Abstract:
There is disclosed a cryptographic key management concept for a user domain, in which a local rights manager (LRM) is provided a cryptographic generation key for validly generating or converting only a limited amount of rights objects. In case a compromise of the LRM is detected, the cryptographic generation key is not renewed after the limited amount of ROs have been generated by the LRM. Otherwise, i.e. in case no compromise of the LRM has been detected, i.e. in case the LRM may be considered trustable, the LRM is provided a new (different) cryptographic generation key for generating a further limited amount of rights objects.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to digital rights management and, more particularly, to a concept for managing cryptographic keys in such a DRM system, particularly for managing keys for a local rights manager (LRM) being responsible for aspects of data import. 
         [0002]    Digital rights management (DRM) describes a concept by which media providers enforce limitations on usage and distribution of digital media content. Presently, there are number of DRM schemes in use. For example, mobile content providers use the Open Mobile Alliance (OMA) DRM system to protect digital mobile media content. 
         [0003]    The OMA DRM family comprises digital rights management standards that are developed by the Open Mobile Alliance. To date, the OMA DRM family comprises: 
         [0004]    OMA Digital Rights Management 1.0 (DRM v1.0), 
         [0005]    OMA Digital Rights Management 2.0 (DRM v2.0), 
         [0006]    OMA Digital Rights Management 2.1 (DRM v2.1), 
         [0007]    OMA DRM v2.0 Extensions for Broadcast Support (XBS), 
         [0008]    OMA Secure Removable Media (SRM), 
         [0009]    OMA Secure Content Exchange (SCE). 
         [0010]    The OMA DRM system enables content issuers to distribute DRM protected content and rights issuers (RIs) to issue rights objects (ROs) for the DRM protected content. The DRM system is independent of media object formats, operating systems, and run-time environments. Contents protected by DRM can be of a wide variety, including games, ring tones, photos, music clips, video clips, streaming media, etc. For a user consumption of the content, users acquire permission to DRM protected content by contacting rights issuers, i.e. an entity that issues rights objects to DRM conformant devices. Rights issuers grant appropriate permission for the DRM protected content to use it on DRM conformant devices. The content is cryptographically protected when distributed and, hence, will not be usable without an associated rights object (RO) issued for the user&#39;s device. 
         [0011]    A user may also wish to operate the digital media content on other DRM conformant devices owned by him. Therefore, according to SCE, a concept of a “user domain” was introduced. A user domain may include other DRM conformant devices owned, operated, controlled, or under responsibility of the user. The user may add devices to the user domain and may use a certain device in the user domain to obtain digital media content usable in the user domain. Further, the user may share content between DRM conformant devices in the user domain via network connectivity or via a storage memory suitable for transferring content between DRM conformant devices. Thus, a user domain refers to a group of DRM conformant devices that may share DRM protected content. A content provider may allow replication and use of content among devices in the user&#39;s user domain. Further, the content provider may limit and/or prohibit distribution and use of such content to devices outside the user domain. 
         [0012]    One of the biggest criticisms in current digital rights management systems is the lack of interoperability between competing systems. The Open Mobile Alliance would like to address this problem through the use of a trusted device in the user&#39;s control called local rights manager (LRM). The LRM will have the functionality to convert non-OMA protected content to OMA DRM protected content for a user domain or for devices outside the user domain. This process will consist of two parts, namely the conversion of the protected data format (if needed) and the conversion of the used license format. 
         [0013]    DRM interoperability is an open challenge in the DRM community. Many academic solutions like Heileman, G. L. and Jamkhedkar, P. A.: “DRM interoperability analysis from the perspective of a layered framework”, Proceedings of the fifth ACM Workshop on Digital Rights Management, Co-Located with ACM CCS 2005 (2005), R. Safavi-Naini and M. Yung, Eds., ACM, pp. 17-26 and Jamkhedkar, P. A. and Heileman, G. L.: “DRM as a Layered System”, Proceedings of the Fourth ACM Workshop on Digital Rights Management, Co-Located with ACM CCS 2004 (2004), A. Kiayias and M. Yung, Eds., ACM, pp. 11-21 are focused on the entire architecture that may be needed to support interoperable DRM. But, as discussed by Koenen, R. H., Lacy, J., Mackay, M., and Mitchell, S.: “The long march to interoperable digital rights management”, Proceedings of the IEEE 92, 6 (2004), 883-897 and by Schmidt, A. U., Tafreschi, O., and Wolf, R.: “Interoperability challenges for DRM systems”, IFIP/GI Workshop on Virtual Goods (2004), there is another form of interoperability that can be achieved in short term: Connected interoperability (also called intermediated DRM). In connected interoperability, a trusted third party is used to convert between two rival formats. The LRM in OMA DRM proposes to allow this form interoperability. 
         [0014]    Currently, the Coral Consortium is the main proponent of a service that is based on this principle. In their scheme (which is very similar to the scheme proposed by Schmidt et al.), users first upload protected content and use licenses (ROs) to their servers, then these files are converted to the target DRM system format, finally the user can download the converted data. The LRM in OMA DRM is different, in that it is a trusted device within the user&#39;s control. For this reason, associated cryptographic key management systems that may be needed are more complex and those used by the Coral Consortium are not applicable. 
         [0015]    Currently, OMA DRM v2.0/v2.1 allow users to group their devices to form user domains. RIs have the capability to admit new devices to the user domain and allow devices to leave the user domain. A RI can issue ROs to access protected content for a user domain, such that any device in that user domain can access the media content associated to the RO. 
         [0016]    In SCE, user domain management is done by an entity called domain enforcement agent (DEA) for enforcing certain domain policies. The DEA may also be referred to as domain authority (DA) in other specifications or older versions of the SCE-specification. Hence, the terms DEA/DA may be used interchangeably. A RI can issue ROs for OMA DRM content to any user domain managed by the DEA. The LRM is proposed to work in conjunction with a user domain for the import of non-OMA DRM content and is restricted to that particular user domain, i.e., it cannot provide imported content to other user domains. The import function works first to converting non-OMA DRM data into OMA DRM data. For example, a device conformant to OMA DRM may attempt to play non-OMA DRM data. In this case, the non-OMA DRM data should be converted or imported into OMA DRM data by a LRM according to the OMA SCE requirements. Thus, the LRM imports the non-OMA DRM data into DRM content format (DCF) and imports a RO for the OMA DRM, which are called imported DCF and imported RO, respectively. Imported DCF and imported RO, which support OMA DRM, can be used by a DRM agent in a device compatible with OMA DRM according to OMA SCE requirements. In the sequel of this specification, ROs generated or converted by a LRM shall be denoted as LRM-ROs. These are similar to RI-generated ROs, except for some LRM-specific properties. 
         [0017]    In both OMA DRM v2.0/v2.1 and SCE, a content encryption key (CEK) may usually not be transmitted unencrypted from the RI to a DRM conformant device, since it may be revealed and used by other devices not possessing a related digital rights object. The CEK hence has to be transferred from the RI to the DRM conformant device in an encrypted manner. The OMA DRM specifications use public key methods for this reason. For a RO meant to be used on one single DRM conformant device, the OMA DRM method works in the following way: 
         [0018]    The DRM conformant device has attached to it a device certificate which binds a device ID to a public encryption key (a pair (m,e) of natural numbers). A corresponding private en-/decryption key d (also a natural number) is only known to the DRM conformant device. 
         [0019]    The RI checks the device certificate and generates a rights encryption key (REK), a message authentication code key (MK) and a random number Z in the range between 0 and m−1. The key MK is used to protect the rights object of changes. 
         [0020]    The RI generates a key encryption key (KEK) by means of a hash function of Z. Z is encrypted to first encrypted information C 1  by means of the public key (m,e). Further, a concatenation of REK and MK is encrypted to second encrypted information C 2  by means of KEK. Further, CEK is encrypted to third encrypted information C 3  by means of REK. CEK is that cryptographic key with which data content of associated digital media is encrypted. Finally, the RO comprising the encrypted data C 1 , C 2  and C 3  is sent from the RI to the DRM conformant device. 
         [0021]    Encrypted media content in a digital media object is typically not obtained from the RI, but via a different communication channel. The DRM conformant device now has access to an encrypted digital media object and an associated digital rights object with the cryptographic data C 1 , C 2  and C 3 . In order to be able to decrypt the encrypted media content, the DRM conformant device performs the following steps: 
         [0022]    Firstly, Z is decrypted by means of C 1  and the DRM conformant device&#39;s private key d. Then, the key encryption key KEK is derived from Z in the same way as it has been described above for the RI. By means of the derived KEK, the DRM conformant device decrypts the cryptographic keys REK and MK. By means of MK, the DRM conformant device may verify, whether the rights object has remained unchanged. By means of the rights encryption key REK, the DRM conformant device may decrypt the content encryption key CEK. Finally, knowing CEK, the DRM conformant device may now decrypt and replay the encrypted digital media content. 
         [0023]    Since the LRM is under the user&#39;s control there is a chance that it could be compromised, e.g. by way of a cryptographic attack. 
         [0024]    Once a compromised LRM is detected it would be desirable to minimize the amount of legitimate LRM-ROs that the LRM can generate for OMA DRM systems. However, legitimate LRM-ROs that were produced by the LRM before the compromise was detected should not be rejected by devices in the user domain as being illegitimate. 
         [0025]    Also, a compromised LRM should not lead to a compromise of legitimate protected content available to the user domain to which the LRM is attached to. Thus, in the event of a compromise of the LRM, it should be possible to isolate the LRM from the user domain without affecting the normal operation of the other devices in the user domain. 
         [0026]    Further, the LRM should possess most of the functionalities offered by a normal RI. Functionalities such as free movement of protected content and licenses (ROs) in the user domain, and the backup of protected content and licenses should also further be possible. 
         [0027]    Yet further, it should be possible for the LRM to have limited functionality without a connection to an external network entity. For example, the LRM could be in an area without consistent access to the internet. 
       SUMMARY 
       [0028]    According to an embodiment, a domain enforcement agent entity for issuing a domain policy for a domain, the domain having a plurality of DRM conformant devices and a local rights managing entity for importing digital media contents and/or rights object related to said digital media content into the domain, may have: a decider for deciding whether the local rights managing entity can be considered trustable; a provider for providing, in case a decision yields that the local rights managing entity can be considered trustable, a cryptographic generation key together with an identifier related to the local rights managing entity and a natural number to the local rights managing entity, such that the cryptographic generation key enables the local rights managing entity to generate a limited amount of rights objects specified by the natural number. 
         [0029]    According to another embodiment, a cryptographic key management method for providing a cryptographic generation key to a local rights managing entity for importing digital media content and/or rights objects related to said digital media content may have the steps of: deciding whether a specific local rights managing entity can be considered trustable; and providing, in case the decision yields that the specific local rights managing entity can be considered trustable, the cryptographic generation key together with an identifier related to the local rights managing entity and a natural number to the local rights managing entity, such that the cryptographic generation key enables the local rights managing entity to generate a limited amount of rights objects specified by the natural number. 
         [0030]    According to another embodiment, a computer program may have a program code for carrying out, when the computer program runs on a computer/and or microcontroller, a cryptographic key management method for providing a cryptographic generation key to a local rights managing entity for importing digital media content and/or rights objects related to said digital media content, wherein the method may have the steps of: deciding whether a specific local rights managing entity can be considered trustable; and providing, in case the decision yields that the specific local rights managing entity can be considered trustable, the cryptographic generation key together with an identifier related to the local rights managing entity and a natural number to the local rights managing entity, such that the cryptographic generation key enables the local rights managing entity to generate a limited amount of rights objects specified by the natural number. 
         [0031]    According to another embodiment, a local rights managing entity for importing digital media content and/or rights objects related to said digital media content and for converting the imported digital media content to a content conformant to a DRM-system may have: a device for generating a rights object based on a variable cryptographic key, the variable cryptographic key being dependent on a natural number and a cryptographic generation key provided to the local rights managing entity by a domain enforcement agent entity, the cryptographic generation key enabling the local rights managing entity to generate a limited amount of rights objects specified by the natural number and, the variable cryptographic key being dependent on an indicator for indicating how many rights objects have been generated by the local rights managing entity using said cryptographic generation key. 
         [0032]    According to another embodiment, a method for generating a rights object related to digital media content may have the steps of: determining a variable cryptographic key based on a natural number and a cryptographic generation key provided by a domain enforcement agent entity, the cryptographic generation key enabling the local rights managing entity to generate a limited amount of rights objects specified by the natural number and based on an indicator for indicating how many rights objects have been generated by the local rights managing entity using said cryptographic generation key; and generating the rights object using the determined variable cryptographic key. 
         [0033]    According to another embodiment, a computer program may have a program code for performing, when the computer program runs on a computer and/or microcontroller, a method for generating a rights object related to digital media content, wherein the method may have the steps of: determining a variable cryptographic key based on a natural number and a cryptographic generation key provided by a domain enforcement agent entity, the cryptographic generation key enabling the local rights managing entity to generate a limited amount of rights objects specified by the natural number and based on an indicator for indicating how many rights objects have been generated by the local rights managing entity using said cryptographic generation key; and generating the rights object using the determined variable cryptographic key. 
         [0034]    According to another embodiment, a DRM conformant device for replaying digital media content related to a received rights object from a local rights managing entity, the rights object having verification information having an indicator for indicating how many rights objects have been generated by the local rights managing entity using a specific cryptographic generation key may have: a verifier for verifying the verification information included by the rights object; and a signaller for signaling that the local rights managing entity is not trustable, in case the verification information is identical to a previously received verification information or, in case the indicator indicates that more than a limited amount of rights objects have been generated by the local rights managing entity using the specific cryptographic generation key. 
         [0035]    According to another embodiment, a method for installing a received rights object from a local rights managing entity to a DRM conformant device, the rights object having verification information having an indicator for indicating how many rights objects have been generated by the local rights managing entity using a specific cryptographic generation key, may have the steps of: verifying the verification information included by the rights object; and signaling that the local rights managing entity is not trustable, in case the verification information is identical to a previously received verification information or, in case the indicator indicates that more than a limited amount of rights objects have been generated by the local rights managing entity using the specific cryptographic generation key. 
         [0036]    According to another embodiment, a computer program may have a program code for performing, when the computer program runs on a computer and/or microcontroller, a method for installing a received rights object from a local rights managing entity to a DRM conformant device, the rights object having verification information having an indicator for indicating how many rights objects have been generated by the local rights managing entity using a specific cryptographic generation key, wherein the method may have the steps of: verifying the verification information included by the rights object; and signaling that the local rights managing entity is not trustable, in case the verification information is identical to a previously received verification information or, in case the indicator indicates that more than a limited amount of rights objects have been generated by the local rights managing entity using the specific cryptographic generation key. 
         [0037]    According to some aspects of the present invention, computer programs are provided for executing the steps of the provided inventive methods. 
         [0038]    The present invention is based on the finding, that the above-mentioned objectives may be solved by a cryptographic key management concept for a user domain, in which the LRM is provided a cryptographic generation key for validly generating or converting only a limited amount of rights objects. In case a compromise of the LRM is detected, the cryptographic generation key is not renewed after the limited amount of ROs have been generated by the LRM. Otherwise, i.e. in case no compromise of the LRM has been detected, i.e. in case the LRM may be considered trustable, the LRM is provided a new (different) cryptographic generation key for generating a further limited amount of rights objects. 
         [0039]    For the provision of the cryptographic generation key embodiments of the present invention provide a domain enforcement agent entity (DA) for issuing a domain policy for a domain managed by the domain enforcement agent (DEA), the domain comprising a plurality of DRM conformant devices and a local rights managing entity (LRM) for importing digital media contents and/or rights object related to said digital media content into the domain. The domain enforcement agent entity comprises means for deciding whether the local rights managing entity can be considered trustable, and means for providing, in case a decision yields that the local rights managing entity can be considered trustable, a cryptographic generation key to the local rights managing entity, the cryptographic generation key enabling the local rights managing entity to generate a limited amount of rights objects. 
         [0040]    According to a further aspect, there is provided a local rights managing entity (LRM) for importing digital media content and/or rights objects related to said digital media content and for converting the imported digital media content and/or rights objects to comply with a specific DRM-system. The local rights managing entity comprises a device for generating a rights object based on a variable cryptographic key. The variable cryptographic key is dependent on a cryptographic generation key provided to the local rights managing entity by a domain enforcement agent entity and on an indicator for indicating how many rights objects have been generated by the local rights managing entity using said cryptographic generation key. The cryptographic generation key enables the local rights managing entity to generate only a limited amount of rights objects and has to be renewed if the local rights managing entity shall be allowed to generate a further limited amount of rights objects. 
         [0041]    Yet further, there is provided a DRM conformant device for replaying digital media content related to a received rights object from a local rights managing entity (LRM), the rights object comprising verification information including an indicator for indicating how many rights objects have been generated by the local rights managing entity using a specific cryptographic generation key. The DRM conformant device comprises means for verifying the verification information comprised by the rights object. and means for signaling that the local rights managing entity is not trustable, in case the verification information is identical to a previously received verification information related to a previously received rights object. Likewise, it is signaled that the local rights managing entity is not trustable, in case the indicator indicates that more than a limited amount of rights objects have been generated by the local rights managing entity using the specific cryptographic generation key. 
         [0042]    The inventive domain enforcement agent entity (DEA), the local rights managing entity (LRM) and the DRM conformant device may be part of an OMA DRM user domain. All the parties in the proposed scheme, the RI, the DEA, the LRM and the individual DRM conformant devices make use of public key cryptography and they have their own public/private key pairs. Every local rights managing entity (LRM) will have its own public/private key pair, possibly installed during its manufacture. The domain enforcement agent entity (DEA) will issue the LRM and the DRM conformant devices in the user domain with a LRM-specific user domain key (LRM-UDK) being different to the UDK issued by the DEA to access RI-generated ROs to user domains. If there are multiple LRMs in a user domain, the DEA issues a different LRM-UDK for each LRM. It shall be noted that the user domain key (UDK) or the LRM-specific user domain key (LRM-UDK) may also be called master domain key (MDK) or the LRM-specific master domain key (LRM-MDK), respectively, in other specifications. Hence, the terms LRM-UDK/LRM-MDK and UDK/MDK be used interchangeably, respectively. 
         [0043]    The DEA will also issue the LRM with the cryptographic generation key (GK), which can be used by the LRM to generate at most N LRM-ROs, where N is a natural number. LRM-ROs generated with the same GK shall be called LRM-ROs from the same generation, i.e. from the generation of the GK. After the LRM has generated N LRM-ROs, a new generation has to be commenced and hence a new GK may be needed for the LRM to further operate. 
         [0044]    When the DEA generates a GK, it creates a protected tuple (LRM-ID, GK, N), wherein LRM-ID denotes a unique device identifier of the LRM, and delivers it to the LRM. A protection of the tuple consists of the encryption of GK using the LRM-specific user domain key LRM-UDK and the cryptographic signing of the complete tuple (LRM-ID, GK, N) with the DEA&#39;s private key. The LRM-ID and N need not necessarily be encrypted. 
         [0045]    Each rights object generated by the LRM, i.e. each LRM-RO, contains a valid and unique tuple (LRM-ID, GK, n) (e.g. n=1, 2, . . . , N) in case the LRM has not been compromised. 
         [0046]    In order to protect the REK that is enclosed in a newly generated LRM-RO, the LRM generates a variable diversified domain key (DDK) denoted by LRM-DDK GK,n  and uses that as a variable encryption key for the LRM-RO. The LRM-DDK GK,n  used for the n-th LRM-RO in the generation of the GK is calculated by some one-way function f of the LRM-UDK, GK and n, i.e. LRM-DDK GK,n =f (LRM-UDK, GK, n). Thereby a one-way function means a function that is relatively easy to compute but impossible or very hard to invert. 
         [0047]    A generated LRM-RO also contains a variable n representing the round of the GK&#39;s use. The variable n may be a non-negative integer, e.g. such that 0≦n&lt;N or 0&lt;n≦N. Each LRM-RO should be based on a unique (GK, n) pair. Further, the LRM may sign all LRM-ROs it generates using its private key. 
         [0048]    A DRM conformant device that can render LRM-generated content is adapted to ensure that there are not installed two different LRM-ROs with the same (LRM-ID, GK, n) tuple and that 0≦n&lt;N or 0&lt;n≦N. Otherwise the DRM conformant device may inform the DEA about a possible compromise of the LRM. 
         [0049]    If the DEA wants to revoke a LRM, it may not send out any new GKs for that LRM, thereby limiting the number of legitimate LRM-ROs the LRM can still generate after the revocation. I.e., once an LRM is revoked, it can at most generate m more legitimate ROs, wherein m is the number of non-used (GK, n) pairs at the time of the revocation. However, legitimate content created by the compromised LRM before its compromise has been disclosed, is not affected. 
         [0050]    Other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]    Embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which: 
           [0052]      FIG. 1  shows a schematic diagram of a domain enforcement agent entity according to an embodiment of the present invention; 
           [0053]      FIG. 2  shows a flow chart of a method for a cryptographic key management method according to an embodiment of the present invention; 
           [0054]      FIG. 3  shows a schematic block diagram of a local rights managing entity according to an embodiment of the present invention; 
           [0055]      FIG. 4  shows a schematic block diagram of a DRM conformant device according to an embodiment of the present invention; 
           [0056]      FIG. 5  shows a flow chart of a method executed of the DRM conformant device according to  FIG. 4 ; and 
           [0057]      FIG. 6  shows a schematic user domain comprising a domain enforcement agent entity, a local rights managing entity and a DRM conformant device according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0058]    The following description sets forth specific details, such as particular embodiments, procedures, techniques, etc. for purposes of explanation and not limitation. But it will be appreciated by one skilled in the art that other embodiments may be employed apart from these specific details. For example, although the following description is facilitated using non-limiting example applications to different OMA DRM embodiments, the technology may be employed to any type of DRM system. In some instances, detailed descriptions of well-known methods, interfaces, circuits, and devices are omitted so as not to obscure the description with unnecessary details. Moreover, individual blocks are shown in some of figures. Those skilled in the art will appreciate that the functions of those blocks may be implemented using individual hardware circuits, using software programs and data, in conjunction with a suitably programmed digital microprocessor or general purpose computer, using application-specific integrated circuitry (ASIC), and/or using one or more digital signal processors (DSPs). 
         [0059]      FIG. 1  schematically shows a domain enforcement agent entity (DEA)  100 , which is used for issuing a domain policy for a DRM user domain comprising a plurality of DRM conformant devices and a local rights managing entity, wherein the local rights managing entity serves for importing digital media content and/or rights objects to the user domain. 
         [0060]    DEA  100  comprises means  102  for deciding whether the local rights managing entity (LRM) can be considered trustable or trustworthy, and means  104  for providing, in case a decision yields that the LRM can be considered trustable, a cryptographic key GK to the LRM, wherein the cryptographic key GK enables the LRM to generate or convert a limited amount N of rights objects. 
         [0061]    According to one aspect of the present invention, the decision formed by means of  102  is based on verification information delivered to the DEA  100  from a DRM conformant device of the user domain. However, the verification information may also stem from other sources, like for example the internet. In any case it provides information on which basis the DEA  100  may decide whether to deliver a new GK to the LRM the next time it request for it. 
         [0062]    The means  102  for deciding hence may be coupled to a DRM conformant device belonging to the user domain in order to obtain the verification information from said DRM conformant device, wherein the verification information indicates that the LRM can or cannot be trusted. That is, the verification information input to the means  102  indicates whether the LRM has been compromised or not. Based on the verification information, means  102  may then decide, whether means  104  shall provide the LRM with a new GK or not. 
         [0063]    The verification information delivered from DRM conformant devices attached to the DEA  100  may, according to one embodiment of the present invention, comprise an actual or present tuple (LRM-ID, GK, n), wherein n should fulfill 0≦n&lt;N or 0&lt;n≦N. Such a tuple may be delivered to the DEA from a DRM conformant device which has just received a LRM-RO comprising said tuple. The means  102  may compare the delivered verification information in form of the tuple (LRM-ID, GK, n) with stored, previously delivered verification information or tuples in order to check whether the presently delivered tuple (LRM-ID, GK, n) is identical to one of the stored tuples. In that case the means  102  may decide that the LRM has been compromised and hence is not trustable anymore. The same decision is made in case the indicator n does not fall in the interval 0≦n&lt;N or 0&lt;n≦N. 
         [0064]    According to other embodiments, the above-described verification can also be performed within the DRM conformant devices themselves. In this case, a DRM conformant device just transmits the result of the verification to the DEA  100 . If the DEA  100  receives a verification result from a DRM conformant device indicating that the LRM has been compromised, means  104  stops providing new cryptographic generation keys GK to the LRM. 
         [0065]    However, in case it is decided that the LRM is trustable, means  104  generates a protected tuple (LRM-ID, GK, N) using the LRM-specific user domain key (LRM-UDK) and the DEA&#39;s private key. According to an embodiment of the present invention the protection of the tuple (LRM-ID, GK, N) consists of the encryption of the cryptographic generation key GK using the key LRM-UDK and signing of the complete tuple with the DEA&#39;s private key. The identifier LRM-ID and the limited amount N need not be encrypted. 
         [0066]    Turning now to  FIG. 2 , a method  200  carried out by the DEA  100  shall be summarized. 
         [0067]    In a first step  202  the DEA  100  obtains verification information from attached DRM agents of the user domain or from other sources, like for example information from the internet. Step  202  may be considered optional, since also no information may be provided to the DEA  100  in case the LRM has not been attacked or compromised. Based on the obtained information (wherein the information may also be no information) means  102  decides in a step  202 , whether the LRM is trustable or not. In case the decision yields that the LRM is not trustable, no new generation key GK is provided to the LRM. In the other case, i.e. decision  204  yields that the LRM is trustable, method  200  proceeds with a step  206  of generating a new cryptographic generation key GK for the LRM. After GK has been generated and secured as has been described before, it is provided to the LRM in step  208  together with N indicating the amount of LRM-ROs that may be generated by the LRM. Thereby, all communication between different parties of the user domain may be done on the basis of adequate data communication protocols. I.e., the LRM may send a request for obtaining a new GK to the DEA  100 . In response to that request the DEA  100  may send a message to the LRM, the message comprising a new GK, in case the LRM is still trustworthy. 
         [0068]    Turning now to  FIG. 3 , a local rights managing entity (LRM)  300  shall be explained in more detail. 
         [0069]    As explained before, LRM  300  serves as a trusted device under the control of the user in the domain. The LRM  300  has the functionality to convert non-OMA protected digital media content to OMA DRM protected digital media content. Therefore it is adapted to receive non-OMA protected digital media content  302  and/or non-OMA licenses  304  from sources outside the user&#39;s domain. A device  306  is used for generating an OMA-conformant RO based on a variable cryptographic key LRM-DDK GK,n  being dependent on the cryptographic generation key GK provided to the LRM  300  and based on an indicator n for indicating how many ROs have been generated by the LRM  300  using said provided cryptographic key GK. Of course, the LRM-RO may only be generated also on the basis of the content of the imported non-OMA license  304  containing the CEK for the associated, imported digital media content. 
         [0070]    Once the LRM  300  has generated a RO, i.e. a LRM-RO, it is transferred (together with the converted DRM content) to the DRM devices of the user domain. 
         [0071]    According to an embodiment, the device  306  for generating is adapted to determine the variable cryptographic key LRM-DDK GK,n  which is used for the n-th LRM-RO in the generation of the GK, on the basis of a one-way function f. The one-way function f is dependent on the provided cryptographic key GK, the indicator n (0≦n&lt;N or 0&lt;n≦N) and a (public) LRM-specific user domain key LRM-UDK, i.e., LRM-DDK GK,n =f(LRM-UDK, GK, n). The variable key LRM-DDK GK,n  is used to protect the REK that is enclosed in the newly generated LRM-RO, wherein the REK in turn protects the CEK of the associated, imported digital media content. 
         [0072]    Further, the device  306  for generating is adapted to cryptographically sign the generated LRM-RO by using the LRM&#39;s private key before sending the generated or converted LRM-RO to a DRM conformant device of the user domain. 
         [0073]    Turning now to  FIG. 4 , a third component of a DRM user domain shall be described, namely a DRM conformant device  400  according to an embodiment of the present invention. 
         [0074]    The DRM conformant device  400  typically comprises a so-called DRM agent possibly in form of a piece of software. The DRM conformant device  400  may be used for replaying digital media content related to a LRM-RO received from the LRM  300 . As explained before, the LRM-RO comprises information, in form of a tuple (LMR-ID, GK, n), on how many LRM-ROs the LRM  300  has already generated using a specific cryptographic key GK, GK enabling the LRM to generate a limited amount N of LRM-ROs. The DRM conformant device  400  comprises means  402  for verifying the information of the LRM-RO and means  404  for signaling that the LRM  300  is not trustable, in case the indicator n is greater than the limited amount N of LRM-ROs or, in case the tuple (LMR-ID, GK, n) is identical to the tuple of a previously received LRM-RO. 
         [0075]    For that purpose the DRM conformant device  400  needs to have knowledge on the limited amount N related to GK. One possibility is to get this knowledge directly from the DEA  100 . Another possibility is to encapsulate the limited amount N into the LRM-RO, thereby reducing signaling traffic in the user domain. A third alternative is to have N as a fixed system parameter. 
         [0076]    In case the verification performed by means  402  yields that the LRM  300  is not trustable, means  402  may trigger means  404  to transmit an information signal to the DEA  100 , the signal indicating that the LRM  300  has been compromised. In case the verification of the tuple (LMR-ID, GK, n) comprised by the received LRM-RO does not indicate a compromise of the LRM  300 , the LRM-RO may be decrypted by means  402  in order to obtain the CEK for replaying the attached DRM content. 
         [0077]    A method executed by the DRM conformant device  400  shall be now explained referring to  FIG. 5 . 
         [0078]    In a first step  502  the DRM conformant device  400  receives a LRM-RO related to an imported DRM content. The LRM-RO comprises information in form of a tuple (LRM-ID, GK, n). This tuple of the newly received LRM-RO is compared with stored tuples of already installed LRM-ROs in a step  504 . In step  506  it is checked, whether the tuple of the newly received RO is identical to any of the previously received tuples. If this the case, and the newly received RO is not identical to the previously received RO, an information may be transferred to the DEA  100  in step  508 , the information indicating that the LRM  300  has been compromised or manipulated. If the check of step  506  yields that the current tuple is not identical to any of the previously received tuples, method  500  proceeds with step  510 , in which it is checked whether n exceeds N. If n does not fall in the window 0≦n&lt;N or 0&lt;n≦N, an information is sent to the DEA  100  in step  508 , the information indicating that the LRM  300  has been manipulated. On the other hand, if the check in step  510  yields a reasonable value for n then the received LRM-RO is installed to the DRM conformant device in step  512 . 
         [0079]    According to an embodiment of the present invention the DRM conformant device  400  takes a role of a guard for surveying the LRM  300 . By looking at the tuples comprised by received LRM-ROs, the DRM conformant device  400  may find out whether a manipulation of the LRM  300  has taken place. Hereby it is assumed, that an attacker would try to manipulate the counter n or the upper limit N in the LRM. By checking the whole tuple other manipulations, like for example manipulating the LRM-ID or the GK may also be revealed. 
         [0080]    An interaction of the three previously described components, i.e. DEA  100 , LRM  300  and DRM conformant device  400 , is schematically depicted in  FIG. 6 . 
         [0081]    After having provided the LRM-UDK to both the LRM  300  and the DRM conformant devices  400  the DEA  100  delivers generation key information GK to the LRM  300 , on which basis it can generate and deliver LRM-ROs to the DRM conformant devices  400  of the user domain  600 . By verifying the tuples (LRM-ID, GK, n) comprised by the LRM-ROs, the DRM conformant devices may verify whether the LRM  300  has been manipulated and send verification information back to the DEA  100  which may then revoke the LRM  300  by not sending out any new GKs for that LRM, thereby limiting the amount of legitimate LRM-ROs the LRM  300  can generate after the revocation by the DEA  100 . Once the LRM  300  is revoked, it can at most generate m more legitimate ROs, wherein m is a number of non-used (GK, n) pairs at the time of revocation. Legitimate content created by a compromised LRM before its compromise is revealed, is, however, not affected. A DRM conformant device  400  cannot install LRM-ROs after an LRM revocation (beyond the m legitimate ROs). 
         [0082]    LRM-ROs that have been issued to the user domain may be backed up and used regardless of the status of the LRM. 
         [0083]    A stand-alone LRM does not have access to the user domain key (UDK) and thus a compromise of the stand-alone LRM does not affect the performance of other devices in the user domain. If a LRM is compromised, the security of other LRMs in the same user domain is not affected since each LRM has a unique LRM-UDK. 
         [0084]    The LRM-ROs allow normal functionality within the user domain like ROs generated by a rights issuer. Even if the LRM is compromised, the LRM-ROs are limited to DRM conformant devices that have the LRM-UDK, i.e. to devices that are part of the user domain to which the LRM-UDK is distributed. 
         [0085]    The LRM necessitates only limited connectivity with external network entities, i.e., it only needs to contact the DEA  100  to acquire new GKs. Hence, according to an embodiment of the present invention, the LRM  300  is adapted to request a new GK after it has created at most N LRM-ROs. In response, the DEA  100  transfers a new GK to the LRM  300  in case there is no indication for its compromise. 
         [0086]    The DEA  100  remains uninformed about which content is being converted by the LRM  300 , safeguarding the user&#39;s privacy. 
         [0087]    While this invention has been described in terms of several embodiments related to OMA DRM systems, there are alterations, permutations and equivalents within the scope of this invention. It should also been noted that are many alternative ways of implementing the described methods and compositions of the present invention. 
         [0088]    Depending on the circumstances, the inventive methods may be implemented in hardware or software. The implementation may be done on a digital storage medium, particularly a disc, CD or DVD with electronically readable control signals, which may cooperate with a programmable computer system such that the respective method is executed. In general, the invention thus also consists in a computer program product with a computer program code stored on a machine-readable carrier for performing the inventive method when the computer program product runs on a computer. In other words, the invention may thus be realized as a computer program with a program code for performing the method when the computer program runs on a computer and/or microcontroller. 
         [0089]    While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations and equivalents as fall within the true spirit and scope of the present invention.