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SECURING XML-BASED MULTIMEDIA CONTENT Ernesto Damiani Dipartimento di Tecnologie dell'InJol'mazione Universitd di Milano, Crema, Italy [email protected]
Sabrina De Capitani di Vimercati Dipartimento di Tecnologie dell'InJormazione Universitd di Milano, Cl'ema, Italy [email protected]
Abstract
Multimedia content delivery applications are becoming widespread thanks to increasingly cheaper access to high bandwidth networks. Also, the pervasiveness of XML as a data interchange format. has given rise to a number of standard formats for lIlultimedia, such as Sl\HL for multimedia presentations, SVG for vector graphics, VoiceXML for dialog, and MPEG-21 for video. Innovative programming paradigms (such as the one of Web services) rely on the availability of XML-based markup and metadata in the multimedia flow in order to customize and add value to multimedia content distributed via the Net. In this paper, we present. a general access control model for multimedia information that allows for expressing complex filtering conditions on a wide range of properties of multimedia data. Access control policies' syntax and enforcement is illustrated by applying the model to some of XML-based multimedia data formats for multimedia.
now becoming even more widespread thanks to the increasingly cheaper access to high bandwidth networks. However, the importance of complementing binary multimedia data with descriptive metadata in the form of XML tagging has been fully realized only recently. XML-based standard formats for multimedia representation such as SMIL [8Jand VoiceXML [9]make the internal structure of multimedia flows available to a variety of consumer applications and devices. More ambitious efforts, such as MPEG-21, are aimed at achieving the same results for video. Intelligent applications like search indexes, topic maps or browsable directories can use XML tagging and auxiliary metadata to identify the structural or semantics-related components of multimedia flows, such as keywords, key frames, audiovisual summaries, semantic concepts, color histograms, shapes, as well as recognized speech. XML formats are paving the way to a new generation of applications even in more traditional fields such as image processing. For instance, raster graphical formats (e.g., GIF or JPEG) have severe limitations, inasmuch they do not carry any information that can be queried, re-organized, or searched through. Scalable Vector Gr'aphics (SVG) [7Jis an XML-based language for describing two-dimensional vector and mixed vector/raster graphics that works well across platforms, across output resolutions and color spaces. Also, SVG cleanly specifies the image structure, allowing applications to p'rocess data at a finer level of granularity. The wealth of semantics- and structure-related information carried by new XML-based multimedia formats suggests that the time has come to enrich traditional, coarse-grained access control models with a number of new concepts that are specific to the nature and meaning of multimedia data. In our approach, all semantics-related and structural components of multimedia data, such as a room in the vector-based map of a building or a range in the frequency spectrum of an audio clip are considered first-class citizens when writing fine-grained access and fruition policies for lIlultimedia. An interesting consequence of using XML for writing our fine-grained access policies is that XML elements used to refer to multimedia components inside policies can be mapped to XML tags and metadata in the data flow. Policy-to-data mapping can be customized to the particular XML-based multimedia format under discussion, achieving fast and effective enforcement either via data filtering or even by encryption. In this paper, we use some popular XML-based formats for multimedia to illustrate our approach, showing fine-grained access and fruition policies referring to specific components of the multimedia data, and discussing how they can be efficiently enforced via on-line data transformation.
SecU7'ing XML-based Multimedia Content
FiguTe 1,
2.
G3
General structure of an XML- based Illultimedia document
Preliminaries
We now describe the document abstraction model on which our approach is based and give some preliminary concepts that will be llsed throughout the papeL Specifically, we consider the general problem of protecting multimedia data associated with XML-based metadata formats, such as SVG [7] for vector graphics, VoiceXML [9] for dialog, and SMIL [8] for creating interactive multimedia presentations, XML-based metadata formats for multimedia differ ill terms of the type of binary data they organize and represent; however, they do share a comlllon structure and semantics, As depicted in Figure 1, XML-based documents attached to multimedia data follow the same general structure of all XML-based documents, They include a root element (e,g"