DE102010031960A1 - Method for transmitting secret image information to receiver, involves magnifying viewing angle corresponding to respective portions of image with respect to eye of viewer is magnified by refraction effect of transparent medium - Google Patents

Abstract

The method involves providing a transparent medium with curved and/or sloping surface portions, such as prism (B) and lens. The viewing angle corresponding to the respective portions of the image (C) with respect to eye of the viewer (A) is magnified by refraction effect of transparent medium when image is viewed through transparent medium. The remaining portion of image is hidden from eye of the viewer.

Description

The present invention relates to a method for secure encryption for image-representable information, in particular for written texts.

The new method is inspired by the so-called visual cryptography, which was presented in 1994 by the scientists Naor and Shamir at a cryptography conference. Visual Cryptography is a method of encrypting visual information.

The basic principle of visual cryptography is to provide the recipient of a message with a foil that is labeled with transparent and non-translucent areas so that parts of an image on which the foil is placed are transmitted to the recipient's eye and other parts to be covered. Under the knowledge of the film, the sender can thus create an image for the recipient such that after the film has been placed on the film, the image appears as the overall image information that the sender intended to send. Skillful (pseudorandom) random filling of the remaining parts of the image ensures that no information can be found on the image created by the sender that the recipient sees after placing the film.

Since the introduction of visual cryptography, several dozen scientific papers have been written about it. The patent EP-2006-068081 re-invents Visual Cryptography. The patent specification 10-2007-018802 (this inventor) uses visual cryptography to also be able to send in the reverse direction than the original secret messages.

Visual Cryptography is a variant of the Cardano cryptography invented in the 16th century by G. Cardano, also called Richelieu or cardigan cryptography. The patents WO-2006-020096 and EP-2006-068081 re-invent the Cardano cryptography. Cardano cryptography also works on the principle that the receiver receives a card with some passages transparent and others opaque. Visual cryptography applies the principle of Cardano cryptography on a small scale, ie at the level of pixels.

Visual cryptography has the disadvantage that the contrast of the image read by the recipient decreases by at least half. This disadvantage of visual cryptography is in principle, d. H. he can not fix it. Another disadvantage is that it is difficult to encrypt so that a slide can be used for multiple encryptions without an eavesdropper being able to gather enough information from the images created by the transmitter to be able to tap into the label of the slide and thus listen in on further uses of the same slide. Another disadvantage is that the adjustment of the film on the image created by the transmitter can be very tedious. In particular, in the version in which the image created by the sender is displayed on a screen, the exact adjustability of the film on this image is so problematic that the variant slide-on-screen in practice can not be applied - and would be the most promising application of visual cryptography, see the above mentioned patents.

The present invention is inspired by visual cryptography, but is novel and seeks to overcome the above-mentioned disadvantages, at least in each case individual disadvantages.

The following refers to foils, although generally meant a clear medium, such as a glass or plexiglass sheet.

In refraction-type cryptography, as with visual cryptography, an image created by the transmitter is viewed by the receiver through a transparent film. The basic principle is that the light rays that reach the eye of the beholder, are deflected by refraction of light on the film surface. That is, for a location of the image that appears to the viewer, the angle of the film surface determines the origin of the light beam that reaches its eye. Overall, therefore, the viewing of the image through the film gives a new visual information for the viewer. The viewer "sees" this information on the slide. For example, in the the view of the observer (A) is guided by different prisms (B) to different areas with the inscriptions "1", "2", ... or "5". Since an attacker is unaware of the surface structure of the film (B), he can not conclude which pictorial information appears to the viewer.

With visual cryptography, the only possibility for a pixel on the front foil is to be transparent or not transparent. In refraction-type cryptography, the surface of the medium (film) for a pixel, as in FIG have many different angles, which cause the viewer's gaze to be diverted to different areas of the image behind it. Thus, a key in the form of such a refraction film may contain much more information than a comparable film in visual cryptography. This in turn makes it harder for the attacker to see who is on the screen the images shown, to combine information about the key. This makes it possible to use a film several times, or - in certain circumstances - even as often as safely.

The contrast is not limited in the refraction-cryptography, because the original colors of the image are transmitted without loss of contrast through the film to the eye of the beholder. For the same reason, the transmission of colored pictorial representations is directly and easily possible.

If a corresponding fragment of a lens is used for a pixel instead of a prism, the view is even focused on a single point in the area of the image. This has the advantage that the positioning of the film on the screen together with the parallax, which may arise from the change in the position of the viewer, only within the size of an area in the image must remain.

pictures

Depending on the inclination of the prisms (B) applied to the transparent film, a different information of the image (C) reaches the eye of the observer (A).

: Lens applied to the film enlarges some parts of the image (C) from the perspective of the viewer's eye (A), while other parts of the image do not reach the viewer's eye.

(See Embodiment 1): The eight prism faces on the film (B) each have a random tilt, which are respectively aligned with one of the 4 digits. With the slide, the viewer can see the sent information (C), but without the slide, little information is to be taken from the image (A).

(See Embodiment 2): By accidentally applied to the medium lenses (B) get some dark points of the image (A) enlarged by certain lenses to the eye of the beholder, while other lenses represent only bright parts of the image. For the viewer, this results in a picture with information content.

Embodiment 1

On a transparent, slightly thicker Kunstofffieie in credit card size are stamped by stamp parts of the surface. For example, there is only one stamp with a square stamp face that creates a skew at a certain angle, along the square diagonal, see , Thus, the film can be divided into a grid of corresponding squares, and with each such square there are 4 ways to stamp the square area, corresponding to the 4 possible rotation angles of the punch.

Such a corresponding (pseudo-) randomly generated slide is sent to the user of a real-world access, in order to be able to communicate new passwords for access online at regular intervals in the future, but secretly.

For the transmission of a new password, the user is shown on the computer screen a picture that consists of as many squares as his slide. Each square is divided into 4 sub-squares, each displaying one character. Each of the 4 sub-squares corresponds to one of the possibilities of stamping. Now the user keeps the film at a certain distance from eye to slide and slide to screen on the image, whereby he can still adjust the size of the image on the screen by intuitively pulling with the computer mouse. The angle of the slope of the punch was chosen so that the user sees exactly one sign at each of the squares by the refraction of light, namely, where the refraction of light at the slope guides his gaze - the other 3 characters the user does not see.

The server can thus transmit a new password as follows. The server knows from each of the squares the rotation angle of the stamp, and thus knows which of the 4 possible sub squares of a square the user's gaze will be directed to. So he labels these sub squares successively with the letters of the password. The remaining sub squares he fills out at random.

The method is suitable for safe, d. H. inaudible online transmission of passwords.

Embodiment 2

It should be encrypted texts with characters. The images are displayed on a screen and viewed by the receiver with a refraction film.

For this purpose, a film is created with many small applied lenses. Ideally, a targeted placement of lens fragments that focus on the respective area of the film on a specific point on the screen would be ideal. However, this may be difficult to achieve technically.

Easier is the application of initially liquid and then hardening drops, which are similar to the shape of a lens. This could either be done selectively with an inkjet printer or the like or by spraying the drops, so that the positioning of the drops is determined by physical coincidence.

In the latter case, the position of the drops can be determined with a scanner or, better, by the combined use of a screen on which the film is placed and a camera connected to the computer, which assumes the later position of the eye. In this case, individual pixels are temporarily turned on and determined by means of the camera, whether they are in the focus of a drop or are covered by the remaining surface of the drop. So are z. For example, d and j on the screen in the focus of the drops on the film (B) while b, c, e, f, h, i, k, l are obscured. The non-drip area, e.g. B. a, g, m on the screen (C), can be hidden by placing a black printed film there.

If now a secret black and white picture (eg the representation of a Y in ), in the image A transmitted to the screen, the areas (pixels) that are in the focus of a drop are colored with the color that predominantly has the image to be displayed in the area of the drop. To compensate, a nearby, but covered by a drop area is colored with the opposite color. In this way, the information of the secret image in image A is obscured.

Looking at the picture thus created, the receiver sees the sender's text message. An attacker can not see anything without the slide from the picture on the screen.

With appropriate design of the slide and the matching images, the process with the same slide can be repeated several times, if not as often as desired, without an attacker being able to hear the text. This is possible because the area available for concealment is large compared to the area whose pixels reach the viewer's eye.

QUOTES INCLUDE IN THE DESCRIPTION

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

Cited patent literature

• EP 2006-068081 [0004, 0005]
• WO 2006-020096 [0005]

Claims (6)

A method of transmitting secret information from a transmitter to a receiver, wherein the receiver views an image provided by the transmitter through a transparent medium having curved and / or sloping surface portions, such that only the light source has the refractive effect on the surface portions of the transparent medium parts of the image corresponding to the respective viewing angle reach the eye of the recipient or amplified or enlarged reach the eye of the recipient, and other parts of the image remain hidden from the eye of the recipient or get attenuated or reduced to the user's eye. Method according to one of the preceding claims, wherein the transparent medium on its surface consists of juxtaposed lenses or prisms, or such lenses or prisms are applied to a transparent base material. Method according to one of the preceding claims, wherein the image provided by the transmitter at the locations which are not or only reduced or attenuated reach the eye of the observer, are designed in a manner which the information which reaches the eye of the receiver for obscure a viewer who does not have the transparent medium, even with multiple use of the transparent medium. Method according to one of the preceding claims, wherein the image provided by the transmitter is displayed on a screen. Method according to one of the preceding claims, wherein the curved and / or inclined surface parts of the transparent medium are generated by physical chance. Method according to one of the preceding claims, wherein the refractive properties of the transparent medium are detected by an automatic computer program.

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