KR20050091732A - Key synchronization in a visual cryptographic system - Google Patents

Abstract

The system for visual cryptography comprises a server 1 for encrypting a series of images using a set of keys, a terminal 2 for displaying the encrypted images, and transmission of encrypted images from the server to the terminal 2. Transmission medium 4, and a decryption apparatus 3 for decrypting the encrypted image displayed on the terminal. The key identification is generated by the server 1 and displayed by the terminal 2. The decryption apparatus 3 detects the key identification and selects the corresponding key from the key set.

Description

Key synchronization in a visual cryptographic system

 The present invention relates to key synchronization of cryptographic systems. More particularly, the present invention relates to a method and system for synchronizing a first key set of an encryption device with a second key set of a decryption device, wherein the encryption device can encrypt images and the decryption device decrypts the images. can do.

It is well known to use key sets in cryptographic systems, and subsequent messages are encrypted using different keys of the key set. The use of different keys for different messages makes it much more difficult for a fraudster to decrypt any messages. In addition, the knowledge of a single key allows only a single message to be decrypted, keeping all other messages secret.

Of course, there is a need to synchronize the key sets, i.e., both the encryption device and the decryption device must ensure that the keys of the same key set are used to encrypt or decrypt the same message. If this synchronization fails, you will not be able to decrypt the messages correctly.

It is also known to encrypt the image to prevent it from being recognized by unauthorized persons or to prevent the content from being read. One technique for encrypting an image is disclosed, for example, in US patent application EP 0 260 815. This technique, also known as visual cryptography, uses two patterns, each of which is not individually recognizable and the patterns are overlaid to produce a recognizable image. To this end, the original image is converted into two random parts or patterns, none of which contain any perceivable image information. One of these patterns is printed on a transparent pattern or displayed on an at least partially transparent display to act as a decryption key. When these patterns are overlaid, these patterns are combined to "decode" in the viewer's eyes.

When viewing large quantities of individually encrypted images, it has been proposed to use a decoding (decryption) device rather than working with annoying transparent drawings. Two types of image decoding devices can be divided into transparent devices and opaque devices.

Transparent decryption devices essentially mimic the transparent sheets used in the prior art and display an encrypted image of one pattern (“shared”). Since the decoding device is at least partially transparent, different patterns of images can be seen through the device and the two image patterns are combined in the viewer's eyes as before. An advantage of using a transparent device instead of a transparent sheet is that the device can display multiple image portions rather than a single image portion. Thus, subsequent images may use different keys. Transparent decryption devices advantageously use LCD (liquid crystal display) screens, two of which are overlaid to "decrypt" the encrypted image to reconstruct the original image. Suitable examples of transparent devices employing LCD screens are disclosed in US patent application 02075527.8 [PHNL020121]. In the device of the European patent application, the use of the polarization rotating effect of the liquid crystal cells of the liquid crystal display device is made. This allows for the encryption and decryption of very convenient black and white images. European patent application 02078660.4 [PHNL020804] discloses a transparent decoding device which also allows color images to be decoded as well.

The opaque decryption devices may detect the encrypted image, perform decryption, and display the decrypted image. Decryption is performed on the device itself and the display shows the completed decrypted image, and the encrypted image is masked by the device. An example of such a decoding device is disclosed in European Patent Application No.02079579.5 [PHNL021058]. The decoding device may use the key to decrypt the image.

An image decoding apparatus will generally need at least one key to decrypt an image. However, to encrypt and decrypt multiple images in a cryptographically secure manner, one must employ a set of keys in which different keys are used to decrypt subsequent images. However, the use of a key set introduces the problem of key set synchronization. Even if a particular key sequence is predetermined, the encryption device and the decryption device may accidentally change the keys at different moments, or one of the devices may not change the keys at all, causing the key synchronization to fail. This will prevent the decryption device from decrypting the encrypted image.

1 schematically illustrates a cryptographic system according to the invention.

2 shows schematically a cross section of a decoder for use in the system of FIG.

3 schematically illustrates an example of an image used in a system and method in accordance with the present invention.

It is therefore an object of the present invention to provide a method and apparatus for establishing synchronization of an encryption apparatus and a decryption apparatus in a simple effective manner.

Another object of the present invention is to provide a method and apparatus for establishing synchronization of an image encryption apparatus and an image decryption apparatus.

Accordingly, the present invention provides a method of synchronizing a first key set of an encryption apparatus and a second key set of the decryption apparatus:

The cryptographic device generating an encrypted image and associated key identification using the keys of the first key set,

The cryptographic device sending the encrypted image and its associated key identification to a display device;

Displaying, by the display device, the encrypted image and its associated key identification;

The decoding device detecting the key identification;

The decrypting device decrypting the encrypted images using a key of the second key set corresponding to the detected key identification, and

And displaying, by the decoding apparatus, the decoded image.

That is, the cryptographic device uses the key of the key set to encrypt the image and generates a key identification corresponding to the key used to encrypt the image. Both the encrypted image and the key identification are sent to the display device allowing the decryption device to detect the key identification. The decryption apparatus uses the key identification to identify the key of the key set and then decrypts the encrypted image using the key thus identified. Preferably, the decoding device is used in the form of both sensing means for sensing the (encrypted) image and display means for displaying the (decoded) image.

By sending the encrypted image and the key identification, it is possible to always maintain key synchronization. Although it is possible to send all encrypted images and key identifications, it is not necessary to do so. Instead, the key identification may be sent periodically, for example only after a certain number of encrypted images have been transmitted or after a certain amount of time has elapsed. Alternatively, the key identification can be sent only upon request. It will be appreciated that generating the key identification may be omitted when no transmission is required.

It is possible for the key identification to be the same as the actual key. However, this is not cryptographically secure when key identification can be intercepted in transit. For this reason, it is preferable that the key identification is a code (eg, key number) associated with the key. It is also desirable that the key identification is a code derived from the key. This provides a degree of tamper protection.

In a preferred embodiment, the key identification is a hash value. Hash values are values that can be derived from a source value such as an encryption key using a hash function, and the form of the function is well known in the cryptography field. Typically, a hash function is a function that cannot determine a one-way function, that is, an inverse function. As a result, the hash value of the key can be easily determined, but it is impossible to derive the key from the hash value. In this way, interception of key identification does not compromise the key itself. In addition, any (unauthenticated) key changes will result in different hash values and will prevent unauthorized decryption of the encrypted image.

In a particularly advantageous embodiment, the step of the decoding device detecting the key identification is:

A substep of the decoding apparatus detecting the hash value and storing the hash value as the detected hash value;

And the decoding device calculates hash values of the second key set and compares each of the calculated hash values and the detected hash value until a match is found.

The exact key can be easily found by matching the hash value of the key set of the decryption apparatus with the detected hash value.

Of course, it is possible to precalculate and store the hash value of the second key set of the decryption apparatus. However, this requires a significant amount of memory space. We found that it is desirable not to store hash values because hash values can be calculated quickly.

Although the key identification may be sent separately, the key identification is preferably part of the encrypted image. This provides both simple transmission of key identification and easy detection by the decryption device. The key identification may form a sub-image of the encrypted image. This sub-image may be a symbol, code, or the like. The sub-images can be encrypted using additional keys that are preferably the same for a series of images.

In a preferred embodiment, the key identification is displayed on a display device such as a barcode. Bar codes can be easily recognized and read by the decoding device. However, other forms of code may also be used. In particular, a time multiplexing code can be used where parts of the code are displayed sequentially. These portions may or may not be constructed by barcodes.

The images used for synchronization according to the invention can be monochrome images or color images. Various techniques for rendering color images in visual cryptography and similar applications can be used, but liquid crystal display techniques are particularly suitable in European Patent Application No. 02078660.4 [PHNL020804EPP].

The invention also provides a system for synchronizing a first key set of an encryption device and a second key set of a decryption device:

An encryption device for generating an encrypted image and associated key identification using the keys of the first key set, and for transmitting the encrypted image and its associated key identification to a display device;

A display device for displaying the encrypted image and its associated key identification, and

And a decryption device for detecting said key identification, decrypting said encrypted images using a key of said second key set corresponding to said key identification, and displaying said decrypted image. .

With this type of system, synchronization of key sets can be easily achieved.

The present invention provides a decryption apparatus for use in the defined system, comprising: sensing means for detecting an encrypted image and key identification, key selecting means for selecting a key based on the detected key identification, and detecting the detected encrypted image. Decrypting means for decrypting using the selected key; And display means for displaying the decoded image.

Advantageously, the sensor means is part of an LED (light emitting diode) circuit, preferably an OLED (organic light emitting diode) circuit.

The invention will be further described below with reference to exemplary embodiments shown in the accompanying drawings.

The system shown by way of non-limiting example only in FIG. 1 includes a server 1, a terminal 2, a decoder (or decoder) 3 and a communication network 4. The server 1 generates and encrypts images transmitted to the terminal 2 via the communication network 4. The communication network 4 may consist of a LAN, a telephone network (POTS), the Internet, or a simple cable or wire. Both the server 1 and the terminal 2 can be dedicated devices or, at least in the case of the terminal 2, can be composed of general-purpose computers with a display screen 21. The decryptor 3 is an encryption device which will be described in more detail below. Both server 1 and decoder 3 are provided with at least one key set consisting of a plurality of cryptographic keys. These keys are used for proper cryptography such as DES. The specific cryptographic processing used is not essential.

As shown in the exemplary embodiment of FIG. 2, the decoder 3 includes a processor 31 for sensing the displayed image, and a processor 32 having an associated memory for performing cryptographic operations on the sensed image. And display elements 33 forming a display screen (34 in FIG. 1) for displaying the decoded image. Electrical conductors or optical fibers connect to the detectors 31, the processors 32 and the display elements 33. The set of cryptographic keys is stored in processor memory. Thus, the decoder 3 can detect the encrypted image, decrypt the image, and display the resulting decrypted image. The terminal 2 is a non-trusted device, and the decoder 3 is a trusted device which is preferably carried by the user and stored in a safe place when not in use. In this way the keys stored in the decoder are not compromised.

Synchronization of the key sets of the system of FIG. 1 is accomplished as follows. The server (cipher apparatus) 1 encrypts an image using a key of a key set. This image is transmitted to a terminal (display device) 2 displaying the image. If the terminal 2 does not own the keys, it cannot decrypt the encrypted image. The displayed encrypted image does not contain recognizable information and may have a phenomenon of a random image (“snow”).

The user places a decoder (decoding device) 3 so that the decoder can detect an image. The encrypted image shown schematically in FIG. 3 has two image parts, the first image part 5 comprises an encrypted image part and the second image part 6 comprises a key identification. The decoder 3 is preferably arranged to sense both images at the same time and to determine if the part of the image shown on the display screen 21 is the second image part 6. In the preferred embodiment, the screen section 21 is assigned to the second (key identification) image part 6 so that this image part is recognizable based on its position.

In the example shown, the second image portion 6 comprises a barcode. Allows the decoder 3 to "scan" the display 21 and detects the barcode using well known electronic image scanning techniques. In this case, it is not necessary to assign a specific portion to the second image portion. Instead of a barcode, other codes or symbols (in combination) may be used. It is also possible that these codes can be recognized by the decoder 3 using pattern recognition techniques. The entire code does not need to be displayed at one moment, but portions of the code are displayed sequentially (ie at different moments) so-called time multiplexing codes can be used. This can be accomplished by flashing (or flashing) certain display elements. Portions of the code may be represented by themselves by a barcode or by any other suitable code.

It is possible to time multiplex the actual images and the key identification, ie to show the first image 5 and the second image 6 alternately, but not simultaneously.

In the embodiment shown, the decoder 3 recognizes and decrypts the barcode contained in the second image part 6 to obtain a key identification or a code representing the key identification. In a preferred embodiment, the (bar) code contained in the two image portion 6 is a hash value of the key.

The decoder 3 calculates the hash value of the key, compares it with the detected hash value, and continues with the next key if the detected hash value does not match the calculated hash value. Try to match one of the If no match is found, then an error must have occurred. If a match is found, the decoder uses the associated key to decrypt the first image portion 5 and display the resulting decrypted image. At the position of the first image portion 6, a mask area (eg blank area) can be inserted by the decoder to mask the key identification.

In order to allow small read or transfer errors, it can be determined that "matching" is found even if the detected hash value and the calculated value are not the same but quite similar. This can be determined, for example, by determining an appropriate maximum acceptable "distance" between the detected value and the calculated value using well known Hamming difference measurements. However, in the preferred embodiment, the difference is zero, thus requiring the values to be the same.

In order to prevent any transmission errors that may cause incorrect key identification to be displayed, the actual key identification may optionally be extended to a Cycical Redundancy Check (CRC) value or similar check that allows error detection.

The key sets of the server and the decoder are virtually identical, ie each key of the server key set when used for server encryption generates an image that can be decrypted using the associated key of the decoder key set when used for decryption processing of the decoder. . In most embodiments, the server key set and decoder key set will be the same, but this is not required in this case. The relationship between the keys can be described as follows:

K => KID => K '

Where K is the key of the first key set, KID is the corresponding key identification, and K 'is the key of the second key set identified by the key identification. Based on the first key K, a key identification KID is generated that is used by the decoder to identify the corresponding key K '. In most cryptographic systems, K and K 'will be the same.

In the above description, it is assumed that the decoder (decoding device) 3 displays the entire decoded image. This is not necessary in that case, and it is conceivable that the embodiments display only part of the image in order to allow the "visual cryptography" techniques disclosed in, for example, European patent application EP 0 260 815. . In these embodiments, the decoder 3 is at least partly transparent, one part or part of the image is displayed by the decoder and the other part or part is displayed by the terminal display 21. Suitable examples of transparent devices employing LCD screens are described in European Patent Application No. 02075527.8 [PHNL020121]. European patent application 02078660.4 [PHNL020804] discloses a transparent decoding device that allows color images to be decoded. However, these transparent devices should also be provided with detectors (31 in FIG. 2) or other suitable sensing means for sensing the displayed key identification.

The present invention is based on the viewpoint that information identifying the key can be displayed in an encrypted image and allows this information to be detected by the decryption device. In addition, the present invention is based on another aspect that an untrusted device (ie, display device) can be used to provide information related to the keys as the untrusted device does not have any knowledge of the keys themselves.

Although the present invention is particularly applicable to systems that cryptographically deliver images, such as "visual cryptography," it is also applicable to other cryptographic systems where data items other than images may be cryptographically protected. For example, it is contemplated that the present invention may be applied to a computer where encrypted data (files) are transferred between computers and computer screens are used for key synchronization.

It is noted that any terminology used herein should not be configured to limit the scope of the invention. In particular, the words “comprise (s) and comprising” are not meant to exclude any elements not specifically mentioned. Single (circuit) elements may consist of multiple (circuit) elements or their equivalents.

Those skilled in the art will appreciate that the present invention is not limited to the above-described embodiments and that many modifications and additions may be made without departing from the scope of the invention as defined in the appended claims.

Claims (15)

A method of synchronizing a first key set of an encryption device and a second key set of a decryption device: The cryptographic device generating an encrypted image and associated key identification using the keys of the first key set, The cryptographic device sending the encrypted image and its associated key identification to a display device; Displaying, by the display device, the encrypted image and its associated key identification; The decoding device detecting the key identification; The decrypting device decrypting the encrypted images using a key of the second key set corresponding to the detected key identification, and Displaying, by the decoding device, the decoded image. The method of claim 1, The key identification is a code derived from the key. The method of claim 2, The key identification is a hash value. The method of claim 3, wherein Detecting the key identification by the decryption device: A substep of the decoding apparatus detecting the hash value and storing the hash value as the detected hash value; And the decoding device calculates hash values of the second key set and compares each of the calculated hash values and the detected hash value until a match is found. The method of claim 1, And the key identification is part of the encrypted image. The method of claim 5, The key identification is displayed on the display device as a barcode and / or a time multiplexing code. A system for synchronizing a first set of keys of a cryptographic device and a second set of keys of a decryption device: An encryption device for generating an encrypted image and associated key identification using the keys of the first key set, and for transmitting the encrypted image and its associated key identification to a display device; A display device for displaying the encrypted image and its associated key identification, and And a decryption device for detecting the key identification, decrypting the encrypted images using a key of the second key set corresponding to the key identification, and displaying the decrypted image. The method of claim 7, wherein The key identification is a code derived from the key. The method of claim 8, The key identification is a hash value. 10. The apparatus of claim 9, wherein the decoding device is: Detect the hash value and store it as the detected hash value, And calculate the hash values of the second key set and detect the key identification by comparing each calculated hash value with the detected hash value until a match is found. The method of claim 7, wherein And the key identification is part of the encrypted image. The method of claim 7, wherein The key identification is displayed on the display device as a barcode and / or a time multiplexing code. A decoding device for use in the system according to any one of claims 7 to 12, wherein the decoding device comprises: Sensing means for detecting an encrypted image and key identification, key selecting means for selecting a key based on the detected key identification, decrypting means for decrypting a sensed encrypted image using the selected key; And display means for displaying the decoded image. The method of claim 13, And said sensing means is comprised of photodiodes. The method of claim 14, Said sensing means being part of an LED circuit, preferably an OLED circuit.