SUMMARYWe study the applicability and evaluate the decryption fidelity of a spread spectrum image encryption method, for which decryption is accomplishedwith easy operations, and which makes possible the protection of secrecy even in hardcopy form. Decryption in the spread spectrum encryption 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,,, called the boldfacedness, which estimates the boldness of the original image. This is essentially the area of the dark-pixel region normalized by the length of the path surrounding it. We relate the intuitivevisibilityof several kinds of images to the boldfacedness, and confirm its validity. To estimate its applicability, we composed a questionnaire concerning the recognition of simple rectangular images in background noise. The results showed that for these simple rectangular images a B,, 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 images based on the size and type of several kinds of characters. To perform this evaluation we took as input Group 3 facsimile, with the resolution of all document images as a consistent 200 dpi. It became clear that, for document images, the letter-recognition threshold value of B,, 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 characters, verifying the applicabilityof this encryption method for general document usage.Key words: Spread spectrum encryption methods, image boldfacedness; accuracy of the decrypted image.
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