JP2011154212A - Encryption recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an encryption recording medium wherein, while a visual decoding type secret sharing scheme is used, two encrypted images are formed on one and the same base material, and no complicated operations such as precisely overlapping the two encrypted images with each other are necessary in restoring the original image. <P>SOLUTION: Pixels of an encrypted image encrypted by using the visual decoding type secret sharing scheme are further divided, and the encrypted images are formed in a diffraction grating each having different grating angle. The two encrypted images formed on one and the same base material become visually recognizable simultaneously only when irradiated with two types of light sources having the irradiation angle suited to the two types of diffraction gratings when restoration is performed, thereby a concealed image is decrypted and becomes readable. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  In the present invention, the recorded image is concealed so that it cannot be visually recognized in a normal state, and the concealed shape information is visualized under a light source of a specific condition at the time of restoration, and the original shape information can be read. The present invention relates to an encryption recording medium.

  In the normal state, there is a long-standing need to use information recorded in a concealed state when it is needed. For example, concealment seals and crimping postcards are familiar examples. These are one-time use (disposable) and are suitable for temporary use. On the other hand, while maintaining high secrecy when not in use, they can be restored by simple operation when used, and It is very difficult to meet the conflicting requirement of being reusable.

There is a visual decryption type encryption technique called a visual decryption type secret sharing method as one of the techniques for the above requirements. This is done by processing the original image information into an image that cannot be visually recognized by dispersing it into a plurality of images (called shared images) composed of irregular patterns, and then superimposing the shared images. This technique restores the original image information to a visible state.
For example, Patent Document 1 is a representative example of visual decryption encryption technology, and a technology related to the configuration and production of a recording medium that conceals information without using a concealment layer or the like using a visual decryption secret sharing method. Is disclosed.
Various methods have been proposed for the visual decryption secret sharing method, the shared image generation means, and the like, and the principle and generation method are described in detail in Non-Patent Document 1, which is a pioneering paper. .

JP 2001-118122 A

M.Naor and A.Shamir, "Visual Cryptography", Proc. Of Eurocrypt'94 May 1994

  As described above, the visual decryption type secret sharing method is advantageous in that it can be restored by visual manual operation, but on the other hand, since the original image is restored by overlapping a plurality of shared images, In order to restore the original image as many media as the number of shared images is required, an operation of precisely superimposing these plural media is required.

  FIG. 1 is a diagram showing a process of encryption and restoration using the visual decryption secret sharing method. For example, as shown in FIG. 1, “T” of the restored image 13 is obtained from two shared images. In this case, the number of required media is 2, and a plurality of media are used. Moreover, as shown in FIG. 2, since the images are overlapped and visually observed, the overlapping surfaces of the two media need to be smooth so that they are in close contact with each other, and at least one of the media is a transparent sheet like the transparent medium a21. It is essential to be. Further, when two shared images are superimposed, there is a problem that it is difficult to distinguish between an image to be restored and a background because a white portion appears with a slight shift in overlap and a difference in density from the background becomes small.

  Therefore, the present invention can form two shared images on the same medium while using the visual decryption type secret sharing method, and has the trouble of superimposing the two shared images when restoring the original image. It is an object of the present invention to provide an encryption recording medium that does not require an operation.

The present invention for solving the above problems
An encryption recording medium formed by superimposing a first encryption pattern and a second encryption pattern on a base material to generate an original image from an encryption image by a visual decryption secret sharing method,
The first cipher pattern and the second cipher pattern are formed by diffraction gratings having different grating angles.
Each pixel constituting the first cipher pattern and the second cipher pattern is a set of cells obtained by dividing the pixel, the cell constituting the pixel of the first cipher pattern, and the second cipher pattern The cells constituting the pixels of the encryption pattern are arranged so as not to overlap each other.
Further, the two types of diffraction gratings have a grating angle orthogonal to each other.

  According to the encryption recording medium of the present invention, since two share images can be formed on the same medium, the cost of the medium can be reduced, and an operation for superimposing the share images is eliminated. Further, since a transparent medium is not required and the sheet is not required to be a smooth sheet, there are no restrictions on the material and shape of the medium.

Conceptual diagram of visual decryption secret sharing method Diagram showing how the visual decryption secret sharing method is restored Diagram showing encryption pattern generation process Figure showing a restored image Diagram showing the process of generating a new cipher pattern Diagram showing observation state The figure which shows the details of the restoration picture with new encryption pattern

Hereinafter, the details of the encryption recording medium of the present invention will be described with reference to FIGS.
FIG. 3 is a schematic diagram showing the encryption pattern generation process used for the encryption recording medium of the present invention in comparison with the encryption pattern generation process by the conventional visual decryption secret sharing method. FIG. 3A shows an encryption pattern generation process by a conventional visual decryption secret sharing method, and FIG. 3B shows an encryption pattern generation process according to the present invention, focusing on pixels forming each pattern. Explained. In the following description, the former is referred to as a share pattern and the latter is referred to as a new cipher pattern in order to distinguish the cipher pattern according to the conventional visual decryption secret sharing method from the cipher pattern according to the present invention.

First, a share pattern based on the conventional visual decryption secret sharing method will be described.
As shown in FIG. 3A, the share pattern based on the conventional visual decryption secret sharing method is such that the pixels constituting the original image (the pixels 30 of the original image) have a small integer ratio (2 × 2 in the example). It is generated by dividing the area 35 and assigning white or black to each small area 35.
The share pattern generated in this way is a pixel constituting the share image A and the share image B. That is, the share pattern a32a and the share pattern a33a are the pixels of the share image A, and the share pattern b32b and the share pattern b33b are the pixels of the share image B. Then, when the share image A and the share image B are overlapped and the original image is restored, a pixel representing “white” such that the overlay of the two patterns such as the share pattern a 32 a and the share pattern b 32 b is the restored pixel 32. Thus, the overlapping of the two patterns of the share pattern a33a and the share pattern b33b becomes a pixel representing “black” like the restoration pixel 33.

Next, the new encryption pattern in the present invention will be described.
In the generation of the new cipher pattern of the present invention, as shown in FIG. 3B, first, a new divided pattern 41 (FIG. 3) for further dividing the 2 × 2 small area shown in FIG. In the example, 6 × 6) is specified. A new cipher pattern a (a series pattern) and a new cipher pattern b (b series pattern) are generated by assigning white or black to each of the divided areas (referred to as cells).
The process of generating these two new encryption patterns will be described in detail later with reference to FIG.
Further, in FIG. 3B, for the sake of understanding, different hatching is applied to the cells to which black of the a-series and b-series patterns are assigned, thereby distinguishing the new cipher pattern a and the new cipher pattern b. .

  Two kinds of patterns such as the new restoration pattern 42 or the new restoration pattern 43 are formed by combining the two kinds of new encryption patterns generated in this way. For example, when two patterns such as the new cipher pattern a42a and the new cipher pattern b42b are observed at the same time, it becomes a pixel (referred to as a “coarse” pixel) having a distribution of “concentrated” cells like the new cipher pattern 42. Similarly, when the two patterns of the new cipher pattern a43a and the new cipher pattern b43b are observed at the same time, pixels having a distribution of “distributed” cells like the new cipher pattern 43 (referred to as “dense” pixels). Become.

  FIG. 4 is a diagram showing the restored image based on the new encryption pattern generated in this way, compared with the restored image based on the conventional visual decryption secret sharing method, and FIG. 4 (A) shows the conventional visual decryption type. A restored image by the secret sharing method and FIG. 4B show a restored image by the encryption pattern of the present invention.

FIGS. 4A and 4B show how the character part of “secret” can be distinguished from the background part and identified by combining two shared images.
FIG. 4A showing a restored image by the conventional visual decryption secret sharing method observes an image that appears when two shared images are superimposed, and represents an irregular pattern representing “white” and “black”. The character part 36 and the background part 37 are distinguished from each other by the density difference of the fill pattern.

  On the other hand, in the restored image using the encryption pattern of the present invention, the discrimination is made based on the difference in density between the pixel patterns of the character portion 46 and the background portion 47 as shown in FIG. That is, the state where the new restoration patterns 42 in FIG. 3B are gathered is visually recognized as a distribution of “coarse” pixels, and the collection of new restoration patterns 43 is visually recognized as a distribution of “dense” pixels.

  Further, by configuring the a-series pattern and b-series pattern in FIG. 3B with reflective diffraction gratings having different grating angles, a restored image can be obtained only when simultaneously illuminating a light source having a specific irradiation angle. Appearing and the restored image can be invisible (unreadable) under normal viewing conditions. Thereby, it becomes possible to superimpose and form two share images (encryption pattern) on a single base material.

<Generation of new cipher pattern>
FIG. 5 shows a state where a cell of the new encryption pattern of the present invention is generated by dividing the original pixel.
FIG. 5A is a diagram for explaining a template used for generating an encryption pattern according to the present invention, and FIG. 5B is a diagram showing a process of generating a new encryption pattern using the template.
The template 50 in FIG. 5A is obtained by assigning “1” or “2” to the new divided pattern 41 in a staggered pattern, and by applying the templates “1” and “2”, The pixel 40 of the image can be divided into two new cipher patterns without overlapping cells.

A procedure for generating the cell of the new encryption pattern will be described below.
As shown in FIG. 5B, for example, “1” of the template 50 (that is, template 51 “1”) is applied to the share pattern 32a according to the conventional visual decryption secret sharing method, and “ When the cell corresponding to “1” is operated so as to be successful, a cell pattern such as the new cipher pattern a42a is obtained. Similarly, a new encryption pattern b42b is obtained by applying the template 52 “2” to the share pattern b32b, and a new encryption pattern b43b is obtained by applying the template 52 “2” to the share pattern b33b.

  As described above, by applying “1” and “2” of the template 50 to each pixel of the shared image A and the shared image B in the conventional visual decoding secret sharing method, the first and second of the present invention are applied. Two types of encryption patterns corresponding to the encryption patterns can be generated.

The new restoration pattern 42 and the new restoration pattern 43 that are pixels constituting the restored image appear by superimposing the new cipher pattern a and the new cipher pattern b (combination of a series and b series).
That is, as shown in FIG. 5B, a new restoration pattern 42 appears by superimposing the new cipher pattern a42a and the new cipher pattern b42b. A pattern 43 appears.
The new restoration pattern 42 and the new restoration pattern 43 form the character portion 46 and the background portion 47 in FIG. 4B, respectively, and the restoration information can be read.
In FIG. 5B, for the sake of understanding, the new cipher pattern a (a series pattern) and the new cipher pattern b (b series pattern) are distinguished by two types of hatching.

<Observation state of the encryption recording medium of the present invention>
FIG. 6 is a diagram showing an observation state. FIG. 6 (a) shows a state where image information, which is a normal observation condition, is concealed. FIG. 6 (b) shows an encrypted image by irradiation with a predetermined light source. This represents a state where the decoded information appears after decoding.
The information cell 61 shown in the figure is a cell provided with a diffraction grating, and the new encryption pattern a and the new encryption pattern b have different grating angles as shown as the diffraction grating a62 and the diffraction grating b63.

  The appearance of the encryption recording medium 1 according to the present invention will be described with reference to FIGS. 6A and 6B. In the state of FIG. 6A, the cells of the new encryption pattern a and the new encryption pattern b are irregularly formed. A specific image does not appear on the encryption recording medium 1 because it reflects light. On the other hand, as shown in FIG. 6B, the new cipher pattern a and the new cipher pattern b are simultaneously generated under two light sources each having a predetermined irradiation angle with respect to the diffraction grating a62 and the diffraction grating b63. Since it is visually recognized, patterns of “rough” and “fine” appear, and the character portion 46 and the background portion 47 can be discriminated. That is, the character string “ABC” can be read.

Note that the diffraction grating has an angle dependency on the angle of the incident light and the line-of-sight angle due to the effect of diffraction and interference on the incident light. That is, the visual light intensity varies depending on the light source position and the viewpoint position. The predetermined light source means a light source having an irradiation angle at which the light intensity from the cell is viewed most strongly.
For example, in FIG. 6B, when a white light source is used as the light source a65 and the light source a66, and the angle formed by the light source a65 and the diffraction grating a62 and the angle formed by the light source b66 and the diffraction grating b63 are arranged in an orthogonal relationship, The new cipher pattern a and the new cipher pattern b are simultaneously recognized with high light intensity, and the character portion 46 and the background portion 47 can be clearly distinguished.

  Also, assuming that the grating angle of the diffraction grating a62 is the angle a and the grating angle of the diffraction grating b63 is the angle b, the larger the difference between the angle a and the angle b, the clearer the restored image. And the angle b are preferably orthogonal.

FIG. 7 is a diagram showing details of the restored image by the encryption pattern of the encryption recording medium of the present invention.
The restored image 70 indicates that both the first cipher pattern 71 and the second cipher pattern 72 are visually recognized. As a result, the character portion 46 and the background portion 47 can be identified, and “s "Can be read. On the other hand, in a state where either the first cipher pattern 71 or the second cipher pattern 72 can be seen alone, each irregular pattern appears and the character portion 46 and the background portion 47 cannot be distinguished.
The enlarged boundary image 76 is an enlarged view of the boundary portion between the character portion 46 and the background portion 47. As a result, the cell distribution between the background portion and the character portion is “rough” and “dense”. You can see the difference.

In FIG. 7, the first cipher pattern 71 and the second cipher pattern 72 are expressed separately for the sake of explanation, but actually they are formed so as to overlap on the same base material.
The enlarged image 75 is an enlarged view of the cell pattern of the encryption pattern.

  As described above, in order to obtain the restored image 70 from the first cipher pattern 71 and the second cipher pattern 72 formed on the same substrate, both patterns have different lattice angles in the cipher recording medium of the present invention. Formed by a cell.

  As described above, according to the encryption recording medium of the present invention, since two encryption patterns can be formed on the same base material, when obtaining a restored image, an operation for overlaying secret images as in the prior art is performed. Do not need. Further, the transparent medium is not required, and the medium is reduced, so that the cost of the medium can be reduced and the surface does not need to be smooth because it does not need to be in close contact. Since there is no need for a sheet, there are no restrictions on the material and size of the base material.

In order to produce the encryption recording medium of the present invention, the first encryption pattern and the second encryption pattern may be formed on the substrate by a reflective diffraction grating. In order to form a diffraction grating, as a well-known method, for example, a transparent ultraviolet curable resin composition is applied to the surface of a resin base material to a thickness of about 3 μm, and a mold surface of a diffraction mold for replication is brought into contact. The diffraction grating is formed by curing the transparent ultraviolet curable resin composition by irradiating ultraviolet rays as it is, and then a metal such as aluminum is vacuum-deposited on the surface on which the diffraction grating is formed to have a thickness of about 100 nm. A reflective layer.
As the base material, various materials such as paper and resin can be used. For example, for card applications, polyvinyl chloride resin, amorphous polyethylene terephthalate (PET-G) resin, polycarbonate resin, polylactic acid resin. and so on. Further, the substrate may be transparent or opaque.

1 Encryption recording medium 10 Original image 11 Share image a
12 share image b
13 Restored image 21 Transparent medium a
22 Medium b
30 pixels of original image 31 division pattern 32 share pattern 33 share pattern 40 pixels of original image 41 new division pattern 42 new restoration pattern 43 new restoration pattern 45 cell 46 character part 47 background part 50 template 61 information cell 62 diffraction grating a
63 Diffraction grating b
70 restored image 71 first cipher pattern 72 second cipher pattern

Claims (3)

An encryption recording medium formed by superimposing a first encryption pattern and a second encryption pattern on a base material to generate an original image from an encryption image by a visual decryption secret sharing method,
The encryption recording medium, wherein the first encryption pattern and the second encryption pattern are formed by diffraction gratings having different grating angles.   Each pixel constituting the first cipher pattern and the second cipher pattern is composed of a set of cells obtained by dividing the pixel, the cell constituting the pixel of the first cipher pattern, and the second cipher pattern 2. The encryption recording medium according to claim 1, wherein the cells are arranged so as not to overlap with cells constituting the pixels of the pattern. 3. The encryption recording according to claim 1, wherein a grating angle of a diffraction grating forming the first encryption pattern and a grating angle of a diffraction grating forming the second encryption pattern are orthogonal to each other. Medium.

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