A Single-Keyed DRM System For Scalable Multimedia Preserving Fine Granularity

With rapid advances of digital technologies and the Intemet industry,more and more multimedia contents are released and distributed over the Internet.Many types of multimedia data are encoded in a scalable manner,i.e.,the original contents are encoded with respect to various criterions into a progressive manner in quality,space,or time. This allows part of the codestream to be discarded to fit into the current channel bandwidth or the device's processing and display capability.On the other side,the fact that digital contents are very easy to copy makes piracy across the Internet a vital problem. This gives birth to the DRM system,a suite of technologies that manages licensing and access control of digital contents.In this paper we design a"What You See Is What You Buy" DRM prototype system for scalable multimedia contents,exemplified by JPEG 2000 and Motion JPEG 2000.We first propose a syntax-compliant encryption scheme that fully preserves the scalabilities of the original multimedia content.Syntax compliance allows a normal player to successfully decode the cipherstream,especially without being crashed.If selected parts of the codestream get properly encrypted,users may be able to get a free preview of the original content.With all scalabilities preserved,the cipherstream can be directly transcoded by some intermediate processing node along the transmission path with no need of the decryption key.The node even doesn't have to be trusted.This is called "encrypt once,decrypt many ways".Compared with other typical encryption schemes,our method features fine granularity of scalability,superior processing speed, excellent bit-error resilience,and negligible file-size overhead.Then we propose a general key management scheme that is flexible and convenient. All kinds of scalabilities are assorted into two common types,represented by toset and poset,respectively.The entire multimedia content is modeled as the Cartesian product of all tosets and posets,which is a poset itself and can be represented by a Hasse diagram. Access control of the scalable multimedia content is equivalent to access control of the product poset.With the help of a hash-based key derivation method,the decryption key for each vertex can be calculated from the key of its parent vertex and some public auxiliary information.The one-wayness of the hash function ensures key derivation can't be inverted.Users can freely choose and combine the scalability parameters to get a custom version of the original content.For each version a single sub-root key is required to derive all child keys.This is called "protect once,access many ways".Lastly,we combine the encryption scheme and the key scheme to implement our DRM prototype system on top of Microsoft's WMRM DRM framework.A user only has to pay for what he would like to access.For each access.request from the user, the DRM license server only has to calculate and send a single access key.All keys for child vertices can be derived from the access key and some public auxiliary information. The DRM server doesn't need large storage or powerful processing capability,which in return enhances its reliability and availability.