How to Increase the Security of Digital Rights Management Systems Without Affecting Consumer's Security

The paper starts with a description of the fundamental principles of modern Digital Rights Management Systems. This is the basis for the discussion of their most important security aspects from the provider’s view on the one hand and the customer’s view on the other hand. The second half of the paper focuses the new DRM standard from the Open Mobile Alliance (OMA) and its implementation on “open” systems like Windows. The security anchor of the OMA DRM is the device private key. As long as no trusted storage facilities for open systems work effectively, techniques for software obfuscation could be a solution. Therefore the obfuscation of the device private key and its secure download is described. Currently on Windows PCs there is no chance for a full tamper-proof solution, but the authors try to make the job of an attacker as hard as possible, without affecting the consumer’s security. 1 Motivation and Introduction After the appearance of illegal services like Napster the music industry needed a few years and technology partners like Apple and Microsoft to setup legal download services which brought digital rights management (DRM) onto end users’ devices. DRM systems (DRMS) now allow the rights owners to restrict and control the usage of the music a consumer has downloaded. Content will only be delivered encrypted. In order to render the content an appropriate license is needed. A license includes the content encryption key and some usage rights. The license has to be delivered and stored on the consumer’s device in a secure manner. Only the DRM controller which resides on the consumer’s device is able to apply the key if the according usage rights allow this. The DRM technology was developed to increase the security of the business models of the content industry. This security is based on the proper function of the DRM controller. But it is very hard to secure a DRM controller, because it resides on the device of the user which cannot be trusted by the DRMS provider. In spite of the conflict between DRMS providers and device owner, the providers must not affect the security needs of ordinary PCs or mobile phone users. The implementation of a 2 Jürgen Nützel and Anja Beyer DRMS must not weak the security of the device against attacks and intrusion from external. Although the DRM standard of the Open Mobile Alliance (OMA) [1] was originally designed for mobile phones the version 2 became now attractive to adopt it for environments like Windows XP. The description of the implementation of such an adoption shows several security problems to solve. Some are critical for the device owner and some are critical for the content owner. 2 Digital Rights Management In times when virtual goods, such as novels, music and movies where bound to a physical medium (e.g. books, CDs, cassettes) the publishers did not have to worry about people misusing their intellectual property [2, p. viii]. By purchasing a book or a CD the complete usage rights (e.g. reading, copying, giving away and retailing) devolved to the customer. It took some time and effort to take a copy of a book or a CD. Nowadays, in times of digitization, it needs nearly no time duplicating virtual goods (digitized goods) and is low in costs. In order to cut down the illegal transmission the vendors established the so called Digital Rights Management systems (DRMS). With DRMS they try to pass the usage rights granularly to the customer. With DRMS the content provider is able to allow its customers only certain operations which are defined in a license file (e.g. take five copies, print two times, etc.) [3, p. 59]. The rights are expressed machine readable using a rights expression language (REL). Several REL exist in parallel. Their common basis is XML (Extensible Markup Language). In [4] a family tree of most popular REL is given. ODRL (Open Digital Rights Language) [5] for instance is the REL of the new DRM standard developed by OMA [3, p. 63]. 2.1 Some Definitions for DRM There is still no standardized definition for the term Digital Rights Management. Iannella differentiates between DRM of the first and second generation. While for him the first generation only applies to copy protection, „[t]he second-generation of DRM covers the description, identification, trading, protection, monitoring and tracking of all forms of rights usages over both tangible and intangible assets including management of rights holders relationships. Additionally, it is important to note that DRM is the "digital management of rights" and not the "management of digital rights". That is, DRM manages all rights, not only the rights applicable to permissions over digital content” [6]. With his three-legged stool with the legs law, business and technology Nils Rump [7] shows that the domain is complex and not only narrowed on technical issues [3, p.60]. Rüdiger Grimm apprehends DRM as procedures that help to protect the rights of the virtual goods in a way that we are accustomed from the intellectual products How to Increase the Security of Digital Rights Management Systems 3 bound to physical media. Copy and transfer shall be linked to the rules of the rights holder, thus the content provider [8]. 2.2 The DRM Reference Model In [2] a DRM reference model was introduced which well describes the fundamental structure and functions of most of the existing DRMS [3, p. 60]. The reference model contains three major components: the content server, the license server and the DRM client (see figure 1). Fig. 1 The DRM reference model with usage counter and device key pair (Public and Private Key) for device identification. Figure 1 describes the DRM reference model. Prior to a download the content server has to prepare the encrypted content for distribution. Therefore, the DRM packager located in the content server encrypts the content and puts some additional metadata (like a unique content ID and the address of the license server) to the content package and hands the applied content encryption keys (or a seed information to retrieve the keys from) over to the license server (OMA calls this the rights issuer). The license server stores the encryption keys and provides them on request (3 step in figure 1) together with the appropriate usage rights. This will be done in the DRM license generator which creates the licenses (OMA calls this rights objects) containing identity, rights specifications and encryption keys. The client of the DRM system is located at the user’s side. It contains the DRM controller, the decoder to render the content and the user’s or device’s identification mechanism. In the OMA DRM standard (version 2) a RSA key pair identifies the device. In [2, p. 82] the DRM controller is described as “...the real nerve center of the DRM system.” It enables the user to exercise his rights, to render the content and it organizes the communication with the content and the license server [2, p. 79ff]. content