Method and system for sustainable digital watermarking
Digital marks (so-called fingerprints and watermarks) serve two basic purposes: (1) Investigative: the owner reads a fingerprint to determine how the marked entity leaked; and (2) Legal: the owner must prove in court that (a) there is a watermark (a concealed copyright message), and (b) it is the owner's. The main difficulty of item (2) is that the first use of the watermark software reveals the watermarking method to the public so that hostile parties are equipped to remove or damage its watermarks. The invention uses tamper-resistant software encoding techniques to protect the digital mark extractor algorithm, frustrating the attacks of hostile parties in two ways: the resulting code is obscure (that is, its inner workings are incomprehensible; and chaotic (that is, a modification at any point will almost certainly produce a nonsense program.
1. A method of ensuring that a digital document is verifiable as being from a specific source, thereby rendering ownership protection for content of the digital document, the method comprising the steps of:
(a) providing a watermark injector in the form of a watermark injection algorithm embodied in stored on a computer readable storage medium, the watermark injector operating to steganographically embed a watermark into the digital document whereby a key is employed in the embedding of such watermark;
(b) providing a complementary watermark extractor in the form of a watermark extraction algorithm embodied in stored on a computer readable storage medium, the watermark extractor operating to reverse the watermark injector's steganographic embedding of a watermark by extracting such embedded watermark from the digital document;
(c) injecting a digital watermark into the digital document using the watermark injector and a randomly selected key;
(d) converting the watermark extractor into tamper-resistant software (TRS) form, whereby the TRS form of the watermark extractor is obscure and has a property of responding chaotically to modification;
(e) publishing the TRS form of the watermark extractor; and,
(f) registering the TRS form of the watermark extractor with a trusted third party, whereby the trusted third party validates that the TRS form of the watermark extractor correctly identifies watermarks injected using the watermark injector but does not identify a watermark in content of a digital document which either does not possess it or which was embedded with the watermark using a different injection algorithm and/or a different key;
whereby in a legal proceeding, the owner of content of the digital document has available as evidence to prove such ownership, each of the following; (i) the prior publication of the TRS form of the watermark extractor; (ii) the third party validation of the correctness of the TRS form of the watermark extractor; and, (iii) an ability to provide in such legal proceeding a demonstration of the TRS form of the watermark extractor extracting the watermark from the content; and, without it being necessary to reveal the injection algorithm, the extraction algorithm, or the key used in embedding the watermark.
2. A method according to claim 1 whereby the provided watermark injector is selected from a plurality of distinct watermark injectors and the provided complementary watermark extractor is selected from a corresponding plurality of distinct complementary watermark extractors, the selection of the provided injector and extractor for the content being random, and whereby varied injectors and extractors are provided for different content thereby making it necessary for an attacker to select from a plurality of published TRS forms of the plurality of extractors for any particular content targeted by such attacker and increasing the difficulty for such an attacker to determine appropriate injection and/or extraction algorithms for such targeted content.
3. A method according to claim 2 whereby the randomly selected key is embedded in the watermark injection algorithm and in the complementary watermark extraction algorithm.
4. A method according to claim 3 whereby the conversion of the watermark extractor is performed using a highly secure method of conversion to TRS whereby the TRS form of the watermark extractor has a high computational overhead and the degree of obscurity and chaotic response to modification of the TRS form of the watermark extractor are increased, thereby further increasing the difficulty for such an attacker to determine appropriate injection and/or extraction algorithms for such targeted content, and without preventing the owner of the content from proving ownership thereof.