Patent ID: 7310429

Claim:
An unauthorized-alteration detecting method comprising: a step in which a processing section specifies a modulus P, an order N which is an even number equal to or larger than 2, and a root α, which are parameters of number theoretic transform; a step in which the processing section reads from a storage section an original-image block f i,j (x, y) obtained by block-dividing an original image [f] to which embedding is to be applied; a step in which the processing section uses the modulus P, the order N, and the root α specified, to apply the number theoretic transform to the original-image block f ij (x, y) to calculate the number-theoretic-transformed block F i,j (x, y) of the original-image block; a step in which the processing section determines an embedding position (x′, y′) of a signature image in each block according to a predetermined randomizing function which uses information of a left adjacent block, a right adjacent block, or a surrounding predetermined block as a parameter; a step in which the processing section reads from the storage section a pixel value g i,j of the signature image to be embedded; a step in which the processing section obtains an integer having the minimum absolute value and satisfying F i,j (x′, y′)+δ=g i,j (mod ε) as an embedding amount δ in each block from the number-theoretic-transformed block F i,j (x′, y′) of the original-image block at the embedding position, the pixel value g i,j of the signature image, and embedding strength ε; a step in which the processing section adds or subtracts the embedding amount δ to or from the number-theoretic-transformed block F i,j (x, y) of the original-image block, based on (x, y) to obtain the number-theoretic-transformed block H i,j (x, y) of an embedding-applied-image block; a step in which the processing section applies inverse number theoretic transform to the number-theoretic-transformed block H i,j (x, y) to obtain the embedding-applied-image block h i,j (x, y); and a step in which the processing section obtains the embedding-applied-image block h i,j (x, y) for each of all (i, j) blocks to obtain an embedding-applied image [h], and stores it in the storage section and/or outputs it from an output section or an interface.