| 摘要 |
In watermarking systems, hackers may try to remove the watermark using a so-called collusion attack. If the attacker has access to multiple identical signals with different watermarks (this typically occurs in electronic content delivery systems), simply averaging the signals will remove the watermark energy. A known solution to this problem is phase modulation. By modulating the phase of the signals, the averaging attack will cause phase cancellation to occur and annoying artifacts to be introduced. In the prior art, said phase modulation is carried out in the base-band domain, prior to watermark embedding. The present invention enables phase modification efficiently to be applied to transform coded signals, in particular DCT or MDCT coded signals such as MP3 or AAC audio signals, or MPEG2 video signals. The bitstream is partially decoded (4) and re-encoded (7). In accordance with the invention, the phase modification is now performed by an appropriately modified version of the forward or inverse (M)DCT transform algorithm (9). The phase modulation is thus an integral part of the decoding process (before watermark embedding) or the encoding process (after watermark embedding), which is very efficient. The phase modulation is controlled by parameters provided by a control function (5). |
