WO2014051518A1

WO2014051518A1 - Security film for revealing a passcode

Info

Publication number: WO2014051518A1
Application number: PCT/SG2013/000417
Authority: WO
Inventors: Chee Fatt CHAN; Beng Soon Tan; Choon Chye GAY; Gim Han LAW; Gareth Ee Ho TANG; Weng Kit CHAN
Original assignee: Temasek Polytechnic
Current assignee: Temasek Polytechnic
Priority date: 2012-09-25
Filing date: 2013-09-25
Publication date: 2014-04-03
Anticipated expiration: 2015-03-25

Description

Security film for revealing a passcode

FIELD OF INVENTION The invention relates generally to a security film for revealing a passcode using visual authentication.

BACKGROUND A passcode is typically required to authorise a person requesting access to restricted information or to a restricted area. Examples of restricted information may include bank account information or employee personal particulars. As for access to a restricted area, a panel may require the passcode to be entered before entry to the restricted area is permitted.

Certain electronic systems only require single factor authentication, in which only one passcode is entered to gain access. For added security, especially for access to banking transactions, two factor authentication is employed. In two factor authentication, a first passcode is entered. A second passcode is then requested even after the first passcode is determined to be correct.

In one exemplary way of implementing two factor authentication, a security token (for example, from RSA) is operated to generate a random passcode as the second passcode, which is based on an algorithm. In another exemplary way of implementing two factor authentication, a SMS (short message service) containing the second passcode is sent to a user's handphone.

However, the above implementations have the following drawbacks. The algorithm upon which the second passcode is generated may be cracked. The transmission with the SMS containing the second passcode may be intercepted. In this way, security may be compromised.

A need therefore exists to address the above drawbacks or provide alternatives to enhance the security of a passcode.

SUMMARY

According to one aspect of the invention, there is provided a security film for revealing a passcode present in content of a document when the security film is correctly aligned over the document, the security film comprising a substrate comprising a plurality of first portions; and a plurality of second portions, wherein the plurality of second portions have a higher degree of optical attenuation compared to the plurality of first portions, wherein, within a designated area of the substrate, each of the plurality of first portions is arranged to alternate with each of the plurality of second portions in a pattern designed to allow light from segments of the passcode to transmit through the substrate, via at least one of the plurality of first portions within the designated area, to create specific viewing zones having sufficient overlap of such light to reveal the passcode from the remaining content of the document.

According to another aspect of the invention, there is provided a method of forming a security film for revealing a passcode present in content of a document when the security film is correctly aligned over the document, the method comprising providing a substrate comprising a plurality of first portions; and a plurality of second portions, wherein the plurality of second portions have a higher degree of optical attenuation compared to the plurality of first portions; patterning a designated area of the substrate so that each of the plurality of first portions is arranged to alternate with each of the plurality of second portions to allow light from segments of the passcode to transmit through the substrate, via at least one of the plurality of first portions within the designated area, to create specific viewing zones having sufficient overlap of such light to reveal the passcode from the remaining content of the document. BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention, in which:

Figures 1A to 1C illustrate the operation of a security film, according to a first embodiment of the invention, to reveal encrypted content.

Figure 1 D shows a schematic illustrating the working principle of the security film, according to various embodiments of the invention.

Figure 2A shows a correct viewing distance and a correct viewing angle to view a passcode revealed by a security film, according to various embodiments of the invention.

Figure 2B shows a screen having a passcode which will not be revealed as a security film is absent.

Figure 2C shows an example of heavily hiding a passcode using noise characters. Figures 3A and 3B describe a cryptography technique used by a security film, according to a second embodiment of the invention.

Figure 4 shows the cross-sectional structure of an optical grating used to realise a security film according to a preferred embodiment.

Figure 5 shows the cross-sectional structure of an optical grating that can also be used to realise a security film, according to various embodiments of the invention.

Figure 6 shows a trend of display resolution, measure in pixels per inch (PPI), for modern displays.

Figure 7 shows a flowchart of a method to fabricate a security film, according to various embodiments of the invention.

Figure 8 shows fabrication of a security film using nano imprinting lithography (NIL) and reactive ion etching (RIE) techniques.

Figure 9 shows a sequence of steps that may be followed when incorporating a security film, according to various embodiments of the invention, for internet banking.

Figure 10 shows application of a security film, according to various embodiments of the invention, over an image displayed by a smartphone application to reveal the passcode therein.

Figure 11 shows a sequence of steps that may be followed when incorporating a security film, according to various embodiments of the invention, for enterprise security.

Figure 12 shows a security film, according to various embodiments of the invention, for automated teller machine (ATM) applications.

Figure 13 shows a security film, according to an embodiment of the invention, for granting door access.

DEFINITIONS The following provides sample, but not exhaustive, definitions for expressions used throughout various embodiments disclosed herein.

The phrase "security film" may mean a thin film which modulates light that transmits through the thin film. This thin film contains one or more optical elements, being of (for example) nano-size, to modulate such transmitting light. Examples of optical elements include lenses, mirrors, prisms, nanowire grid polarizers and parallax barriers. The word "reveal" or "revealing" may mean allowing for a desired portion of a document, where important information (such as a passcode) is present, to be made distinguishable from the remainder of the document using visual authentication through the security film. This may be achieved, for example, by such information having a different colour from the remainder of the document when the document is viewed through the security film; or such information being perceived to be brighter than the remainder of the document when the document is viewed through the security film. In another embodiment, it is only the passcode portion that can be seen when the document is viewed through the security film.

The word "passcode" may mean a portion of content in a document which appears to be normal and blends with the remainder of the content, but appears different (such as being contrastable or distinguishable) from the remainder of the content when viewed through the security film. For the same document, different portions may become the passcode, depending on the manner in which the security film is designed. The word "passcode" may be meant to be understood in the context of security, so that "passcode" may mean a string of information (such as one or more letters, numbers, symbols or segments of a drawing/picture/figure) that allows access to a computer, interface, or system.

The word "content" may mean any one or more of letters, words, phrases, numbers, symbols, drawings, pictures or figures found in a document. This content contains the passcode, which is revealed when the content is viewed through the security film.

A "document" need not necessarily be in physical form (such as a print out) and may also be in electronic form, which is shown in a display.

The phrase "correctly aligned" may mean that a sufficient area of the security film, containing a segment designed to reveal a passcode, is placed over a document containing the passcode. A correct alignment does not necessarily require the security film to be centralised with respect to the document.

The word "substrate" may mean a structure, fabricated from material such as glass or plastic, serving as a base for the optical elements of the security film that allows for a passcode to be revealed from content in a document.

The phrase "remaining content" may mean portions of content in a document that is irrelevant to the passcode, i.e. portions of the document content not containing the passcode.

The term "protrusion" may mean a structure that extends from a surface of a substrate. DETAILED DESCRIPTION

In the following description, various embodiments are described with reference to the drawings, where like reference characters generally refer to the same parts throughout the different views.

Figure 1A shows a document 140 having content 142 comprising four rows of letters. In this embodiment, the document 140 is in electronic format and displayed in an LCD (liquid crystal display) screen. As seen from Figure 1A, no portion of the content appears outstanding or of particular significance. The content 142 appears to be letters that are arranged in a random manner.

Figures 1 B and 1C show the same document 140, with Figure 1 B depicting a security film 100, according to a first embodiment of the invention, being placed over the document 140 and Figure 1C depicting the security film 100 being centralised over the document 140. With the security film 100 placed over the document 140, a portion 144 of the document 140 becomes more visible when compared to content surrounding the portion 144. In the embodiment shown in Figures 1A to 1C, the security film 100 is designed to distinguish the portion 144 from its surroundings through any area of the security film 00, so that a correct alignment of the security film 100 to reveal the portion 144 is achieved without having to centralise the security film 100 as shown in Figure 1 C. Thus, repositioning the security film 100 would still reveal the phrase "fhm", as long as a portion of the security film 100 is placed over the document 140. However, it will be appreciated that, in other embodiments, a security film may be designed such that only a specific area will distinguish a portion of a document.

This portion 144, in the embodiments shown in Figures 1A to 1C, blends with the remainder of the document 140 when viewed with the naked eye, but when viewed through the correctly aligned security film 100, becomes distinguishable from the remainder of the document 140. In one possible application, the phrase "fhm" appearing in the distinguished portion 144 may be used as a passcode to gain access to a computer, interface, or system.

Figure 1D shows a schematic illustrating the working principle of the security film 100 when positioned as shown in Figure 1C, i.e. when the security film 100 is correctly aligned over the document 140 to reveal a desired portion of the document 140, where this desired portion may contain a passcode as described above. Figure 1 D shows a segment of the security film 100 containing a designated area of the security film 100 designed to reveal the passcode contained in the document 140. It will be appreciated that the working principle, described with reference to Figure 1 D, is present in security films according to the various embodiments mentioned below.

The security film 100 has a substrate, which is not shown for the sake of clarity (but see below with respect to Figure 4). The substrate has a plurality of first portions 102 and a plurality of second portions 104. The plurality of second portions 104 have a higher degree of optical attenuation compared to the plurality of first portions 102 (so that, vice versa, the plurality of first portions 102 have a lower degree of optical attenuation compared to the plurality of second portions 104). Within a designated area of the substrate (being a portion of the security film 100 shown in Figure 1 D), each of the plurality of first portions 102 is arranged to alternate with each of the plurality of second portions 104 in a pattern 106 designed to allow light from segments 108 of the passcode to transmit through the substrate, via at least one of the plurality of first portions 102 within the designated area. Specific viewing zones 110 are created where there is sufficient overlap of such light to reveal the passcode from the remaining content of the document 140.

The passcode is most clearly reveajed within these specific viewing zones 110 because these are regions where light from segments 108 of the passcode - after transmitting through the substrate via at least one of the plurality of first portions 102 within the designated area - converges to an extent which is sufficient for the brain to make sense of the images seen by the human eye. The brain is presented with images (transmitted by the human eye) containing sufficient segments of the passcode, which have been distinguished from the parts of the document 40 irrelevant to the passcode by the security film 100, to be able to coherently reconstruct the passcode and thereby reveal the passcode. These segments of the passcode are distinguished because in these specific viewing zones 110, light from parts of the document 140 irrelevant to the passcode transmits through the plurality of second portions 104 and therefore experiences greater optical attenuation compared to light transmitting through the plurality of first portions 102. Accordingly, the passcode is revealed by being brighter than the remaining content of the document 140 from observation through the security film 100 due to the lower degree of attenuation of the plurality of first portions 102 in the designated area.

In other positions, such as zones 1 12 and 1 14, the human eye may still see portions of the document 140 where the passcode is present (i.e. the segments 108 of Figure 1 D). However, in such zones 112 and 114, the light from these segments 108 is not from light filtered by the pattern 106, so that the segments 108 are not perceived to be any different from the remaining content of the document 104 (i.e. the passcode is not revealed); or even if the zones 112 and 114 receive a portion of the light filtered by the pattern 106, it is insufficient for the brain to distinguish the passcode from the remainder of the document 140, so that the passcode is still not revealed. Accordingly, in the zones 1 2 and 114, a noise view, having jumbled content such as letters, characters or images, is seen by the human eye.

Thus, with reference to Figure 2A, a security film 200, according to various embodiments of the invention, is made with precision alignment such that line of sight to the passcode 208 will only be possible at a correct viewing distance d from a screen 240 as well as a correct viewing angle Θ. On the other hand, without the security film 200 (compare Figure 2B), the passcode will not be revealed. The security film 200 may be designed to reveal passcodes that are heavily hidden in the screen 240 showing pixels with noise characters (see Figure 2C).

The introduction of factors such as the correct viewing angle Θ and the correct viewing distance d provide a specific viewing zone (also see reference numeral 110 in Figure 1 D), which spans over a region that is bounded by the correct viewing angle Θ and the correct viewing distance d. This specific viewing zone enhances privacy and security because the passcode is not exposed or revealed to an unauthorised party who may be nearby, but whose eyes will not be positioned within these specific viewing zones. In contrast to privacy filters which provide only a restricted viewing angle, the security film 200 provides an additional level of privacy, by restricting visibility of the passcode within the correct viewing angle Θ and the correct viewing distance d.

Consider a scenario where the screen 240 is a LCD, whereby appropriate operation of the LCD pixels enables generation of the content displayed in the LCD. From a viewer's perspective, pixels are understood to be in an activated state when they are seen to be differentiated from other pixels (which are understood to be in a deactivated state), i.e. the brain perceives the differentiated pixels to contain the passcode, while the remainder of the pixels are irrelevant. The substrate of the security film 200 can be patterned (refer to the pattern 106 shown in Figure 1 D) to provide one or more specific viewing zones, within which pixels can be distinguished to be either in an activated or deactivated state. When the viewer's left and/or right eyes are not within these specific viewing zones, all the pixels are perceived to be in a deactivated state. The security film 200 may be designed to reveal one or more separate passcodes from a document, i.e. the security film 200 can reveal two separate passcodes from the document to access a computer, interface, or system. The designated areas of the security film 200 may be designed to isolate these one or more separate passcodes, by having one or more specific viewing zones for each of the one or more separate passcodes. For instance, there may be one or more specific first viewing zones for a first passcode and one or more specific second viewing zones for a second passcode, where the second passcode is not revealed when the document is observed in the one or more first specific viewing zones. On the other hand, in a scenario where all of the one or more separate passcodes may be seen in the one or more specific viewing zones, markers (not shown) may be placed on the security film 200 to assist a viewer to isolate the one or more first specific viewing zones. Such markers include margins (whereby each of one or more separate passcodes is located within two such margins); or boundaries (having shapes like a capsule, a circle or a rectangle within which each of one or more separate passcodes is located).

The patterning of the substrate is such that light from sufficient segments of the passcode is allowed to transmit through portions of the substrate that have a lower degree of optical attenuation for the passcode to be revealed. The design of the pattern is not constrained to only reveal passcodes in number (i.e. pixels that form "0" to "9"), letter (i.e. pixels that form "A" to "Z") or symbol format (such as pixels that form β, λ, μ or a), but can also allow reveal passcodes that are images (such as pixels that form drawings, pictures or figures). The pattern on the substrate can also have dimensions that decompose the pixel to a subpixel level, i.e. the passcode is revealed from a combination of light from a fraction of a first pixel and a fraction of a second pixel.

Even within the specific viewing zones, different viewing positions are provided, where each presents a different image to the left and/or right eyes, whereby the combined image is processed in the brain to perceive the revealed passcode. Thus, the patterning of the substrate preferably observes the following design conditions, described in Table 1 below.

Table 1 : Design conditions for consideration when patterning substrate

Condition Left Eye Right Eye

a) Black Passcode

b) Passcode Black

c) Half Passcode Half Passcode Under conditions a) or b), light is angled to reach the eye such that the passcode is revealed to either the left or the right eye. With both eyes open, the passcode is less clearly perceivable, but will be clearly revealed when either the left or the right eye is respectively closed. Under condition c), half of the passcode is seen in either the left or the right eyes, where the both images are combined in the brain so that the passcode is clearly revealed.

A cryptography technique used by a security film 300, according to a second embodiment of the invention which visually decrypts or reveals a passcode in a display that uses pixels to create numbers from 0 to 9, is described with reference to Figures 3A and 3B.

In this cryptography technique, the security film 300 is divided into a 6X6 grid, i.e. a grid having 6 rows and 6 columns. The security film 300 is aligned over a display 340 having numbers arranged also in a correspondingly sized 6X6 grid. Each box (330, 332) in the security film 300 grid has a unique pattern formed from n patterns, each designed to reveal an assigned passcode portion when the unique pattern matches the content in an aligned box (342, 344) of the display 340 grid, i.e. the content of the aligned box (342, 344) is visually observed, through the security film 300, to be different from other boxes in the same column of the security film 300 grid. With reference to Figure 3B, the security film 300 grid is designed to reveal a passcode segment in each of its six columns, so that the remainder of the passcode is found from the remaining five columns of the security film 300 grid, whereby the entire passcode will be understood to be a combination of six passcode segments. Within each column of the security film 300 grid, a box 330 will be seen to be set apart from the remaining boxes 332 of the same column. There will be other boxes 330, each in its own respective column, that have the same appearance. The remaining boxes 332, while appearing the same as each other, will have an appearance that is different to those of boxes 330 and the security film 300 grid is also patterned such that these remaining boxes 332 appear less prominent than the boxes 330. In this manner, a viewer will appreciate that boxes 332 contain the entire passcode.

The cryptography technique described in Figures 3A and 3B allow the passive security film 300 to display a passcode in any sequence and position in its column x and row y grid. With reference to Figure 3B, each box (330, 332) in the security film 300 grid is designed to reveal a pre-assigned number from 0-9, thereby providing 10 unique n patterns. Accordingly, the security film 300 grid can provide different combinations of 6 windows from 36 grid boxes to reveal a six-digit passcode contained in the display 340. In the cryptography technique illustrated in Figures 3A and 3B, the security film 300 is able to generate n!*row^*col^* P_y permutations, for 5.08x10¹⁵ ((10!) x 36 X P₆ = 5.08 x 10¹⁵) permutations of six-digit passcodes, wherein P represents pattern and the subscript y representing the pattern number.

The example shown in Figures 3A and 3B uses one of 10 patterns P₀ to P₉ for each box (330, 332) in the security film 300 grid, as reflected in the table 360 of Figure 3B. Taking the boxes (330, 332) that have the pattern P₀ as an example, they are designed to reveal the digit "5", so that if the aligned box (342, 344) from the display 340 grid has the digit "5" as its content, the digit "5" will be seen more prominently through the security film 300. On the other hand, if the aligned box (342, 344) from the display 340 grid does not have the digit "5" as its content, that digit (such as "6", "7", "8", etc.) will be seen less prominently through the security film 300. In the example shown in Figures 3A and 3B, the six-digit passcode that will be seen is "091595". It will be appreciated that while Figure 3A shows that the security film 300 and the display 340 both use a matching 6X6 grid, it is not a requirement that the 10 patterns P₀ to P₉ match a grid against which the content in the display 340 is arranged or that the size and shape of the security film 300 matches that of the display 340. Essentially, the patterns P₀ to P₉ are designed to reveal a pre-assigned number, so that the security film 300 and the display 340 can have mismatching shapes and sizes. The content of the display 340 may also be designed to be compatible with the security film 300. For instance, the display 340 may also be modulated so that light from sufficient pixels, that form the passcode, will transmit through the security film 300 to reveal the passcode to the viewer.

Figures 1A to 1D, 2A to 2B and 3A to 3B illustrate use of a security film (100, 200 and 300) on content that may be dynamic (because the content is generated by an LCD). However, the security film (100, 200 and 300) will also work on static content, such as those found in physical documents (such as a printed document, a picture or a painting). Such physical documents, being passive, are not able to generate light. Rather, light that is used to reveal the passcode is from ambient light that transmits through the security film (100, 200 and 300), reflects off these physical documents and again transmits through the security film (100, 200 and 300) in an opposite direction.

A security film, according to various embodiments of the invention, comprises of a combination of optical structures having nano and micro size dimensions. Each of these optical structures act as a lens, where in one embodiment, over 90,000 lenses can be found in one square inch, with each lens measuring about 3-10 nanometers. By alternating these structures in a specific manner, unique patterns can be designed within any desired area of the security film, depending on the contents of a document that is to contain a passcode. Cross-sectional structures of an optical grating, which may be used for such a security film, are shown in Figures 4 and 5.

Figure 4 shows the cross-sectional structure of an optical grating 400 used to realise a security film according to a preferred embodiment.

The optical grating 400 includes a substrate 402 comprising a plurality of protrusions 404. A space 406 exists in between any two adjacent protrusions 404. A cap 418 is provided on at least one of the plurality of protrusions 404 at an end 403 that is furthest from the substrate 402. The cap 418 has a higher degree of optical attenuation compared to the substrate 402 material.

Cross-referencing Figure 4 against Figure I D, the portion of the substrate 402 that spans across the space 406 provides the plurality of first portions 02 of the security film 100 of Figure 1 D. The cap 418 provides the plurality of second portions 104 of the security film 100 of Figure 1 D. Thus, the interaction between the cap 418 and the portion of the substrate 402 that spans across the space 406 (as explained above in the description concerning both the plurality of second portions 104 and the plurality of first portions 102 in respect of Figure 1D) creates specific viewing zones that reveal a passcode within a document (not shown). Specifically, in these specific viewing zones, light from parts of the document irrelevant to the passcode is attenuated by the cap 418 to a greater extent compared to light from the passcode that transmits through the portion of the substrate 402 that spans across the space 406. In these specific viewing zones, there is sufficient convergence of light passing through this portion of the substrate 402 to create images seen by the human eye that can be reconstructed in the brain to reveal the passcode.

The combination of each protrusion 404 and the respective cap 418 thereon may have a generally symmetric cross-sectional profile. To realise this symmetry, the cross-sectional profile 412 of each of the sidewalls 408, 410 of each of the protrusions 404 has a convex shape relative to a longitudinal axis 414 extending along the centre and intersecting the base 416 of the respective protrusion 404. While an arced cross-sectional profile is shown in Figure 4, other profiles such as square shaped or round shaped are possible, as long as the overall cross-sectional profile of the cap 418 and the respective protrusion 404 has symmetry. Further, the cross-sectional arc profile 412 need not necessarily extend over the entire surface of the sidewalls 108, 110, but may be present only along a portion of the entire sidewalls 108, 110. While Figure 4 shows that each of the plurality of protrusions 404 has a cap 418 at an end 403 that is furthest from the substrate 402, it will be appreciated that the cap 418 is only provided on a selected number of the plurality of protrusions 404, i.e. there will be protrusions 404 that do not have the cap 418.

The plurality of protrusions 404 may be arranged in an array that achieves the pattern to reveal a passcode in a document. To achieve such an array, one embodiment has any two adjacent protrusions 404 spaced apart about equally (i.e. the space 406 between any two adjacent protrusions 104 is around the same). Each protrusion 404 may have a width from a range 10nm to 300nm, with a suitable width being for example 70nm. The distance for the space 406 may be from a range 10nm to 300nm, with a suitable distance being for example 70nm. In another embodiment, the array may be realised by the optical grating 400 having a repetitive pattern, whereby two adjacent protrusions 404 are spaced 406 a first distance apart and another two adjacent protrusions 404 are spaced 406 a second distance apart, the size of the first distance being different from that of the second distance. In yet another embodiment, this array may not have a repetitive pattern, whereby a first distance separating two adjacent protrusions 404 and a second distance separating two adjacent protrusions 404 each have respective lengths that are not duplicated.

Figure 5 shows the cross-sectional structure of an optical grating 500 that can also be used to realise a security film, according to various embodiments of the invention.

The optical grating 500 includes a substrate 402 comprising a plurality of protrusions 404. A space 406 exists in between any two adjacent protrusions 404. A cap 518 is provided on at least one of the plurality of protrusions 404 at an end 403 that is furthest from the substrate 402. The cap 518 has a higher degree of optical attenuation compared to the substrate 402 material.

Accordingly, the difference between the optical grating 500 and the optical grating 400 of Figure 4 lies in the cap 518. The combination of each protrusion 404 and the respective cap 518 thereon has an asymmetric cross-sectional profile.

With reference to Figures 4 and 5, the plurality of protrusions 404 may be fabricated simultaneously with the substrate 402 and are therefore integral with the substrate 402 and made of the same material as the substrate 402. The substrate 402 may be made from direct imprinting onto a plastic sheet (which includes all types of thermoplastic films, e.g. polycarbonate (PC), polymethylmethacrylate (PMMA) and polyethylene) so that the substrate 402 is flexible. The cap 418/518 may be made from different material from the substrate and comprise material that has a higher degree of optical attenuation, compared to the substrate 402 material. The cap 418/518 is preferably opaque. Such an opaque overcap can be made using metals (e.g. aluminum, gold and chromium) or dielectric with light-absorbent additive molecules. The degree of opacity (i.e. degree of optical attenuation) in metals is controlled by the thickness of the metals. Using metals to realise the cap 418/518 provides the cap 418/518 with reflective properties. The degree of opacity (i.e. degree of optical attenuation) in a dielectric is controlled by the amount of light-absorbent additive molecules (e.g. pigments, dyes, colorants and photosensitive emulsions, etc.) present in the dielectric. Using a dielectric to realise the cap 418/518 provides the cap 418/518 with non-reflective properties.

The physical properties of the cap 418/518 imparted by the material used to fabricate the cap 418/518 allows the optical grating 400/500 to function as a nanowire grid polarizer in the following manner. Given that a non-reflective overcap does not reflect TE polarized light, an opaque and reflective overcap for the cap 418/518 and the portion of the substrate 402 that spans across the space 406 allow the optical grating 400/500 to function as a nanowire grid polarizer according to the operating principle described below.

The cap 418/518 provides sub-wavelength gratings on the transparent substrate 400/500. The submicron size gratings provide a series of alternate opaque and transparent slits. In principle, the optical characteristics of a nanowire grid polarizer enables incident light with polarization parallel to the submicron size gratings to be reflected with little amount of this incident TE-polarized light transmitted through the polarizer, while enabling the incident light with perpendicular polarization to be transmitted through the polarizer.

Cross-referencing the security film 100 described above with respect to Figure 1 D, the portion of the substrate 402 that spans across the space 406 provides the plurality of first portions 102 (of the security film 100) with a different direction of polarisation compared to the direction of polarisation of the plurality of second portions 104 (of the security film 100) realised by the cap 418/518. This has the effect of revealing a passcode within the document 140 to have a different colour compared to the remaining content of the document 40 from observation through the security film 100. In more detail, the document 140 may have coloured and non- coloured (black and white) portions. When viewed through the security film 100, the passcode may be perceived to have a colour different from its original. A security film, according to various embodiments of the invention, takes advantage of the trend that pixels per inch (PPI), for modern displays, has already exceeded visual acuity (the human eye has difficulty in differentiating detail beyond 300 PPI) and is still increasing, as shown in the chart 600 of Figure 6. PPI is an absolute measure of display resolution that is independent of the size of the monitor.

Techniques used to fabricate a security film, according to various embodiments of the invention, comply with fabrication process that are able to meet pixels and/or sub pixels precision requirements in already available displays and future displays built following this increasing PPI trend.

Figure 7 shows a flowchart 700 of a method to fabricate a security film, according to various embodiments, for revealing a passcode present in content of a document when the security film is correctly aligned over the document.

In step 702, a substrate comprising a plurality of first portions; and a plurality of second portions is provided. The plurality of second portions has a higher degree of optical attenuation compared to the plurality of first portions.

In step 704, a designated area of the substrate is patterned, so that each of the plurality of first portions is arranged to alternate with each of the plurality of second portions, to allow light from segments of the passcode to transmit through the substrate, via at least one of the plurality of first portions within the designated area. This creates specific viewing zones having sufficient overlap of such light to reveal the passcode from the remaining content of the document. The patterning of the designated area of the substrate may be performed by a photoresist technique.

The substrate may comprise a plurality of protrusions with a space in between any two adjacent protrusions. A cap may be provided on at least one of the plurality of protrusions at an end that is furthest from the substrate. The cap may have a higher degree of optical attenuation compared to the substrate material.

In one embodiment (see Figure 4), the combination of each protrusion and the respective cap thereon has a generally symmetric cross-sectional profile. In another embodiment (see Figure 5), the combination of each protrusion and the respective cap thereon has an asymmetric cross-sectional profile.

The substrate may be fabricated from flexible material that comprises any one or more of the following: plastic, polymethylmethacrylate, polycarbonate and polyethylene.

The method shown in Figure 7 is described in greater detail below with reference to Figure 8, which illustrates fabrication of a security film from a wire grid polarizer (WGP) using nano imprinting lithography (NIL) and reactive ion etching (RIE) techniques. The publication, "Fabrication of a 50 nm half-pitch wire grid polarizer using nanoimprint lithography" by Ahn, S. W., Lee, K. D., Kim, J. S., Kim, S. H., Park, J. D., Lee, S. H. & Yoon, P. W., Nanotechnology, 16, 1874-1877 (2005) provides detail on exemplary NIL and RIE techniques that can used.

The fabrication shown in Figure 8 has two stages A and B. The first stage A comprises of steps 801 , 809, 811 , 813 and 815. The second stage B comprises of steps 817 and 821.

Stage A:

In step 801, a metal layer 806, in this case aluminium (Al), is coated onto a base flexible substrate 802, such as polycarbonate, in an evaporator chamber. A poly resin polymethylmethacrylate (PMMA) 808, such as an imprint resist from Microresist Technology GmbH, is spin-coated to the substrate 802. The structure is then baked in an oven chamber to remove any remaining stamp residual and trapped moisture.

In step 809, a mould 810 is provided that acts as a silicon imprinting stamp to impart the desired WGP structure (in the pitch range of 120nm to 160nm) on the substrate 802. The mould 810 may be fabricated using an electron beam by a commercial nano structure stamp manufacturer such as NTT-AT (Japan). The grating patterns on the stamp should have an aspect ratio as high as possible (such as aspect ratio 4) in order to facilitate the fabrication of Al gratings using RIE in the step 809. The fabricated stamp is pre-treated by an anti-sticking layer (1 H, H, 2H, 2H-perfluorodecyl trichlorosilane) to prevent the stamp from adhering to the imprint pattern during a NIL process that occurs in step 809. An NIL technique is then used to transfer the WGP structure to the PMMA 808 using the WGP mould 810

In step 811 , a residual layer 808r, that results from the PMMA 808 being patterned by the WGP mould, is trimmed using 0₂ RIE.

Step 8 3 shows the end result from the 0₂ RIE etch of step 811. This 0₂ RIE exposes the Al layer 806 which is etched using BCI₃/CI₂ plasma enhanced by a magnetic field in step 815. The Al layer 806 may be etched using BCI₃ of 40 seem and Cl₂ of 15 seem for 40s with rf power of 100 Wand and a working pressure of 10 mT. The etch selectivity of mri-8020 versus Al is higher than 1 :3. After the etch, the remaining polymer 810 on top of the Al pattern 806p is removed using a photo stripper. At the end of stage A, a WGP structure will be formed, being an optical grating that comprises the substrate 802 with the Al pattern 806p formed thereon.

Stage B:

Once the WGP structure is completed, security patterns can be patterned on the WGP structure. The surface of the WGP structure is first degreased using common degreaser such as isopropyl alcohol. Next, a layer of photoresist 818 (any common photo resist) is spin coated onto the substrate 802, followed by a hot plate soft baking process for dehydration of resist trapped moisture. The substrate 802 is than allow to cool down to room temperature over a period of 3 hours. This slow cooling down process allows the photoresist 818 to achieve a higher side wall gradient, which will later facilitate better distinct security patterns.

Step 817 shows the substrate 802 with the photoresist 818 after cooling.

Using a conventional lithography UV patterning machine, security patterns which are pre drawn onto a glass mask 820 is patterned at i-line UV range followed by developing in a normal resist developer. The whole process is completed in step 821 with the stripping of the residual photoresist 818 using a diluted photo resist stripper of ratio 1 :2. The result would be a security film 800 comprising the substrate 802 having a grating thereon (formed after stripping of the residual photoresist 818) that is patterned to create specific viewing zones that reveal a passcode within a document. The grating may be an array of aluminum protrusions arranged in a pattern to create such specific viewing zones, whereby the protrusions may each have a generally rectangular cross-sectional shape. With reference to the security film 100 of Figure 1 D, the portion of the substrate 802 between two aluminum protrusions correspond to the plurality of first portions 102 of the security film 100, while the aluminum protrusions correspond to the plurality of second portions 104 of the security film 100 of Figure 1 D. The aluminum protrusions provide a different direction of polarisation compared to the substrate 802, so that when observing a document through the security film 800, a passcode is revealed to have a different colour compared to the remaining content of the document. While the steps above describe that aluminum is used, another metal such as gold may instead be used. Gold is also able to provide a different direction of polarisation compared to the substrate 802.

A security film, according to various embodiments of the invention, can be used in the following security applications:

1 ) Internet banking using smartphone secure applications and Multimedia Messaging Service (MMS);

2) Enterprise Security;

3) Authentication access for Automated Teller Machine (ATM); or

4) Access control panel

Further details of these applications are provided below.

1) Internet banking using smartphone secure applications and Multimedia Messaging Service (MMS) Figure 9 shows a sequence of steps that may be followed when incorporating a security film, according to various embodiments of the invention, for internet banking.

In step 1 , a user, using a smartphone application, accesses a protected resource and triggers an authentication process. In step 2, a web server requests for login information, such as a userid and a password. This userid and passcode act as a first factor authentication. In step 3, a user submits his userid and password.

In step 4, the web server authenticates the user. In one exemplary implementation, when authenticating the user, the server may determine (from the userid and password information) a serial number of the security film. The web server may contain a database that contains serial numbers of security films, with each being registered to a unique userid and password. The database may be populated from pre-registration of security films each belonging to a respective user. Thus, with the userid and password information, the serial number of the registered security film will be retrieved from the database. The database may also store, from prior registration, a display specification associated with the security film 100 having that serial number, the display specification storing the design parameters of the security film. In step 5, a Lightweight Directory Access Protocol (LDAP) server retrieves the user contact number and the associated display specification. With information on the user contact number and the design parameters of the security film from the the associated display specification, a token cache and passcode generator in the web server generates, in step 6, a passcode that is stored in a Cross Domain Authentication Service (CDAS) token cache and which will serve as a second factor authentication. The passcode is created in a format that is compliant to the design parameters of the security film, i.e. the security film is able to decipher the passcode after it is hidden in an image (see step 7). In step 6, the web server informs the user to enter a passcode for the second factor authentication. In step 7, the web server deploys the second factor authentication by hiding the passcode into an image via a secure embedded system. The image is then sent to a MMS or SMS (short message service) gateway in step 8. In step 9, the smartphone application will receive the image, whereby a user will align the security film over the image to unveil the passcode (see Figure 10). A passcode is then submitted in step 10 via the smartphone application to the web server. A CDAS module will then verify whether the passcode entered in step 10 matches the passcode saved in the CDAS token cache.

2) Enterprise Security Figure 11 shows a sequence of steps that may be followed when incorporating a security film, according to various embodiments of the invention, for enterprise security.

In step 1102, a user enters his employee ID and personal password pin. This employee ID and personal password pin act as a first factor authentication.

In step 1104, after the employee ID and personal password pin are entered, an image appears which contains a passcode that will serve as a second factor authentication. A field is also provided to enter the passcode hidden in the image.

In step 1106, the user will align the security film, which may be provided in his employee ID card, over the image to unveil the passcode.

3) Authentication access for Automated Teller Machine (ATM)

Figure 12 shows an ATM that may be in communication with a host computer and a bank computer. As an additional security measure, a display containing passcode to be revealed by a security film, according to various embodiments of the invention, may be shown before any transactions are performed before the ATM. A user will align the security film over the display to unveil the passcode.

4) Access control panel

Figure 13 shows a panel that may be used to grant door access. The panel may have a screen 1305 that can generate images that each contains a passcode to be revealed by a security film 1300, according to an embodiment of the invention. The security film 1300 is aligned over the screen 1305 to unveil the passcode.

In addition to being able to reveal passcodes, a security film, according to various embodiments of the invention, may also double up as a protection layer over a display.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the embodiments without departing from a spirit or scope of the invention as broadly described. The embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.

Claims

1. A security film for revealing a passcode present in content of a document when the security film is correctly aligned over the document, the security film comprising a substrate comprising

a plurality of first portions; and

a plurality of second portions, wherein the plurality of second portions have a higher degree of optical attenuation compared to the plurality of first portions,

wherein, within a designated area of the substrate, each of the plurality of first portions is arranged to alternate with each of the plurality of second portions in a pattern designed to allow light from segments of the passcode to transmit through the substrate, via at least one of the plurality of first portions within the designated area, to create specific viewing zones having sufficient overlap of such light to reveal the passcode from the remaining content of the document.

2. The security film of claim 1 , wherein the plurality of first portions further has a different direction of polarisation compared to the plurality of second portions, so that the passcode is revealed to have a different colour compared to the remaining content of the document from observation through the security film.

3. The security film of claim 2, wherein the different direction of polarisation comprises using aluminum or gold for the plurality of second portions.

4. The security film of any one of the preceding claims, wherein the passcode is revealed by being brighter than the remaining content of the document from observation through the security film due to the lower degree of attenuation of the plurality of first portions in the designated area.

5. The security film of any one of the preceding claims, wherein the substrate comprises

a plurality of protrusions with a space in between any two adjacent protrusions; and

a cap provided on at least one of the plurality of protrusions at an end that is furthest from the substrate, wherein the cap has a higher degree of optical attenuation compared to the substrate material.

6. The security film of claim 5, wherein the combination of each protrusion and the respective cap thereon has a generally symmetric cross-sectional profile.

7. The security film of claim 5, wherein the combination of each protrusion and the respective cap thereon has an asymmetric cross-sectional profile.

8. The security film of any one of the claims 5 to 7, wherein the cap comprises different material from the substrate.

9. The security film of any one of the claims 5 to 7, wherein the cap is opaque.

10. The security film of any one of the claims 5 to 9, wherein the cap comprises one or more metals.

11. The security film of any one of the claims 5 to 8, wherein the cap comprises dielectric materials with a light-absorbent additive.

12. A method of forming a security film for revealing a passcode present in content of a document when the security film is correctly aligned over the document, the method comprising

providing a substrate comprising

a plurality of first portions; and

a plurality of second portions, wherein the plurality of second portions have a higher degree of optical attenuation compared to the plurality of first portions;

patterning a designated area of the substrate so that each of the plurality of first portions is arranged to alternate with each of the plurality of second portions to allow light from segments of the passcode to transmit through the substrate, via at least one of the plurality of first portions within the designated area, to create specific viewing zones having sufficient overlap of such light to reveal the passcode from the remaining content of the document.

13. The method of claim 12, wherein the substrate comprises

a plurality of protrusions with a space in between any two adjacent protrusions; and a cap provided on at least one of the plurality of protrusions at an end that is furthest from the substrate, wherein the cap has a higher degree of optical attenuation compared to the substrate material.

14. The method of claim 13, wherein the combination of each protrusion and the respective cap thereon has a generally symmetric cross-sectional profile.

15. The method of claim 13, wherein the combination of each protrusion and the respective cap thereon has an asymmetric cross-sectional profile.

16. The method of any one of claims 12 to 15, wherein the substrate is fabricated from flexible material.

17. The method of any one of claims 12 to 16, wherein the substrate comprises any one or more of the following materials: plastic, polymethylmethacrylate, polycarbonate and polyethylene.

18. The method of any one of claims 12 to 17, wherein the patterning of the designated area of the substrate comprises a photoresist technique.