Patent Grant
7769880
Digital Rights Management (DRM) refers to techniques that are used to protect content, such as by controlling or restricting the use of digital media content on electronic devices. One characteristic of DRM is that it can bind the media content to a given machine or device. Thus, a license that pertains to a particular piece of content and which defines rights and restrictions associated with the piece of content will typically be bound to the given machine or device. As a result, a user will not typically be able to take the piece of content and move it to another machine in order to playback the content.
There are some technologies that permit DRM-protected content to be moved to other machines to enable playback of the content on those machines, but such technologies can tend to use non-real time protocols for content transfer that are unsuitable for simultaneous transferring and playback of the content.
Various embodiments utilize methods of protecting content, such as Digital Rights Management (DRM), to enable secure playback of content on machines and devices within a local network, such as a home media network. In at least some embodiments, messages and content are delivered using, respectively, a control protocol for streaming and a transport protocol. In at least some embodiments, the control protocol for streaming is Real Time Streaming Protocol (RTSP), and the transport protocol is Real Time Transport Protocol (RTP).
Various embodiments described herein utilize method for protecting content, such as Digital Rights Management (DRM), to enable secure playback of content on machines and devices within a local network, such as a home media network. In at least some embodiments, messages and content are delivered using, respectively, a control protocol for streaming and a transport protocol. In at least some embodiments, the control protocol for streaming is Real Time Streaming Protocol (RTSP), and the transport protocol is Real Time Transport Protocol (RTP). In these embodiments, protocol extensions are introduced which enjoy advantages offered by RTSP/RTP, including data delivery over User Datagram Protocol (UDP) and bi-directional communication between client and server, as will be appreciated by the skilled artisan.
In particular, in at least some embodiments, a protocol extension securely establishes a session using RTSP, transfers protected data encapsulated in RTP, provides schemes for encrypting and transferring the data depending on the RTP payload format, and various methods for transferring encryption parameters in conjunction with encrypted content data.
In the discussion that follows, a section entitled “Content Security and License Transfer Protocol” is provided and describes one particular system in which the inventive techniques can be employed. Following this, a section entitled “RTSP” is provided to give the reader who is unfamiliar RTSP at least some context for understanding the inventive techniques in the RTSP space. Following this section, a section entitled “Exemplary Implementation Using RTSP” is provided and describes various inventive techniques that employ RTSP for establishing a control flow, and utilize RTP for establishing a data flow.
Content Security and License Transfer Protocol
The following provides a discussion of an exemplary protocol which provides security and transfers licenses for content flowing over digital links. This protocol constitutes but one exemplary protocol with which the various inventive techniques can be employed. It is to be appreciated and understood that other protocols can be utilized without departing from the spirit and scope of the claimed subject matter.
The following cryptographic notation is used in this description:
In this particular protocol, there are five primary procedures: Registration, Revalidation, Proximity Detection, Session Establishment, and Data Transfer.
In the Registration procedure, a transmitter (i.e. a device that has content that is to be transmitted to another device) can uniquely and securely identify an intended receiver (i.e. a device to which content is to be transmitted). In this particular protocol, the transmitter maintains a database with registered receivers and ensures that no more than a small predetermined number of receivers are used simultaneously. During the registration process, the transmitter also employs a Proximity Detection procedure to ensure that the receiver is located “near” the transmitter in the network, in order to prevent wide distribution of protected content.
The Revalidation procedure is utilized to ensure that the receiver continues to be “near” the transmitter. Content is not delivered to receivers unless they have been registered or revalidated within a predetermined period of time in the past.
The Session Establishment procedure is used whenever the receiver requests content from the transmitter. The transmitter enforces that devices must be registered and recently validated before the Session Establishment can be completed.
Once the session is established, the Data Transfer of the requested content can take place in a secure way. The receiver may reuse the session to retrieve specific portions of the content (seeking), but must establish a new session in order to retrieve a different content.
Consider now the Registration procedure in connection with
Here, the receiver sends a registration request message that contains, among other information, the receiver's digital certificate. Responsive to receiving the registration request message, the transmitter validates the receiver's certificate, generates a seed and a random session ID, returning the same in the form indicated above to the receiver in a registration response message. The receiver then validates the transmitter's signature, obtains the session ID and performs the other actions indicated in the figure. The receiver and the transmitter can then undergo a proximity detection process which is described below.
With regard to Revalidation, the same procedures as outlined above are performed, with the difference being that during Revalidation, the receiver is already registered in the database.
With regard to Proximity Detection, consider the following in connection with
During the Proximity Detection procedure, the receiver sends to the transmitter a message containing the Session Id indicated in a Proximity Detection Initialization Message. The transmitter then sends to the receiver a message containing a Nonce (128-bit random value), and measures the time it takes for the receiver to reply with the nonce encrypted using a Content Encryption key. Finally, the transmitter sends a message to the receiver indicating if the proximity detection was successful or not.
The receiver may repeat the process until it has a confirmation that the proximity detection succeeded. When this particular protocol is used over IP-based networks, the proximity detection messages are exchanged over UDP. The receiver learns the transmitter's address via the Registration Response message. The receiver's address does not need to be separately communicated since it can be determined by inspecting the incoming IP header of the UDP packet that carries the Proximity Detection Initialization Message.
The following table describes the messages that are exchanged during Proximity Detection:
With regard to Session Establishment, consider the following in connection with
In this example, a License Request Message is sent from the receiver to the transmitter and contains the information described above. In response, the transmitter can send a License Response Message that contains the information described above.
In this particular example, the License is represented in XMR format and includes a Content Encryption key, a Content Integrity key, a Version of the Transmitter's CRL, a 128-bit Rights Id and a 128-bit Serial Number. The License also contains an OMAC calculated using the Content Integrity key using OMAC.
With regard to the Data Transfer procedure, consider the following in connection with
Having now provided a brief overview of an exemplary protocol with which the inventive embodiments can be employed, consider now some background information on RTSP.
RTSP
The Real Time Streaming Protocol or RTSP is an application-level protocol for control over the delivery of data with real-time properties (i.e. streaming), as it will be appreciated by the skilled artisan. RTSP provides an extensible framework to enable controlled, on-demand delivery of real-time data, such as audio and video. Sources of data can include both live data feeds and stored clips. This protocol is intended to control multiple data delivery sessions, provide a means for choosing delivery channels such as UDP, multicast UDP and TCP, and provide a means for choosing delivery mechanisms based upon RTP.
RTSP establishes and controls either a single or several time-synchronized streams of continuous media such as audio and video. It does not typically deliver the continuous streams itself, although interleaving of the continuous media stream with the control stream is possible. In other words, RTSP acts as a “network remote control” for multimedia servers.
The set of streams to be controlled is defined by a presentation description. In RTSP, there is no notion of an RTSP connection; instead, a server maintains a session labeled by an identifier. An RTSP session is in no way tied to a transport-level connection such as a TCP connection. During an RTSP session, an RTSP client may open and close many reliable transport connections to the server to issue RTSP requests. Alternatively, it may use a connectionless transport protocol such as UDP, as will be appreciated by the skilled artisan.
The streams controlled by RTSP may use RTP, but the operation of RTSP does not depend on the transport mechanism used to carry continuous media.
Consider now a typical RTSP request/response exchange in connection with
Preliminarily, the RTSP requests/responses have headers which, for the sake of brevity, are not described. In RTSP, a client/receiver 500 typically issues what is known as a DESCRIBE request which is directed to retrieving a description of a presentation or media object identified by a request URL from server 502. The server 502 responds with a description of the requested resource which is represented in the SESSION DESCRIPTION PROTOCOL (SDP). The DESCRIBE response (SDP) contains all media initialization information for the resource(s) that it describes.
Next, client 500 sends a SETUP request for a URI that specifies the transport mechanism to be used for the streamed media. In the
At this point, the client can issue a PLAY request which tells the server to start sending data via the mechanism specified in the SETUP. Responsive to receiving a PLAY request, the server can start streaming the content which, in this example, is the audio/video content. In this example, the streaming content is encapsulated using RTP packets and is sent over UDP, as will be appreciated by the skilled artisan.
The RTSP protocol has other methods of interest which include PAUSE, TEARDOWN, GET_PARAMETER, SET_PARAMETER, REDIRECT, and RECORD. For additional background on RTSP, the reader should consult the RTSP RFC, Schulzrinne, H., Rao, A., and R. Lanphier, “Real Time Streaming Protocol (RTSP)”, RFC 2326, available at http://www.ietf.org/rfc/rfc2326.txt, April 1998.
Exemplary Implementation Using RTSP
In the discussion that follows, two primary subsections appear, one entitled “Control Flow” that describes how a control flow for DRM-protected content is established using RTSP, and one entitled “Data Flow” that describes how a data flow for DRM-protected content is established using RTP. Each of these primary subsections has its own associated subsections that describe aspects of the inventive embodiments.
In the discussion that follows, a description is provided of how the Session Establishment and Data Transfer procedures of the above-described protocol are accomplished using RTSP/RTP in accordance with one embodiment. More specifically, in the “Control Flow” section below, a description is provided of how Session Establishment is accomplished using RTSP. In the “Data Flow” section, a description is provided of how the Data Transfer is accomplished using RTP.
Control Flow
In accordance with this embodiment, Session Establishment is initiated by a receiver device which is willing to playback DRM-protected content—that is, content that has an associated license. Recall from the discussion of the Content Security and License Protocol above, that the client/receiver would accordingly send a License Request Message to the server/transmitter, to which the server/transmitter would reply with a License Response Message. The License Response Message, in turn, carries a license which in the example above, is represented in eXtensible Media Rights (XMR). The license contains the policy and content key associated with the content being requested.
Carrying License Request Messages in DESCRIBE Requests
Consider now the confluence of the Content Security and License Protocol and RTSP in connection with
As but one implementation example, consider the DESCRIBE request excerpt just below which incorporates a License Request Message in accordance with one embodiment.
The “Require: com.microsoft.wmdrm-nd” is used, in this example, to indicate that the receiver expects the server to be a particular type of transmitter. The “Content-Type: application/vnd.ms-wmdrm-license-request” is used, in this example, to indicate that the body of the DESCRIBE contains a License Request Message.
Unless there is an error, the transmitter should reply with an SDP description which includes the License Response Message described in the section immediately below.
Embedding License Response Messages in SDP Descriptions
Having received a DESCRIBE request that contains, in the body, a License Request Message, the server can return a License Response Message. In this example, the server returns an SDP description that not only contains the various parameters described before, but also the License Response Message. In this embodiment, the License Response Message, as previously indicated, will carry an XMR license that dictates which policies apply to the content.
As but one implementation example, consider the SDP excerpt just below which incorporates a License Response Message in accordance with one embodiment.
In accordance with one embodiment, the SDP returned by the transmitter includes the License Response Message encoded in a data URL according to the specification in RFC-2397 (http://www.ietf.org/rfc/rfc2397.txt). The data contained in the data URL, in this example, must be Base64 encoded and the MIME type must be set to “application/vnd.ms-wmdrm-license-response”.
As an example of the syntax, consider the following:
The data URL, in this example, must be inserted at the SDP session level using a “a=key-mgmt” attribute, according to the SDP key management extensions specification, which continues to be a work in progress as of this date). The syntax is as follows:
The URL parameter is the data URL described above.
Carrying License Response Messages in ANNOUNCE Requests
Consider now that certain media files contain segments which require different policies to be enforced. Take, as an example, the case of files generated by Windows Media Center Edition for TV recordings. Such files are protected by WMDRM and have multiple policies associated with them. For example, Macrovision may be required for a TV show, but not for the commercial segments that appear within this same recording.
This requirement results in the need to define a mechanism for delivering updated policies during the middle of the stream. In accordance with one embodiment, updated policies can be delivered in the middle of a stream using RTSP's ANNOUNCE request. In this embodiment, the ANNOUNCE requests carry License Response Messages which contain new XMR licenses.
In this example, there are two different instances in which policies associated with streaming media may change. In a first instance, only the policies associated with a particular stream may change. In a second instance, both policies and the content format itself can change.
Consider the first instance in which only the policies associated with the streaming media change. One example of this case would be a switch between a segment of a TV show and a commercial, in which the TV segment requires Macrovision to be enabled on analog outputs while the commercial does not. Notice that in this example only the policy changes: the encoding parameters, such as bitrate, codec, etc. remain the same.
Consider the second instance in which both the policies and the content format changes. An example of this case would also be a switch between a segment of a TV show and a commercial, with the same type of change to the policy. However, in this example, the TV show and the commercial are encoded using different encoding parameters, such as a transition from a High Definition encoding to a Standard Definition encoding. Such scenarios are commonly denominated “format changes”. Another example of this case relates to what is commonly known as “entry changes”. Entry changes are typically a consequence of a switch in media files that are being delivered by the server as part of a “server-side playlist”. These playlists may be composed of a collection of media files which do not necessarily share any encoding parameters or policies.
Whenever policies change but formats do not, as illustrated in the first case, the server only sends a new policy to the client, as part of the body of the ANNOUNCE request. In this case, a License Response Message is included in the body of an ANNOUNCE message. As an example, consider
Whenever policies change and formats do so as well, as illustrated in the second case, the server delivers to the client an updated SDP description. This SDP description is required in order to describe the format changes that took place. In this example, SDP descriptions, in the case of format changes, are also delivered as ANNOUNCE requests. So instead of delivering two consecutive ANNOUNCE requests, one containing the format change, and another containing the policy change, the server may send only one ANNOUNCE request, which carries an SDP description. The policy change is then communicated as a License Response Message embedded in the SDP description. Consider again
The format for embedding License Response Messages in SDP descriptions that are part of ANNOUNCE requests is the same as described previously for embedding SDP descriptions that are part of DESCRIBE responses.
Step 800 attempts to establish a control flow by sending a request for a license for DRM-protected content via a streaming protocol. In the illustrated and described embodiment, this step is executed by a client/receiver. One specific example of a request for a license is the License Request Message which is described above. Other request types or formats can be utilized without departing from the spirit and scope of the claimed subject matter. In addition, one example of a streaming protocol (i.e. RTSP) is described above. Other streaming protocols can be used with departing from the spirit and scope of the claimed subject matter. In the RTSP embodiment, the request is inserted into the body of a DESCRIBE request.
Step 802 attempts to establish a control flow by receiving the request for a license. This step is implemented, in this example, by a server/transmitter. Responsive to receiving the request, step 804 can send a license to the client/receiver using the streaming protocol. One specific example in which a license is returned to the client/receiver is provided above in which a license in the form of a License Response Message is sent to the client/receiver. Other response types or formats can be utilized without departing from the spirit and scope of the claimed subject matter. In addition, one example of a streaming protocol (i.e. RTSP) is described above. Other streaming protocols can be used with departing from the spirit and scope of the claimed subject matter. In the RTSP embodiment, the response is sent in the SDP.
It is to be appreciated and understood that step 804 can also be implemented to send updates to the client/receiver. In this instance and in the context of the RTSP example, updates can be delivered using ANNOUNCE requests as described above.
Step 806 receives the license via the streaming protocol. In the illustrated and described embodiment, this step is implemented by the client/receiver. After receiving the license, the client can access and consume the content pursuant to terms defined in the license.
The data flow that follows the license acquisition process is described just below.
Data Flow
Having described exemplary embodiments of a control flow that utilizes RTSP in connection with DRM-protected content, consider now the data flow that contains or enables communication of the actual DRM-protected content.
In the embodiments described below, DRM-protected content is communicated between a transmitter and a receiver using RTP as a data transfer protocol. That is, DRM-protected content is communicated from the transmitter and communicated to the receiver.
In the particular examples provided, two different approaches are described. In the first approach, the RTP payload format that is utilized supports extensions which, in turn, allows encryption parameters such as key ID extensions and initialization vectors to be included in the RTP packet so that encrypted payload data can undergo a decryption process and be decrypted. In the second approach, the RTP payload format does not support extensions. Hence, in this approach, a Descriptor is defined and associated with the RTP packet that contains the encrypted payload. The Descriptor contains encryption parameters such as key ID extensions and initialization vectors that can be used in a decryption process to decrypt the encrypted payload data.
Carrying Sample-Encrypted Payloads Over Windows Media Payload Format
Packet 900, in this example, comprises an RTP header 902 and a payload format header 904. The payload format header, in this example, allows for extensions. As such, packet 900 further comprises a key ID extension 906 and an initialization vector 908, along with encrypted payload data 910 (either audio or video data) that is associated with and can be decrypted using key ID extension 906 and initialization vector 908. Further, RTP packet 900 can include multiple other encrypted payloads. In this particular example, packet 900 further comprises another payload format header 904a, key ID extension 906a initialization vector 908a along with encrypted payload data 910a (either audio or video data) that is associated with and can be decrypted using key ID extension 906a and initialization vector 908a.
In this particular embodiment, one RTP packet can contain multiple different encrypted payloads. As a specific implementation example in but one specific context, consider the following in connection with Windows Media Audio and Video Content.
When carrying Windows Media content protected by licenses as described above, the following values and fields must be set in the RTP packet.
Sample Encryption
As a further explanation of item 4(c) above, consider the following. In this embodiment, each sample should be encrypted using AES in Counter mode.
In this embodiment, Counter mode creates a stream of bytes that are then XOR'd with the clear text bytes of the media sample to create the encrypted media sample. The Key Stream Generator uses an AES round to generate 16-byte blocks of key stream at a time. The inputs to the AES round are the Content Encryption key (KC) and the 128-bit concatenation of a Sample ID and the block number within the sample.
The output of key stream generator should be XOR'd byte by byte with the data from the corresponding block (i) of the media sample. In the case that the media sample is not evenly divisible by 16 bytes only the valid bytes of the media data from the last block should be XOR'd with the key stream and retained for the encrypted sample.
When encrypting samples from an ASF file, the Sample ID is equivalent to the Sample ID from the payload extension.
Hence, in this embodiment, data is encrypted and decrypted according to “sample” boundaries, which are the natural boundaries for the given media type, e.g. a video frame for a video stream or a block of audio samples for an audio stream.
Carrying Link-Encrypted Payloads Over RTP Payload Format Using Data Segment Descriptors
In this embodiment, the RTP packet with the exception of the RTP header, is treated as a data segment associated with the descriptor 1112. Descriptor 1112, in turn, carries with it the encryption parameters that can be used in a decryption process that enables payload data 1110 to be decrypted. In this particular example, a single policy and content encryption key applies to the payload data 1110.
In accordance with one embodiment, descriptor 1112 comprises a data structure as follows:
In this example, the Flags section is a bit-field indicating attributes of the data. The following values are currently defined: 0x01 indicates encrypted data. When this flag is set, it indicates that the data is in encrypted form. Otherwise, the data is in the clear.
With regard to the Extensions section, the Number of Extensions field indicates the number of variable length extensions included in this descriptor. With regard to the Variable Length Extension field, each extension has the following format:
In accordance with one embodiment, a key ID extension and a data segment ID extension are defined as follows:
Key ID Extension
Extension Type: Must be set to 0x01 for Key ID Extension.
Extension Length: Must be set to 16, which represents 128 bits (16 bytes).
Extension: Must contain the Key ID value for the encrypted media delivered in conjunction with this descriptor. This extension is only used when the Encrypted Data flag is set.
Data Segment ID Extension
Extension Type: Must be set to 0x02 for Data Segment ID Extension.
Extension Length: Must be set to 8, which represents 64 bits (8 bytes).
Extension: Must contain the Data Segment ID for the encrypted media delivered in conjunction with this descriptor. This extension is only used when the Encrypted Data flag is set.
With regard to the Length section, in this embodiment, this section must contain the total length of the Data Segment descriptor in bytes. This length does not include the size of the media data delivered in conjunction with this descriptor.
Various embodiments described above utilize methods for protecting content, such as Digital Rights Management (DRM), to enable secure playback of content on machines and devices within a local network, such as a home media network. In at least some embodiments, messages and content are delivered over Real Time Streaming Protocol (RTSP) and Real Time Transport Protocol (RTP), and protocol extensions are introduced which enjoy advantages offered by RTSP/RTP, including data delivery over User Datagram Protocol (UDP) and bi-directional communication between client and server.
Although the invention has been described in language specific to structural features and/or methodological steps, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or steps described. Rather, the specific features and steps are disclosed as preferred forms of implementing the claimed invention.
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