Method of inserting a watermark and authenticating a digital signal

Method of inserting an additional information item (W) for authenticating a digital signal (I) decomposed into a set of coefficients (Xij, Iij), each coefficient being represented in L bits, and said additional information item (W) being represented by a set of binary values (Wij) of the same size as said set of coefficients (Xij, Iij), characterised in that it includes the following steps:

• calculating, for at least one coefficient (Xij, Iij), a check bit (Cij) in accordance with a predetermined operation as a function of L minus M(L−M) first bit planes of said coefficient (Xij, Iij), wherein said M is the number of bit planes to be substituted;
• calculating M last bit planes (Xij0, X0ij;, I0ij) according to a predetermined rule which is reversible as a function of the check bit (Cij) and a binary value (wij) of the additional information (W);
• and
• substituting the last M bit planes calculated (X0ij, X0ij;, I0ij) for the last M bit planes of said coefficient (Xij, Iij).

Device for inserting an additional information item (W) for authenticating a digital signal (I) decomposed into a set of coefficients (Xij, Iij), each coefficient being represented in L bits, and said additional information item (W) being represented by a set of binary values (wij) of the same size as said set of coefficients, characterised in that it has:

• means of calculating (21) a check bit (Cij) in accordance with a predetermined operation as a function of L minus M(L−M) first bit planes of a coefficient (Xij, Jij), wherein said M is the number of bit planes to be substituted;
• means of calculating (22) M last bit planes (X0ij, X0ij, I0ij) according to a predetermined rule which is reversible as a function of the check bit and a binary value of the additional information item;
• and
• means of substituting (23) the last M bit planes calculated (X0ij, X0ij, I0ij) for the last M bit planes of said coefficient (Xij, Iij).