As used herein, the terms content or digital content may include, but are not limited to, audio, video or multimedia content. Content or digital content can be thought of as a digital signal. Watermarking is the process of modifying the content in order to embed information into the content and the corresponding process of recovering that information from the modified content. One example of such watermark information is a digital forensic code added to or embedded in content after production and before or during distribution. In this case, the watermark or digital forensic code is intended to apply a unique identifier to each of many copies of a multimedia work that are otherwise identical. In one application, this can be used to identify the source of an illegally copied content. Watermarking digital content, such as digital cinema, is one technique to deter thieves from misappropriating a copy of the content and then illegally redistributing it. This technique also encourages authorized distributors of digital content to maintain high security standards because watermarking can identify the specific authorized dealer from which the misappropriated copy originated. For example, if an illegal copy of the digital content is confiscated, the watermark information within the digital content can be used to determine the identity of the authorized distributor and, perhaps, the time and place of the public showing or sale of the digital content by the authorized distributor via the use of serial numbers in the forensic code. With this information, an investigation can begin at the identified authorized distributor to determine the conditions under which the misappropriation occurred.
In many applications, a unit of digitally watermarked content may undergo some modification between the time it is embedded and the time it is detected. These modifications are called “attacks” because they generally degrade the watermark and render its detection more difficult. If the attack is expected to occur naturally during the process of authorized or unauthorized distribution, then, the attack is considered “non-intentional”. Examples of non-intentional attacks can be: (1) watermarked content that is cropped, scaled, JPEG compressed, filtered etc. (2) watermarked content that is converted to NTSC/PAL SECAM for viewing on a television display, MPEG or DIVX compressed, re-sampled etc. On the other hand, if the attack is deliberately done with the intention of thwarting the purpose of the watermark, then the attack is “intentional”, and the party performing the attack is a thief or pirate. The three classes of intentional attack are unauthorized embedding, unauthorized detection, and unauthorized removal. This invention is concerned with unauthorized removal; removing the watermark or impairing its detection (i.e. the watermark is still in the content but cannot be easily retrieved by the detector). Unauthorized removal attacks generally have the goal of making the watermark unreadable while minimizing the perceptual damage to the content. Examples of attacks can be small, imperceptible combinations of line removals/additions and/or local rotation/scaling applied to the content to make difficult its synchronization with the detector (many watermark detectors are sensitive to de-synchronization).
One type of attack is a collusion attack where different copies are combined in an attempt to disguise or scramble the different digital watermark information contained in each. Attackers may also perform additional processing on the colluded copy before re-distributing the processed colluded copy. The additional processing may cause errors in the detected bits of the forensic codes. Without careful design, the forensic watermarking system can be easily broken by an attack by two or three colluders.
Prior art works on forensic marking codes design by Boneh-Shaw and Tardos are designed to resist collusion attacks. However, the Boneh-Shaw approach has the drawbacks of requiring a very long code and providing low collusion resistance when applied to multimedia signals, i.e. only a few colluders can break the system. The Tardos approach has good collusion resistance and requires shorter code length. However, its computational complexity and storage consumption during the code generation and detection is ten thousand times of that of a comparable Error Correcting Code (ECC) based forensic code. The ECC-based forensic marking scheme proposed by He and Wu uses Gaussian spread spectrum embedding to carry the code symbol rather than using a binary inner code. Since that scheme specifically uses spread spectrum embedding, it may not be applicable to other embedding schemes. It would be useful to develop a technique for constructing a binary inner code for generation of collusion-resistant watermarks that is also computationally efficient and has a reasonable length.