Security of Invertible Media Authentication Schemes Revisited
Dittmann, Katzenbeisser, Schallhart and Veith (SEC 2005) introduced the notion of invertible media authentication schemes, embedding authentication data in media objects via invertible watermarks. These invertible watermarks allow to recover the original
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bstract. Dittmann, Katzenbeisser, Schallhart and Veith (SEC 2005) introduced the notion of invertible media authentication schemes, embedding authentication data in media objects via invertible watermarks. These invertible watermarks allow to recover the original media object (given a secret encryption key), as required for example in some medical applications where the distortion must be removable. Here we revisit the approach of Dittmann et al. from a cryptographic viewpoint, clarifying some important aspects of their security definitions. Namely, we first discuss that their notion of unforgeability may not suffice in all settings, and we therefore propose a strictly stronger notion. We then show that the basic scheme suggested by Dittmann et al. achieves our notion if instantiated with the right cryptographic primitives. Our proof also repairs a flaw in the original scheme, pointed out by Hopper, Molnar and Wagner (TCC 2007). We finally address the issue of secrecy of media authentication schemes, basically preventing unauthorized recovering of the original media object without the encryption key. We give a rigorous security statement (that is, the best security guarantee we can achieve) and prove again that the scheme by Dittmann et al. meets this security level if the right cryptographic building blocks are deployed. Together our notions of unforgeability and of secrecy therefore give very strong security guarantees for such media authentication schemes.
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Introduction
The transition from analog to digital media facilitates many tasks but also comes along with continually improved manipulation tools, which allow various modifications of media objects. Thus, it becomes increasingly difficult to distinguish authentic from altered objects. To enable a better distinction it is therefore necessary to apply techniques that guarantee authenticity, integrity and possibly secrecy of data. The straightforward use of digital signatures is not always a satisfying solution to provide authenticity and integrity, because an object and its signature have to be stored separately. This, however, may not be convenient in the area T. Furon et al. (Eds.): IH 2007, LNCS 4567, pp. 189–203, 2007. c Springer-Verlag Berlin Heidelberg 2007
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of multimedia data. To counter this problem fragile watermarks were proposed, which can be used to embed a signature directly into an object, such that any (significant) modification will destroy the watermark and thereby invalidates the signature. Unfortunately, this approach comes with the disadvantage that it always leads to irrevocable distortions in the authenticated object, which may not be acceptable in all applications, e.g., X-ray imaging objects are extremely sensitive to modifications. One solution is to use invertible watermarking schemes, which are special fragile watermarks addressing the need to re-obtain the original media object by allowing a complete removal of the embedded data. Media Authentication Schemes. Using invertible watermarking schemes in combination wit
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