The present invention relates to methods for applying forensic watermarks to media and methods for identifying media through the forensic watermarks.
Owners and producers of media (e.g. music, movies or software) often distribute their work to consumers through a copy-protected optical disk (e.g. a DVD). This ensures the owners that the consumers do not copy and illegally distribute the work without the owners getting proper payment. However, many consumers desire to make copies of the work that they have legally acquired on a copy-protected optical disc, such as a DVD. Content owners and/or state law may grant consumers the right to make copies as long as those copies are for personal use only. Preferably, such copies should also play on “legacy” equipment or formats, such as PCs, CD-R discs and mp3-players, which do not support any copy protection system. In order to limit the distribution of the (in that case) unprotected copies, it has been proposed to embed forensic watermarks in the copies. The purpose of forensic watermarks is to enable identification of individuals making the original (first generation) copies. It is expected that the presence of those watermarks will deter consumers from distributing their copies on the Internet. However, it has been questioned whether a forensic watermark can provide conclusive proof as to who would ultimately be responsible for rogue copies that appear on the Internet.
Typically, the consumer's player or recorder (hereafter called the client device) has to embed the forensic watermark in the copy. For this purpose the client device uses an internally stored watermark pattern and provides the consumer's identity as the watermark payload. In this setup all client devices share the same fixed watermark pattern. Eventually a skilled hacker may be able to extract the watermark pattern from a client device as well as the method that is used to construct its (optional) payload. In addition, the hacker may codify this knowledge in a software tool and publish it on the Internet. This tool enables anyone (skilled or unskilled) to embed any watermark carrying any payload. As a result, the watermark no longer provides conclusive proof as to who made the copy and the person responsible for the illegal distribution cannot be found.
Revealing the watermark pattern, which is a global secret known by all client devices, represents a catastrophic failure of the system. The risk that the watermark pattern would be revealed is high when all client devices know how the watermark pattern is constructed, which is typically done by modulating a certain basic watermark pattern. A skilled hacker could easily get access to a client device and thereby hack the device. Furthermore, the so-called collusion attack where a number of traitors collaborate in order to hack the watermarking system is a problem. A number of multiple individuals get hold of the basic pattern and thus could collaborate to jointly defeat the system. Once the basic pattern is known the whole system is compromised, and it is therefore impossible to track illegal copies back to the traitor. The solution is to use independent basic patterns for each copy. However, the problem of this approach is that detection of the traitor becomes too complex as it would require searching all possible patterns.
It is a goal of this invention to improve upon the above.
The invention discloses a watermark generation method for generating watermarks to be embedded in digital media, where said watermark is generated by combining at least two watermark patterns from a set of watermarks patterns. The set of watermark patterns is divided into at least two subsets of watermark patterns, and the subsets are hierarchical related. Hereby a watermark is generated from hierarchical related watermark patterns which make it easier to identify a watermark from different watermarks. Furthermore, different watermarks could be generated because the watermark consists of different watermark patterns that could be combined in different ways. This is an advantage when a unique watermark has to be embedded in a copy of a media.
In one embodiment of the watermark generation method, the watermark patterns in said watermark are directly related watermark patterns, and the watermark patterns in said watermark are directly related watermark patterns. Directly related means that the watermark patterns are related like parent and child in a hierarchical structure, e.g. a tree structure. The direct relation between the watermark patterns makes it possible to make a structured search strategy. The hierarchical structure limits the search results very fast because first the top watermark pattern (the parent) is found, and thereafter the search is limited to the watermarks patterns (the children) directly related to the top watermark.
In one embodiment of the watermark generation method, the watermark patterns in each of the subsets are mutually independent. This makes it more difficult for a hacker to overcome the watermark system and thus illegal distribution of the copies is limited.
In a further embodiment of the watermark generation method, the watermark patterns in at least one subset is generated by modulating a given basic pattern by a payload, and at least one watermark pattern from said subset is used in the generation of said watermark. This makes the search strategy even faster as the top watermark pattern can easily be found from the payloads.
In a further embodiment of the watermark generation method only one watermark pattern from each subset is used to generate said watermark. This ensures that only one watermark pattern from each level in the hierarchy is used to generate the watermark, and the result is that the search strategy is improved because it can be performed from top to bottom of the hierarchy.
In a further embodiment of the watermark generation method, each combination of said watermark patterns is used only once in order to generate a unique watermark. It is hereby possible to embed a unique watermark in every copy made of the media, and the result is that every copy can be uniquely identified.
Further, the invention relates to an apparatus for generation of watermarks to be embedded in digital media, where said apparatus comprises generation means for generation of a watermark as described above. Hereby a watermark can be generated from direct hierarchical related watermark patterns, thus making it easier to identify a watermark form different watermarks, and because of the direct relation between the watermark patterns a structured search strategy can be made. The hierarchical structure limits the search results very fast because first the top watermark pattern is found and thereafter the search is limited to the watermark patterns directly related to the top watermark.
In an embodiment the apparatus described above further comprises embedding means where said embedding means is adapted to embed said generated watermark in digital media. This ensures that the watermark can be integrated in a copy of the media and the watermark can later be extracted from the copy, and the copy can then be traced back to the consumer that made the copy.
In an embodiment the apparatus further comprises storing means, and said storing means is adapted to store said watermark. This ensures that the watermark can be stored in e.g. a database together with information of which consumer made which copy. Thereby the responsible consumer can be found in the case of illegal distribution of the copy.
In the following preferred embodiments of the invention will be described referring to the figures, where
The unique path of watermarks should only be used once so that it is possible to uniquely identify the transaction that approved the copy. The identification of a rogue copy proceeds in n steps. First, the investigator must check if the copy contains one of the m watermark patterns of the top level nodes. If this is the case, the investigator must check if the copy contains one of the m watermark patterns of the appropriate child node—and so on. If the investigator finds the watermark patterns of one of the leaf nodes, the transaction that approved the copy is identified because this watermark pattern is not shared with any other transaction. In this procedure, at most nom watermark patterns have to be checked per rogue copy. In addition, already after the first step, which comprises at most m checks, it is known whether the rogue copy may be identified at all. The unique watermark given as an example above can be identified by first searching through the top level of the tree. Here the 2-watermark is identified and the search continues to the second level of the tree, but the search is limited to the m sub-nodes below the 2-watermark. Now the 2.m-watermark is identified, and the search continues to the third level where the search is limited to the m sub-nodes below the 2.m-watermark. This process proceeds until the lowest level of the three is reached, and the unique watermark is then identified. As an example, with parameters m=128 and n=6 it is possible to support about 4·1012 uniquely identifiable transactions, and rogue copies can be identified with at most 768 checks. In the preferred embodiment, the n watermarks are embedded throughout the copy in random locations.
Where j=1 . . . i and k=1 . . . m the wk watermarks are independent and unique. When i=0, the above system reduces to a system where each transaction is given a strictly unique watermark pattern and detection would require search over m random sequences, where m is in the order of 1012. On the other hand, when m=0 the above system reduces to the classical watermarking system that is based on a single basic watermark pattern. The above system thus provides a flexible watermark structure that can easily be adapted for different complexities without losing the advantage that each transaction gets a unique watermark pattern.
In order to uniquely identify a given peripheral node (watermark tracing), one has to search over m+1 patterns. First, the payload pLj is determined to identify the first layer node. Once this node is determined, search is conducted over the m patterns corresponding to the m-children of the node identified by pLj. Thus, one can arbitrarily choose the values of i and m to satisfy a certain complexity requirement. It is important to note that the value of m can be as low as one, in which case the above system becomes equivalent to the classical payload based watermark system, where each watermark wo[pLj] is masked with a one-time pad (OTP) pattern wk that is unique for each transaction. Here, tracing is entirely conducted using the payload carrying watermark wo[pLj], and wk is used for masking purpose and for verifying the validity of the detected payload.
In one preferred embodiment, the structure of
Number | Date | Country | Kind |
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05112768.6 | Dec 2005 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB06/54881 | 12/15/2006 | WO | 00 | 6/14/2008 |