Key length estimation of zero-bit watermarking schemes

This paper proposes a new definition of Watermarking Security which is more in line with the cryptographic viewpoint. To this end, we derive the effective key length of a watermarking system from the probability of guessing a key equivalent to the original key. The effective key length is then computed for two zero-bit watermarking schemes based on normalized correlation by estimating the region of equivalent keys. We show that the security of these schemes is related to the distribution of the watermarked contents inside the detection region and is not antagonist with robustness. We conclude the paper by showing that the key length of the system used for the BOWS-2 international contest was indeed equal to 128 bits.