1. Field of the Invention
The present invention relates to an image decrypting apparatus, an image encrypting apparatus, and an image decrypting method that decrypt an encrypted image using a decryption key.
2. Description of the Related Art
Leakage of confidential information is a serious problem in the advancement of computerization of the general society and development of a technique that prevents the leakage of the information is desired. For example, as to digital data, a technique has been developed of encrypting data to prevent the content of the data from being viewed when the data is delivered to a third party. The technique is already utilized as a useful means of preventing information leakage.
On the other hand, a technique of preventing information leakage of a printed item printed on a paper medium, etc., has not been fully developed yet and any example thereof is not present. It is also actually said that substantially a half of the information leakage occurs from printed items. Therefore, similarly to the case of digital data, development of a technique of preventing information leakage from printed items is an urgent mission.
Specific examples for which a measure against the information leakage from the printed items is desired include a bill for purchase of merchandise, a statement for a credit card, etc., a clinical chart of a hospital, a report card of a school, and a name list. The present invention is usable as a technique of preventing information leakage by encryption and decryption of important portions of, for example, each of these examples.
Conventionally, a known example that handles encryption of a printed item can be, for example, a technique disclosed in Japanese Patent Application Laid-Open Publication No. 8-179689. According to this '689 publication: an entire original image is divided into a plurality of blocks; images respectively of the divided blocks are rearranged based on a parameter obtained from a password input (encryption key); the image of each of the blocks designated by the parameter are black/white-inversed and mirror-inversed; and, thereby, the original image is encrypted. When the encrypted image is decrypted, a frame for positioning is added to the image in the outer periphery thereof and another password (decryption key) is input. Thereafter, the original image is recovered by performing the encryption in a reverse manner.
Another convention technique can be, for example, a technique of embedding binary data as an image in a printed item, as shown in Japanese Patent Publication No. 2938338. The visualization as an image in this conventional technique is realized by representing the binary data using black-and-white squares each having a designated size and arranging the squares in a matrix. In addition, the printed item is added with a symbol for positioning at a designated position in the matrix such that the position that is visualized as an image can be known in decryption. Using this positioning symbol as the criterion enables shooting of the image using a scanner or a camera and, thereby, decryption of the information embedded.
In the above conventional techniques, a process is performed of decrypting an encrypted image output on an analog medium by printing, etc., into the original image using a reverse procedure of the encryption. However, when the contour of the encrypted image is smeared due to smears of ink and optical deformation resulted from printing a digital image and reading the image using an optical apparatus such as a camera or a scanner, the original pixel values can not be recovered even when the reverse process is performed. Therefore, a problem has arisen that the image quality of the decrypted image is degraded.
As to the image in the encrypted area, this image is encrypted using a method of dividing the image into a plurality of blocks and rearranging (black/white inversion, mirror inversion, and upside-down inversion) the blocks. However, it can be considered that the image present before the encryption is speculated from the feature of the image of each block of the divided blocks when the blocks are only rearranged. Therefore, a problem has arisen that the security needs to be enhanced.