. Thus, printed PUF 408 may allow relatively inexpensive, widely available equipment to be used to authenticate the printed PUF 408 of the present disclosure.

[0054] 5 shows a flowchart of an exemplary method 500 for printing a PUF of the present disclosure. In one embodiment, method 500 may be performed by device 300 or by an apparatus such as device 700 illustrated in FIG. 7 and discussed below.

[0055] In one embodiment, method 500 begins at block 502. At block 504, method 500 applies a clearcoat layer onto a substrate. The clearcoat layer can be any type of colloidal suspension of solid particulates that is compatible with the substrate. For example, the colloidal suspension must be able to adhere to the substrate. An example of a colloidal suspension can include silica particles suspended in deionized water.

[0056] In one embodiment, the clear coat layer may be applied via a spin coating process, a drawdown coating method, aerosol spraying, etc. In one embodiment, the clear coat layer may be applied via a printer. The clear coat layer may be applied to a desired thickness to obtain a desired resolution of the microstructures formed in the clear coat layer, as described below.

[0057] At block 506, the method 500 dries the clear coat layer to form random microstructures in the clear coat layer. The random microstructures may include microcracks that form in the clear coat layer as it dries. The random microstructures may have different shapes, arrangements, and sizes. An exemplary desired resolution of the random microstructures may include microstructures having widths of 3 to 9 micrometers and lengths of 20 to 70 micrometers.

[0058] At block 508, the method 500 dispenses a printing fluid to print a graphic pattern on the clear coat layer. The printing fluid can be a water-based graphic ink. Any type of ink can be used that is compatible with the clear coat layer, adheres to the clear coat layer, and can print the desired pattern of the PUF. The printing fluid can fill in random microstructures around where the printing fluid is dispensed on the clear coat layer.

[0059] At block 510, the method 500 generates a physically unclonable function (PUF) pattern by drying the printing fluid that fills the random microstructures formed in the clear coat layer. For example, the printing fluid may be dried, and the resulting image of the dispensed printing fluid and the printing fluid in the random microstructures may form the PUF pattern. At block 512, the method 500 ends.

[0060] 6 shows a flowchart of an example method 600 for authenticating a printed PUF of the present disclosure. In one embodiment, method 600 may be performed by system 400 or by an apparatus such as apparatus 700 illustrated in FIG. 7 and discussed below.

[0061] In one embodiment, method 600 begins at block 602. At block 604, method 600 captures an image of a printed physical unclonable function (PUF) on a substrate with a camera of a mobile device, where the PUF is formed on the substrate using a printing fluid dispensed into a clear coat, and the clear coat is dried to form random microstructures filled with the printing fluid. In one embodiment, the mobile device can be a cell phone, mobile phone, smartphone, tablet computer, laptop, etc. In other words, any type of device with a camera can be used to capture an image of a PUF for authentication.

[0062] A printed PUF can be formed via random microstructures formed in a clear coat layer dispensed onto a substrate. A printing fluid can be dispensed onto the clear coat and fill the random microstructures within the clear coat layer. The resolution of the PUF can be controlled via the material used for the clear coat and the thickness of the clear coat layer, as described above.

[0063] At block 606, the method 600 compares the image to an image stored on a server. For example, an image of the printed PUF can be captured when the PUF is created. The image can be stored in a database within the network. The database can be managed by an authentication service or the company that printed the PUF. The image captured by the mobile device can be compared to the stored images in the database to determine if a match is found.

[0064] At block 608, the method 600 authenticates the substrate when the image of the PUF matches the image stored on the server. If a match is found, the substrate may be authenticated. For example, a mobile device that captured the image of the printed PUF may receive a message or notification that the substrate has been authenticated. At block 610, the method 600 ends.

[0065] 7 depicts a high-level block diagram of a computer dedicated to performing the functions described herein. As depicted in FIG. 7, the computer 700 comprises one or more hardware processor elements 702 (e.g., a central processing unit (CPU), microprocessor, or multi-core processor), memory 704, such as random access memory (RAM) and / or read only memory (ROM), a module for printing a PUF 705, and various input / output devices 706 (e.g., storage devices including, but not limited to, tape drives, floppy drives, hard disk drives, or compact disk drives, receivers, transmitters, speakers, displays, speech synthesizers, output ports, input ports, and user input devices (keyboard, keypad, mouse, microphone, etc.)). It should be noted that while only one processor element is shown, multiple processor elements may be employed in a computer.

[0066] It should be noted that the present disclosure may be implemented in software and / or a combination of software and hardware, for example, using an application specific integrated circuit (ASIC), a programmable logic array (PLA) including a field-programmable gate array (FPGA), or a state machine deployed on a hardware device, a computer, or any other hardware equivalent, such as computer-readable instructions that may accompany and be used to configure a hardware processor to perform the steps, functions, and / or operations of the methods discussed above. In one embodiment, instructions and data (e.g., a software program including computer-executable instructions) for this module or process 705 for printing a PUF may be loaded into memory 704 and executed by hardware processor element 702 to implement the steps, functions, or operations as discussed above. Also, when a hardware processor executes instructions to perform an "action," this may include the hardware processor performing the action directly and / or facilitating, directing, or cooperating with another hardware device or component (e.g., a coprocessor, etc.) to perform the action.

[0067] A processor that executes computer-readable instructions or software instructions related to the above methods may be recognized as a programmed processor or a specialized processor. As such, the present module 705 (including corresponding data structures) for printing a PUF of the present disclosure may be stored in a tangible or physical (broadly defined, non-transitory) computer-readable storage device or medium, such as volatile memory, non-volatile memory, ROM memory, RAM memory, a magnetic or optical drive, device, or diskette. More specifically, a computer-readable storage device may comprise any physical device that provides the capability to store information, such as data and / or instructions, that may be accessed by a processor or a computing device, such as a computer or application server.

[0068] It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unprecedented alternatives, modifications, variations, or improvements may be made by those skilled in the art, which are intended to be encompassed by the following claims.

Claims

1. 1. A method comprising: applying a clear coat layer onto the substrate; drying the clear coat layer to form a random microstructure in the clear coat layer; dispensing a printing fluid to print a graphic pattern on the clear coat layer, the graphic pattern comprising a bar code, a quick response code, or a pattern of shapes; and generating a physically unclonable function (PUF) pattern by drying the printing fluid filling the random microstructures formed in the clear coat layer.

2. The method of claim 1 , wherein the clearcoat layer comprises a colloidal mixture of water and solid particulates.

3. 3. The method of claim 2, wherein the solid particulate comprises silica in the range of 30 weight percent to 50 weight percent.

4. The applying step comprises: The method of claim 1 , comprising dispensing the clearcoat layer using a print head of a printing device that also dispenses the printing fluid.

5. 10. The method of claim 1, wherein said applying is performed via a spin coating procedure at between 100 revolutions per minute (RPM) and 5000 RPM.

6. The method of claim 1 , wherein the clear coat layer is applied to a desired thickness to form the random microstructures at a desired resolution.

7. The method of claim 6, wherein the desired thickness comprises 5 nanometers (nm) to 100 nm.

8. The method of claim 1 , wherein the printing fluid comprises a water-based graphic ink.

9. A non-transitory computer-readable medium storing a plurality of instructions, the plurality of instructions, when executed by a processor, causing the processor to: applying a clear coat layer onto the substrate; drying the clear coat layer to form a random microstructure in the clear coat layer; dispensing a printing fluid to print a graphic pattern on the clear coat layer, the graphic pattern comprising a bar code, a quick response code, or a pattern of shapes; and generating a Physically Unclonable Function (PUF) pattern by drying the printing fluid that fills the random microstructures formed in the clear coat layer.

10. The non-transitory computer-readable medium of claim 9 , wherein the clear coat layer comprises a colloidal mixture of water and solid particulates.

11. 11. The non-transitory computer-readable medium of claim 10, wherein the solid particulate comprises silica in the range of 30 weight percent to 50 weight percent.

12. The applying step comprises: The non-transitory computer-readable medium of claim 9 , comprising dispensing the clear coat layer using a print head of a printing device that also dispenses the printing fluid.

13. 10. The non-transitory computer-readable medium of claim 9, wherein the applying is performed via a spin-coating procedure at between 100 revolutions per minute (RPM) and 5000 RPM.

14. 10. The non-transitory computer-readable medium of claim 9, wherein the clear coat layer is applied to a desired thickness to form the random microstructures at a desired resolution.

15. 15. The non-transitory computer-readable medium of claim 14, wherein the desired thickness comprises between 5 nanometers (nm) and 100 nm.

16. The non-transitory computer-readable medium of claim 9 , wherein the printing fluid comprises a water-based graphic ink.

17. 1. A method comprising: capturing an image of a printed physically unclonable function (PUF) on a substrate with a camera of a mobile device, the PUF being formed on the substrate using a printing fluid dispensed into a clear coat that is dried to form random microstructures filled with the printing fluid, the clear coat comprising a colloidal mixture of water and silica in the range of 30 weight percent to 50 weight percent; comparing the image of the printed PUF with an image stored on a server; authenticating the substrate when the image of the printed PUF matches the image stored on the server.

Images