BOLDFACEDNESS OF AN IMAGE AND THE APPLICABILITY OF A SPREAD-SPECTRUM IMAGE ENCRYPTION SCHEME

We study the applicability and evaluate the decryption fidelity of a s pread spectrum image encryption method, for which decryption is accomp lished with easy operations, and which makes possible the protection o f secrecy even in hardcopy form. Decryption in the spread spectrum enc ryption method is based on the recognition of the dark-pixel region in the picture's background noise. To evaluate the applicability of this image encryption method, we employ measure B-n, called the boldfacedn ess, which estimates the boldness of the original image. This is essen tially the area of the dark-pixel region normalized by the length of t he path surrounding it. We relate the intuitive visibility of several kinds of images to the boldfacedness, and confirm its validity. To est imate its applicability, we composed a questionnaire concerning the re cognition of simple rectangular images in background noise. The result s showed that for these simple rectangular images a B-n value down to about 0.023 is adequate. Next, since we often want to transmit images of documents in facsimile, we evaluated the fidelity of document image s based on the size and type of several kinds of characters. To perfor m this evaluation we took as input Group 3 facsimile, with the resolut ion of all document images as a consistent 200 dpi. It became clear th at, for document images, the letter-recognition threshold value of B-n is in the neighborhood of 0.015. This is equivalent to the case both for 16-point Mincho-tai characters and for 10-point standard character s, verifying the applicability of this encryption method for general d ocument usage.