“…In general it is difficult to recover the watermark, f ðx; yÞ, directly from the cyphertext gðx; yÞ without using these two keys. The purpose of the cryptanalysis presented in [4][5][6][7][8] is to retrieve part, or all, of the information of these keys with a priori but incomplete knowledge of the plaintext and/or the cyphertext, and eventually retrieve any plaintext encrypted using the same key-set ½/ðx; yÞ; wðu; vÞ. Collision, on the other hand, involves finding another key-set, ½/ 0 ðx; yÞ; w 0 ðu; vÞ, which will encrypt a different f 0 ðx; yÞ to the same gðx; yÞ, the cyphertext of f ðx; yÞ.…”