The invention provides an identification data insertion and detection
system for digital data which improves the detection ratio when an image
is converted into frequency components and electronic watermark data are
embedded in a portion of an image having a strong frequency component and
besides allows a statistical similarly to be calculated with a higher
degree of accuracy. The identification data insertion system for digital
data includes a DCT calculator for extracting blocks of 8.times.8 pixels
from within a current image and performing DCT (discrete cosine transform)
calculation for the blocks, a quantizer for quantizing data outputted from
the DCT calculator, a quantization table for being referred to when the
quantizer performs quantization, an electronic watermark data insertion
element for inserting electronic watermark data into data after
quantization, and an encoder for encoding data outputted from the
electronic watermark data insertion element to produce MPEG data in which
the electronic watermark data are inserted and outputting the MPEG data.
