Abstract:
The present invention discloses an apparatus and method for delivering a revocation list over a one-way broadcast network to receivers with limited memory capabilities. In one example, the revocation list is partitioned to form a first certificate revocation list (CRL) sequence if the number of entries in the revocation list exceeds a predetermined value. Individual identification numbers belonging to a first identification number series are subsequently assigned to partitions of the first CRL sequence. Afterwards, the first CRL sequence is interleaved into a first content transport stream.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/723,487 filed Oct. 4, 2005, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to video-over-networks, e.g., video-over-Internet Protocol (IP) networks that utilize digital rights management functions for securely communicating content to network components. More specifically, the present invention relates to a method and apparatus for delivering a certificate revocation list (CRL) to a one-way client device over a broadcast one-way network. 
     2. Description of the Related Art 
     Digital content information has recently gained wide acceptance in the public. Such content includes, but is not limited to: movies, videos, music, and the like. Consequently, many consumers and businesses employ various digital media devices or systems that enable the delivery of such digital multimedia content via several different communication channels (e.g., a wireless satellite link or a wired cable connection). Similarly, the communication channel may be a telephony based connection, such as DSL and the like. 
     In addition to being used to deliver digital content, a communication channel may be used to distribute a certificate revocation list (CRL) to one-way client devices (e.g., a set top box (STB) that receives a broadcast and does not have an interactive connection to the infrastructure) located in a local network. Typically, a CRL is delivered over an IP network as a communication message that is distinguished from digital content information. This manner of distribution may be an inefficient use of network resources. Furthermore, two-way interactive communications are not available to all receivers, e.g., digital TV set-top boxes without a return channel. Additionally, CRLs may grow to be very large over time while a receiving client device may possess a limited amount of memory. Consequently, the memory may be quickly consumed in the attempt to handle such large CRL objects. Although an attempt to keep the CRLs small could be made, the overall effectiveness of the CRL distribution system may be compromised. For example, in an effort to minimize the size of CRLs, only Certificate Authority (CA) certificates are revoked. Therefore, when a CA certificate is revoked, all device certificates (compromised and uncompromised device certificates alike) issued by that CA are effectively invalidated. 
     Thus, there is a need in the art for a method and apparatus for delivering a CRL to a one-way client device to a local network. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the present invention discloses an apparatus and method for delivering a revocation list. Specifically, the revocation list is partitioned to form a first certificate revocation list (CRL) sequence if the number of entries in the revocation list exceeds a predetermined value. Individual identification numbers belonging to a first identification number series are subsequently assigned to partitions of the first CRL sequence. Afterwards, the first CRL sequence is interleaved into a first content transport stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  depicts a block diagram of a system for facilitating the streaming of digital content over a communications network in accordance with the present invention; 
         FIG. 2  depicts an Intellectual Property Management and Protection (IPMP) tool that carries an encapsulated CRL; 
         FIG. 3  depicts a method for delivering a certificate revocation list in accordance with the present invention; and 
         FIG. 4  is a block diagram depicting an exemplary embodiment of a computer suitable for implementing the processes and methods described herein. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of a content distribution system  100  (e.g., an Internet Protocol rights management (IPRM) system) that utilizes an authenticated key management protocol (e.g., MOTOROLA ESBroker™ protocol) to facilitate the secure transfer of digital rights and content. In general, any secure content distribution system where the delivered content is persistently stored and consumed on one or more devices within the end user&#39;s network can be utilized. 
     In the event the content distribution system used by a content provider utilizes digital certificates, there is a need for end user devices to verify the revocation status of the digital certificates belonging to the content provider&#39;s servers. Furthermore, end user devices may legally exchange content in a protected manner, which commonly requires the local devices to verify each device&#39;s digital certificate. Therefore, end user devices also need to verify the revocation status of each device&#39;s certificate (e.g., deliver CRLs to those end user devices). 
     In one embodiment, the system  100  comprises a content provider  108  (e.g., a streaming server), a communications network  112  (e.g., the Internet), a certificate revocation license (CRL) server  114 , and a local network  102 . Although only one content provider  108 , one CRL server  114 , and one local network  102  are depicted, those skilled in the art realize that any number of content providers, CRL servers, or local networks may be included in the system  100 . 
     The local network  102  may comprise a home network that includes a Home key distribution center (Home KDC)  104  and a plurality of client devices  106   1 . . . N . The devices  106   1 . . . N  may each comprise a set top box (STB), a digital video recorder (DVR), and the like. These devices may be used to provide digital content to viewing devices, such as a television, computer monitor, and the like. In one embodiment, client devices are one-way, although not limited to being one-way. That is, a one-way client device is capable of receiving communication messages (e.g., broadcast) but does not have an interactive connection to the infrastructure. For example, the infrastructure equipment may be responsible for generating a one-way MPEG-2 transport stream that is delivered as a digital broadcast over cable, satellite or a terrestrial network. While the infrastructure may support two-way communications for some client devices with a return channel capability, cheaper devices do not have the return channel and are only capable of receiving a one-way broadcast transport stream. At present, this is the scenario with most cable and satellite digital television networks. In addition, some two-way client devices may not use their interactive capability for all functions, and may instead process broadcast messages, particularly in a hybrid network consisting of both one-way and two-way devices. 
     The Home KDC  104  is typically a single device (e.g., a STB, a DVR, etc.) in a home network that is designated to function as a media gateway. The Home KDC  104  facilitates communication between the local network  102  and the other components of the system  100 . In addition, the Home KDC  104  exchanges messages with the devices  106   1 . . . N  in order to register a client device  106 , provide tickets needed to obtain content from a content provider  122 , and the like. Similarly, the Home KDC  104  in an IPRM-protected domain is configured to process CRLs after an updated CRL sequence is received via broadcast from the CRL server  114 . 
     The content provider  108  may comprise a streaming server that provides the digital content requested by the client devices  106   1 . . . N  (or the Home KDC  104 ). More specifically, the content provider  108  distributes encrypted content to the Home KDC  104  positioned in the local network  102  where the content is ultimately provided to the appropriate client device  106 . In one embodiment, the content provider  108  may be configured to utilize caching servers (not shown) throughout the system  100  to distribute content to the local network(s). 
     The CRL server  114  may comprise a stand-alone server that obtains signed CRL files from a certificate authority (CA) server  110 . Specifically, a Certificate Authority  110  is responsible for recording the serial number of a client device that is deemed to be compromised or when a client device&#39;s certificate is revoked for any reason. Specifically, the serial number of the device is recorded to a CRL  116 . Once a CRL has been created and signed by a Certificate Authority, it can be safely transferred to a CRL server  114  with no additional security necessary. The CRL server  114  is capable of storing a plurality of CRLs in a database or other storage medium. Upon being notified that the CRL needs to be modified, the Certificate Authority updates the CRL sequence to include the additional compromised devices (or remove authorized devices). The updated CRL sequence is subsequently transferred to the CRL server  114  and broadcast to the local network  102 . 
     A CRL sequence is repeated periodically at a configurable interval, e.g., once every few hours. The client devices  106   1 . . . N  are required to remain tuned in to the broadcast of the CRL. If the previously obtained CRL has expired and a new one has not been obtained within a pre-defined interval (e.g., 1 day), a client device  106  temporarily loses its ability to transfer any content to other devices. After an updated CRL has been acquired, the ability to transmit content is regained. In one embodiment, each CRL is an X.509 CRL that conforms to the IETF RFC 3280 standard. A client device  106  inspects the CRL Number parameter and determines if that particular CRL has already been received, so that the CRL does not have to be downloaded again. Because the X.509 CRL encoding positions the CRL Number extension near the end of the CRL, each DER-encoded X.509 CRL is also prefixed by a 4-byte CRL Number at the beginning of the CRL for the sake of efficiency (i.e., easier detection for client device). 
       FIG. 2  depicts an exemplary IPMP Tool Container  200  that is used to encapsulate a CRL. IPMP protocol is defined by MPEG-2 Part 11 in order to carry DRM-related information in an MPEG-2 broadcast stream. IPMP defines a set of MPEG-2 tables that can be included in an MPEG-2 multiplex and can include a construct called an IPMP Container, which may be used to include CRLs. Other standards-based and proprietary containers for broadcasting CRLs are also possible. In one embodiment, the IPMP Tool Container  200  may be used to distribute CRLs in-band over a video transport stream, such as a moving picture experts group (MPEG) broadcast stream (e.g., MPEG-2 broadcast stream) utilizing a specific PID (e.g., PID 3). Each CRL is separated into sections  206   1 . . . M  (e.g., IPMP_Control_Info sections), which are carried by the IPMP Tool Container  200 . A single IPMP_Control_Info section can hold up to 4093 bytes of data. Because a memory-constrained device may run out of memory processing large CRL objects, IPRM protocol limits the size of a single CRL to a maximum of 1024 entries, which amounts to approximately 40 kilobytes. Consequently, a CRL may have to be separated into a maximum of 10 sections (i.e., 40 kilobytes divided by 4093 bytes). The IPMP Tool Container  200  comprises a CRL issuer name  202 , a CRL Number parameter  204 , the CRL itself and a signature  212 . The CRL issuer name  202  is a field that identifies the CRL issuer that signed the CRL, which is commonly a Certificate Authority (and is not the CRL Server). The CRL Number parameter  204  and issuer  202  are fields that a client device may inspect in order to determine if the corresponding CRL has been previously received. The signature  212  is normally included as part of each CRL to validate that the CRL has not been modified after being generated by a legitimate CRL issuer named in the CRL. 
     In order to support a CRL that contains more than 1024 entries, a CRL can be represented as a sequence of CRLs (i.e., CRL “partitions”) that are individually signed (i.e., each individual CRL partition contains its own unique signature that is only associated with that one CRL partition). Notably, a sequence of CRLs may be characterized by a number of factors. In one embodiment, all the CRLs in a given sequence must have the same validity period (i.e., the CRLs expire at the same time). Also, the first CRL in a sequence includes an identification number (e.g., a CRL Number extension) with a value that is a multiple of a number such as 0×10000 (65536). Similarly, each successive CRL in the same CRL sequence includes a CRL Number that is incremented by the same constant value (e.g., “1”). Furthermore, the last CRL in a given sequence must have less than the maximum 1024 entries. If the number of revoked certificates is an exact multiple of 1024, then the last CRL in the sequence must be empty. The present invention utilizes the CRL Numbers to signify the grouping of a particular CRL sequence as well as an indicator of an updated revocation list. 
     For example, in one embodiment, a particular CRL sequence may use a specific “series” of numbers to be used as CRL Numbers. A first sequence of CRLs may employ a 0×10000 series representation wherein the first CRL in the sequence possesses an identification number of 0×10001. Similarly, the second CRL in the sequence would be incremented have an identification number of 0×10002. This method of numbering the identification numbers would continue in like fashion for all the CRLs in a given sequence. However, when a CRL server  114  needs to modify the current revocation list, a new sequence of CRLs is assigned a second series of numbers. For example, the CRL server  114  may assign a 0×20000 series representation to the second sequence of CRLs, wherein the first CRL in the sequence would be numbered 0×20001, the second CRL would be numbered 0×20002, and so on. By changing the entire series of numbers used to identify a modified CRL sequence, the Home KDC  104  and the client devices  106   1 . . . N  in the local network  102  are able to detect an updated CRL. Notably, the device compares the identification number in Section  0  to the last identification number(s) stored in memory and initiates a download of the CRL sequence if a change is detected. 
     The ESB protocol defines the types of messages in which a CRL may be included. During client provisioning with a Home-KDC, the Init Principal Reply message includes a CRL of Home-KDC certificates (i.e., a list of compromised) Home KDCs and like devices. Similarly, a client device may utilize an AS Request message to request that a CRL be included in a corresponding AS Reply from the Home KDC. Client devices need to request a new CRL if the old revocation list has already expired or is about to expire. In one embodiment, each Home-KDC is required to obtain two types of CRLs: (1) client CRL so that the KDC can verify client certificates, and (2) a Home-KDC CRL that is provided to clients within a local network. In one embodiment, CRLs are distributed to each Home-KDC over an MPEG-2 multiplex using an in-band method. For example, when a client sends a request message such as AS Request to the Home-KDC, it first checks the timestamp on its copy of a Home-KDC CRL to see if it is expired. If that CRL appears to be expired, then the client sets a flag in the request message to indicate to the Home-KDC that it needs a fresh CRL. When preparing a normal response message to the client (e.g., AS Reply), the Home-KDC will also include the latest and non-expired copy of the Home-KDC that it obtained from a CRL Server. Once the client receives the reply from the Home-KDC with an updated CRL, it will use it to verify the status of the Home-KDC certificate. 
     After detecting an updated revocation list, the Home KDC (or client device) downloads the new CRL and processes the data. Notably, the Home KDC  104  determines if any client device in the local network  102  is listed on the downloaded CRL. If so, the Home KDC records the id of the client device on a “to-be-revoked” list. When a client device with “to-be-revoked” status contacts the Home-KDC, the Home-KDC may be configured to reject any request from such clients. Consequently, the client device will be denied access to all content in the IPRM-protected domain that is not already stored locally. 
       FIG. 3  illustrates a method  300  for delivering a CRL to a client device in accordance with the present invention. Method  300  begins at step  302  and proceeds to step  304  where a CRL is generated. In one embodiment, the CRL server  114  creates a revocation list after receiving a list of compromised client devices (and/or Home KDCs) from network operators, equipment manufacturers, or some other reporting entity. 
     At step  306 , a determination is made as to whether the CRL contains a number of entries that exceeds a threshold value. In one embodiment, the CRL server  114  ascertains if the compromised devices entries on the revocation list exceeds 1024. If the threshold value is not exceeded, then the method  300  proceeds to step  312 . If the threshold value is exceeded then the method  300  continues to step  308  where a CRL sequence is generated. In one embodiment, the CRL server  114  divides the oversized revocation list into separate CRL “partitions” that contain a maximum of 1024 device entries. 
     At step  310 , identification numbers are assigned to the CRL sequence. Notably, the CRL server  114  assigns each sequence “partition” an identification number that belongs to an identification number series. In one embodiment, the first CRL partition has an identification number that is a multiple of 0×10000 (i.e., 65536). Similarly each sequence CRL in the same sequence has an identification number that increments by 1. 
     At step  312 , the CRL sequence is transmitted. In one embodiment, the CRL server  114  interleaves the CRL sequence into a content transport in-band to a local network. For example, the CRL sequence may be inserted into an MPEG transport stream. Typically, a Home KDC in the local network receives the digital content and CRL sequence. At step  314 , a determination is made as to whether the transmitted CRL sequence has been previously received. Notably, the Home KDC  104  ascertains if the CRL is either a previously received CRL or a new and/or modified CRL. In one embodiment, the Home KDC accomplishes this by inspecting the identification number series of the CRL sequence and comparing the value(s) to a recorded value (i.e., a previously received identification number series). If the CRL sequence has already been received on a prior occasion, the method  300  continues to step  322 , where the Home KDC will ignore the CRL sequence. Alternatively, the method  300  continues to step  316  where the CRL sequence is downloaded. 
     At step  318 , a determination is made as to whether the CRL sequence contains a client device in the local network. In one embodiment, the Home KDC compares the certificate(s) of the client devices in the local network  102  with the CRL sequence entries. If a match is not found, the method  300  continues to step  324  and ends. If a match is found, the method proceeds to step  320  where the device certificate is revoked. In one embodiment, the Home KDC  104  places the compromised client device on a “to-be-revoked” list that is stored locally. If the Home KDC  104  receives any content requests from the client device in question, the requests will be ignored. The method  300  ends at step  324 . 
       FIG. 4  depicts a high level block diagram of a Home KDC or general purpose computer suitable for use in performing the functions described herein. As depicted in  FIG. 4 , the system  400  comprises a processor element  402  (e.g., a CPU), a memory  404 , e.g., random access memory (RAM) and/or read only memory (ROM) and/or persistent memory (Flash), a CRL delivery module  405 , and various input/output devices  406  (e.g., storage devices, including but not limited to, a tape drive, a floppy drive, a hard disk drive, a compact disk drive, a receiver, a transmitter, a speaker, a display, a speech synthesizer, an output port, and a user input device (such as a keyboard, a keypad, a mouse, etc.) and the like. 
     It should be noted that the present invention can be implemented in software and/or in a combination of software and hardware, e.g., using application specific integrated circuits (ASIC), a general purpose computer or any other hardware equivalents. In one embodiment, the CRL delivery module or process  405  can be loaded into memory  404  and executed by processor  402  to implement the functions as discussed above. As such, the present CRL delivery module  405  (including associated data structures) of the present invention can be stored on a computer readable medium or carrier, e.g., RAM memory, magnetic or optical drive or diskette and the like. 
     While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.