To realize a method for inserting additional information aiming at digital data authentication to detect digital data in various spectrums or space expression fields.
The method inserts additional information to data for the purpose of authenticating a digital signal separated into sets of coefficients (Xij), the respective coefficients are expressed in 16 bits, and the method by which the additional information is expressed by sets of binary values (wij) of the same size as that of the sets of the coefficients (Xij) includes steps (E5, E6) where check bits (Cij) with respect to at least one coefficient (Xij) are calculated as a function of first 15 bit planes of the coefficient (Xij) based on a prescribed reversible operation, steps (E7, E8) where the final bit plane (X0ij) is calculated as a function of the check bits (Cij) and the binary values (wij) of the additional information (W), and a step (E7) where the final bit plane of the coefficients (Xij) is replaced with the calculated final bit plane (X0ij). Thus method is especially applicable to the authentication of a compression format picture.