METHOD FOR INSERTING WATERMARK TO DIGITAL IMAGE OR COMPRESSED DIGITAL IMAGE, DIGITAL IMAGE CAPTURING DEVICE, COMPUTER SYSTEM AND METHOD FOR DETECTING ALTERATION OF DIGITAL IMAGE WITH WATERMARK

摘要

PROBLEM TO BE SOLVED: To reduce distortion and to easily verify alteration by partially decoding digital images, generating plural data blocks, judging whether or not to bury watermark bits and burying the watermark bits in the data blocks. SOLUTION: Images are partially decoded, plural blocks are generated and the quantization variable of the conversion coefficient of a highest frequency in the respective blocks is obtained and multiplied with the conversion coefficient. Whether or not to bury the watermark bit is judged based on the coefficient and the remaining watermark bit number of the watermark to be buried, the least significant bit of the conversion coefficient of the highest frequency of the block for which burying is decided is set to zero and a hash value calculated in the last block is turned to the hash value of the entire images. Then, by calculating the watermark from the hash value by using a secret key and digital signature algorithm, a ciphered hash value is obtained and the LSB of the conversion coefficient of the block to perform burying is matched with the watermark bit.