Encrypted Media Extensions

Abstract

This proposal extends HTMLMediaElement providing APIs to control playback of protected content.

The API supports use cases ranging from simple clear key decryption to high value video (given an appropriate user agent implementation). License/key exchange is controlled by the application, facilitating the development of robust playback applications supporting a range of content decryption and protection technologies.

This specification does not define a content protection or Digital Rights Management system. Rather, it defines a common API that may be used to discover, select and interact with such systems as well as with simpler content encryption systems. Implementation of Digital Rights Management is not required for compliance with this specification: only the simple clear key system is required to be implemented as a common baseline.

The common API supports a simple set of content encryption capabilities, leaving application functions such as authentication and authorization to page authors. This is achieved by requiring content protection system-specific messaging to be mediated by the page rather than assuming out-of-band communication between the encryption system and a license or other server.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

Implementers should be aware that this specification is not stable. Implementers who are not taking part in the discussions are likely to find the specification changing out from under them in incompatible ways. Vendors interested in implementing this specification before it eventually reaches the Candidate Recommendation stage should join the mailing list mentioned below and take part in the discussions.

This document was published by the HTML working group as an Editor's Draft. Please submit comments regarding this document by using the W3C's (public bug database) with the product set to HTML WG and the component set to Encrypted Media Extensions. If you cannot access the bug database, submit comments to public-html-media@w3.org (subscribe, archives) and arrangements will be made to transpose the comments to the bug database. All feedback is welcome.

Publication as an Editor's Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

Note: It is an open issue whether and how the spec should do more to encourage/ensure CDM-level interop. See Bug 20944.

Note: This specification contains sections for describing security and privacy considerations. These sections are not final and review is welcome.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

• 1. Introduction
• • 1.1. Definitions
• 2. Media Element Extensions
• • 2.1. Errors
  • 2.2. MediaKeySession States
  • 2.3. Media Element Restrictions
• 3. Events
• • 3.1 Event Definitions
  • 3.2 Event Summary
• 4. Algorithms
• • 4.1. Initialization Data Encountered
  • 4.2. Encrypted Block Encountered
  • 4.3. Queue a "message" Event
  • 4.4. Queue an "error" Event
  • 4.5. Session Close
• 5. Simple Decryption
• • 5.1. Clear Key
• 6. Security Considerations
• 7. Privacy Considerations
• 8. Examples
• 9. Revision History

1. Introduction

This section is non-normative.

This proposal allows JavaScript to select content protection mechanisms, control license/key exchange, and implement custom license management algorithms. It supports a wide range of use cases without requiring client-side modifications in each user agent for each use case. This also enables content providers to develop a single application solution for all devices. A generic stack implemented using the proposed APIs is shown below. This diagram shows an example flow: other combinations of API calls and events are possible.

1.1. Definitions

Text in this font and color is non-normative.

1.1.1. Content Decryption Module (CDM)

This section is non-normative.

The Content Decryption Module (CDM) is a generic term for a part of or add-on to the user agent that provides functionality for one or more Key Systems. Implementations may or may not separate the implementations of CDMs and may or may not treat them as separate from the user agent. This is transparent to the API and application. A user agent may support one or more CDMs.

1.1.2. Key System

A Key System is a generic term for a decryption mechanism and/or content protection provider. Key System strings provide unique identification of a Key System. They are used by the user agent to select the Content Decryption Modules and identify the source of a key-related event. Simple Decryption Key Systems are supported by all user agents. User agents may also provide additional CDMs with corresponding Key System strings.

A Key System string is always a reverse domain name. For example, "com.example.somesystem". Key System strings are compared using case-sensitive matching. It is recommended that CDMs use simple lower-case ASCII key system strings.

Within a given system ("somesystem" in the example), subsystems may be defined as determined by the key system provider. For example, "com.example.somesystem.1" and "com.example.somesystem.1_5". Key System providers should keep in mind that these will be used for comparison and discovery, so they should be easy to compare and the structure should remain reasonably simple.

1.1.3. Key Session

A Key Session, or simply Session, represents the lifetime of the license(s)/key(s) it contains and associates all messages related to them. Sessions are embodied as MediaKeySession objects. Each Key session is associated with a single instance of Initialization Data provided in the createSession() call.

Each Key Session is associated with a single MediaKeys object, and only media elements associated with that object may access key(s) associated with the session. Other MediaKeys objects, CDM instances, and media elements may not access the key session or use its key(s). Key sessions and the keys they contain are no longer usable by the CDM for decryption when the session is closed, including when the MediaKeySession object is destroyed.

1.1.4. Session ID

A Session ID is a unique string identifier generated by the user agent or CDM that can be used by the application to identify MediaKeySession objects. (The underlying content protection client or server do not necessarily need to support Session IDs.)

A new Session ID is generated each time the user agent successfully initializes a MediaKeySession object. It must be valid before the MediaKeySession enters the PENDING or READY states and the user agent fires the associated events.

Each Session ID shall be unique within the browsing context in which it was created. (Note: Some use cases may require that Session IDs be unique within the origin over time, including across browsing sessions.)

1.1.5. Initialization Data

This section is non-normative.

Initialization Data is a generic term for container-specific data that is used by Content Decryption Modules to generate a license request. It should always allow unique identification of the key(s) needed to decrypt the content.

Key Systems usually require a block of initialization data containing information about the stream to be decrypted before they can construct a license request message. This block could be a simple key or content ID or a more complex structure containing such information.

The format of the initialization data depends upon the type of container. Containers may support more than one format of initialization data. The initialization data type is a string that indicates what format the initialization data is provided in. Initialization data type strings are always matched case-sensitively. It is recommended that initialization data type strings are lower-case ASCII strings.

The Encrypted Media Extensions Stream Format and Initialization Data Format Registry provides the mapping from initialization data type string to the specification for each format.

This initialization information may be obtained in some application-specific way or provided with the media data. Initialization data found with the media data is provided to the application in the initData attribute of the needkey event.

1.1.6. Cross Origin Support

During playback, embedded media data is exposed to script in the embedding origin. In order for the API to fire needkey and message events, media data must be CORS-same-origin with the embedding page. If media data is cross-origin with the embedding document, authors should use the crossorigin attribute on the media element and CORS headers on the media data response to make it CORS-same-origin.

2. Media Element Extensions

We extend media element to allow decryption key acquisition to be handled in JavaScript.

enum MediaWaitingFor { "none", "data", "key" };

partial interface HTMLMediaElement {
  // Encrypted Media
  readonly attribute MediaKeys mediaKeys;
  void setMediaKeys(MediaKeys mediaKeys);
  
  attribute EventHandler onneedkey;

  readonly attribute MediaWaitingFor waitingFor;
};

[Constructor(DOMString keySystem)]
interface MediaKeys {
  readonly attribute DOMString keySystem;

  MediaKeySession createSession(DOMString initDataType, Uint8Array initData);
  MediaKeySession loadSession(DOMString sessionId);

  static bool isTypeSupported(DOMstring keySystem, optional DOMString contentType);
};

interface MediaKeySession : EventTarget {
  // error state
  readonly attribute MediaKeyError? error;

  // session properties
  readonly attribute DOMString keySystem;
  readonly attribute DOMString sessionId;

  // session operations
  void update(Uint8Array response);
  void release();
};

partial interface HTMLSourceElement {
  attribute DOMString keySystem;
};

The mediaKeys attribute is the MediaKeys being used when decrypting encrypted media data for this media element.

The setMediaKeys(mediaKeys method provides the MediaKeys to use. When calling this method, the media element must run the following steps:

1. If the MediaKeys object is already in use and the user agent is unable to re-use it with this element, throw a QUOTA_EXCEEDED_ERR exception and abort these steps.

2. Set the mediaKeys attribute of the media element to mediaKeys.

Note: As a best practice, applications should create a MediaKeys object and call setMediaKeys() before providing media data (for example, setting the src attribute of the media element). This avoids potential delays in some implementations.

Note: In some implementations, MediaKeySession objects created by createSession() may not fire any events until the MediaKeys object is associated with a media element with setMediaKeys().

The onneedkey event handler for the needkey event must be supported by all HTMLMediaElements as both a content attribute and an IDL attribute.

The waitingFor attribute indicates what the media element is waiting for, if anything (indicated by the waiting and canplay events). This is described in the Encrypted Block Encountered algorithm.

The MediaKeys(keySystem) constructor must run the following steps:

1. If keySystem is an empty string, throw an INVALID_ACCESS_ERR exception and abort these steps.

2. If keySystem is not one of the user agent's supported Key Systems, throw a NOT_SUPPORTED_ERR and abort these steps. Key system string comparison is case-sensitive.

3. Create a new MediaKeys object.

   1. Set the keySystem attribute to keySystem.

4. Schedule a task to execute the following steps:

   1. Let cdm be the content decryption module corresponding to keySystem.

   2. Load and initialize the cdm if necessary.

   3. If cdm fails to load or initialize, save the appropriate error name and system code internally with the MediaKeys instance being created. This will be used to fire an error against the first session created for this instance. If no system code is provided, use 0.

5. Return the new MediaKeys object to the caller.

The keySystem attribute identifies the Key System being used.

The createSession(initDataType, initData) method must run the following steps:

Note: The contents of initData are container-specific Initialization Data. initDataType is the initialization data type that indicates how to interpret initData.

Note: User agents and CDMs should not treat sessions created with audio data differently than those created with video data. That is, there is no such thing as an "audio session" or a "video session" - all sessions are used for all media streams processed by cdm.

1. If initDataType is an empty string, throw an INVALID_ACCESS_ERR exception and abort these steps.

2. If initData is null or an empty array, throw an INVALID_ACCESS_ERR exception and abort these steps.

3. If initDataType contains an invalid or unrecognized initialization data type, throw a NOT_SUPPORTED_ERR exception and abort these steps.

4. If the content decryption module corresponding to the keySystem attribute does not support the Initialization Data format initDataType, throw a NOT_SUPPORTED_ERR exception and abort these steps.

5. Let session be a new MediaKeySession object, and initialize it as follows:

   1. Set the error attribute to null.

   2. Set the keySystem attribute to the value of the MediaKeys object's keySystem attribute.

   3. Set the sessionId attribute to the empty string.

   4. Let the state be CREATED.

6. Schedule a task to process the initData, providing session, initDataType, and initData.

   The user agent will asynchronously execute the following steps in the task:

   1. Wait for the MediaKeys constructor task to complete.

   2. If error information is saved with the MediaKeys object because of an error during the MediaKeys constructor task:

      1. Run the Queue an "error" Event algorithm on the session, providing the error name and system code that were saved with the MediaKeys object.

      2. Clear the error information saved with the MediaKeys object.

      3. Abort the task.

   3. Let request be null.

   4. Let default URL be null.

   5. Let cdm be the cdm loaded in the MediaKeys constructor.

   6. Use the cdm to execute the following steps:

      1. Process the initData, interpreting it per initDataType.

      2. If a message exchange (e.g. a license request) is required:

         1. Let request be a request generated by the CDM using the initData.

            cdm must not use any stream-specific data, including media data, not provided via the initData.

            Note: request may be a request for multiple keys, depending on the keySystem and/or the initData. This is transparent to the application.

         2. If the initData indicates a default URL relevant to keySystem, let default URL be that URL.

   7. Set the session's sessionId attribute to a unique Session ID string. It may be obtained from cdm.

   8. If any of the preceding steps in the task failed, run the following steps:

      1. Run the Queue an "error" Event algorithm on the session, providing the appropriate error name and system code value, if provided, and 0 otherwise.

      2. Abort the task.

   9. If the associated media element(s) are waiting for a key, queue a task to attempt to resume playback.

      In other words, resume playback if the necessary key is provided.

      The user agent may choose to skip this step if it knows resuming will fail (i.e. no usable key was added).

   10. Follow the steps for the first matching condition from the following list:

       If request is not null

       Run the Queue a "message" Event algorithm on the session, providing request and default URL.

       Otherwise

       1. Let the state of the session be READY.

       2. Queue a task to fire a simple event named ready at the session.

          Note: this step makes it possible for a MediaKeySession to transition from the CREATED state to the READY state. User agents might do this as an optimization but, even if this is done, all MediaKeySession instances must appear distinct regardless of the underlying implementation.

7. Return the new object to the caller.

   Note: User agents should always fire an event at the returned object in response to this call. For some implementations, it is possible - especially the first time a Key System is used - that the first event will be delayed significantly while the client is initialized, the user responds to permission requests, etc. Applications should expect and handle such delays.

The loadSession(sessionId) method must run the following steps:

1. If the content decryption module corresponding to the keySystem attribute does not support loading previous sessions, throw a NOT_SUPPORTED_ERR exception and abort these steps.

2. If sessionId is an empty string, throw an INVALID_ACCESS_ERR exception and abort these steps.

3. Let session be a new MediaKeySession object, and initialize it as follows:

   1. Set the error attribute to null.

   2. Set the keySystem attribute to the value of the MediaKeys object's keySystem attribute.

   3. Set the sessionId attribute to the empty string.

   4. Let the state be CREATED.

4. Schedule a task to load the session, providing session and sessionId.

   The user agent will asynchronously execute the following steps in the task:

   1. Wait for the MediaKeys constructor task to complete.

   2. If error information is saved with the MediaKeys object because of an error during the MediaKeys constructor task:

      1. Run the Queue an "error" Event algorithm on the session, providing the error name and system code that were saved with the MediaKeys object.

      2. Clear the error information saved with the MediaKeys object.

      3. Abort the task.

   3. Let request be null.

   4. Let destination URL be null.

   5. Let origin be the origin of the MediaKeys object's Document.

   6. Let cdm be the cdm loaded in the MediaKeys constructor.

   7. Use the cdm to execute the following steps:

      1. If there is no data stored for the sessionId in the origin, run the following steps:

         1. Run the Queue an "error" Event algorithm on the session, "NotFoundError" and 0.

         2. Abort the task.

      2. Let session data be the data stored for the sessionId in the origin. This must not include data from other origin(s) or that is not associated with an origin.

      3. Load the session data and associate it with the session.

      4. If a message exchange is required:

         1. Let request be a request generated by the CDM based on the session data.

         2. If the session data indicates a destination URL for the request, let destination URL be that URL.

   8. Set the session's sessionId attribute to sessionId

   9. If any of the preceding steps in the task failed, run the following steps:

      1. Run the Queue an "error" Event algorithm on the session, providing the appropriate error name and system code value, if provided, and 0 otherwise.

      2. Abort the task.

   10. If the associated media element(s) are waiting for a key, queue a task to attempt to resume playback.

       In other words, resume playback if the necessary key is provided.

       The user agent may choose to skip this step if it knows resuming will fail (i.e. no usable key was added).

   11. Follow the steps for the first matching condition from the following list:

       If request is not null

       Run the Queue a "message" Event algorithm on the session, providing request and destination URL.

       Otherwise

       1. Let the state of the session be READY.

       2. Queue a task to fire a simple event named ready at the session.

5. Return the new object to the caller.

The isTypeSupported(keySystem, contentType) method returns whether keySystem is supported with the container and codec(s) specified by contentType.

It must run the following steps:

1. If keySystem is an empty string, return false and abort these steps.

2. If keySystem contains an unrecognized or unsupported Key System, return false and abort these steps. Key system string comparison is case-sensitive.

3. If contentType was not provided or is an empty string, return true and abort these steps.

4. If contentType contains an invalid or unrecognized MIME type, return false and abort these steps.

5. Issue 5

   isTypeSupported needs to be updated including using initDataType. This includes the discussion in Bug 24873.

   Let initDataFormat be the container type specified by contentType.

6. If the user agent does not support the Initialization Data format initDataFormat, return false and abort these steps.

7. If the CDM specified by keySystem does not support the Initialization Data format initDataFormat, return false and abort these steps.

8. If neither the CDM specified by keySystem nor the user agent support all codec(s) specified by contentType, return false and abort these steps.

9. Return true.

The error attribute is a MediaKeyError representing the current error state of the session. It is null if there is no error.

The keySystem attribute identifies the Key System of the MediaKeys that created the session.

The sessionId attribute is the Session ID for this object and the associated key(s) or license(s).

The update(response) method must run the following steps:

Note: The contents of response are keySystem-specific.

1. If response is null or an empty array, throw an INVALID_ACCESS_ERR exception and abort these steps.

2. If the session is not in the PENDING state, throw an INVALID_STATE_ERR.

3. Schedule a task to handle the call, providing response.

   The user agent will asynchronously execute the following steps in the task:

   1. Let cdm be the cdm loaded in the MediaKeys constructor.

   2. Let request be null.

   3. Use the cdm to execute the following steps:

      1. Process response.

         Note: When response contains key(s) and/or related data, cdm will likely cache the key and related data indexed by key ID.

         Note: The replacement algorithm within a session is Key System-dependent.

         Note: Keys from different sessions should be cached independently such that closing one session does not affect keys in other sessions, even if they have overlapping key IDs.

         Note: It is recommended that CDMs support a standard and reasonably high minimum number of keys per MediaKeySession object, including a standard replacement algorithm, and a standard and reasonably high minimum number of MediaKeySession objects. This enables a reasonable number of key rotation algorithms to be implemented across user agents and may reduce the likelihood of playback interruptions in use cases that involve various streams in the same element (i.e. adaptive streams, various audio and video tracks) using different keys.

      2. If another message needs to be sent to the server, let request be that message.

   4. If any of the preceding steps in the task failed, run the following steps:

      1. Run the Queue an "error" Event algorithm on this object, providing the appropriate error name and system code value, if provided, and 0 otherwise.

      2. Abort the task.

   5. If the associated media element(s) are waiting for a key, queue a task to attempt to resume playback.

      In other words, resume playback if the necessary key is provided.

      The user agent may choose to skip this step if it knows resuming will fail (i.e. no usable key was added).

   6. Follow the steps for the first matching condition from the following list:

      If request is not null

      Run the Queue a "message" Event algorithm on this object, providing request and null.

      Otherwise

      1. Let the state of this object be READY.

      2. Queue a task to fire a simple event named ready at this object.

The release() method allows an application to indicate to the system that it may release any resources associated with this object. It must run the following steps:

1. If the state of this object is CLOSED then abort these steps.

2. If the state of this object is ERROR, throw an INVALID_STATE_ERR exception and abort these steps.

3. Schedule a task to handle the call.

   The user agent will asynchronously execute the following steps in the task:

   1. Let cdm be the cdm loaded in the MediaKeys constructor.

   2. Use the cdm to execute the following steps:

      1. Process the release request.

         Note: the release() method is intended to act as a hint to the user agent that the application believes the session is no longer needed. However, the CDM determines whether resources can now be released.

      2. If the previous step caused the session to be closed, run the Session Close algorithm on this object.

The keySystem attribute of HTMLSourceElement specifies the Key System to be used with the media resource. The keySystem attribute must be supported by all HTMLSourceElement as both an IDL attribute and also a content attribute named keysystem. The resource selection algorithm is modified to check the keySystem attribute after the existing step 5 of the Otherwise branch of step 6:

6. ⌛ If candidate has a keySystem attribute whose value represents a Key System that the user agent knows it cannot use with type, then end the synchronous section, and jump down to the failed step below.

2.1. Errors

2.1.1. Interface

[Constructor(DOMString name, unsigned long systemCode, optional DOMString message = "")]
interface MediaKeyError : DOMError {
  readonly attribute unsigned long systemCode;
};

2.1.2. Error Names

The error names table below lists all the allowed error names for the name attribute along with a description. In addition, any of the standard error names are allowed.

2.2 MediaKeySession States

This section and its subsections are non-normative.

Each MediaKeySession maintains an internal state that determines what events may be fired.

2.2.1 MediaKeySession State Definitions

The following table describes the possible states:

2.2.2 MediaKeySession State Transitions

The following diagram shows the possible state transitions and the events fired when changing state:

2.3 Media Element Restrictions

This section is non-normative.

Media data processed by a CDM may not be available through Javascript APIs in the usual way (for example using the CanvasRenderingContext2D drawImage() method and the AudioContext MediaElementAudioSourceNode). This specification does not define conditions for such non-availability of media data, however, if media data is not available to Javascript APIs then these APIs may behave as if no media data was present at all.

Where media rendering is not performed by the UA, for example in the case of a hardware protected media pipeline, then the full set of HTML rendering capabilities, for example CSS Transforms, may not be available. One likely restriction is that video media may be constrained to appear only in rectangular regions with sides parallel to the edges of the window and with normal orientation.

3. Events

3.1. Event Definitions

[Constructor(DOMString type, optional MediaKeyNeededEventInit eventInitDict)]
interface MediaKeyNeededEvent : Event {
  readonly attribute DOMString initDataType;
  readonly attribute Uint8Array? initData;
};

dictionary MediaKeyNeededEventInit : EventInit {
  DOMString initDataType;
  Uint8Array? initData;
};

[Constructor(DOMString type, optional MediaKeyMessageEventInit eventInitDict)]
interface MediaKeyMessageEvent : Event {
  readonly attribute Uint8Array message;
  readonly attribute DOMString? destinationURL;
};

dictionary MediaKeyMessageEventInit : EventInit {
  Uint8Array message;
  DOMString? destinationURL;
};

event . initDataType

Returns a string indicating the initialization data type of the Initialization Data related to the event.

event . initData

Returns the Initialization Data related to the event.

event . message

Returns the message (i.e. license request) to send.

event . destinationURL

Returns the URL to send the message to.

3.2. Event Summary

This section is non-normative.

The following event is fired at HTMLMediaElement.

The following events are fired at MediaKeySession.

4. Algorithms

4.1. Initialization Data Encountered

The following steps are run when the media element encounters a source that may contain encrypted blocks or streams during the resource fetch algorithm:

1. Let initData be null.

2. If Initialization Data was encountered and if the media data is CORS-same-origin, let initData be that initialization data. Let initDataType be the string representing the initialization data type.

3. Queue a task to fire a simple event named needkey at the media element.

   The event is of type MediaKeyNeededEvent and has:

   • initDataType = initDataType
     initData = initData

   Firing this event allows the application to begin acquiring the key process before it is needed.

   Note that readyState is not changed and no algorithms are aborted. This event merely provides information.

   Note that if the media is not CORS-same-origin then the initData will be null. This allows applications that can retrieve initData from an alternative source to continue. Applications with no way to retrieve initData may wish to consider aborting playback in this case.

4. Continue Normal Flow: Continue with the existing media element's resource fetch algorithm.

4.2. Encrypted Block Encountered

The following steps are run when the media element encounters a block (i.e. frame) of encrypted media data during the resource fetch algorithm:

1. If the media element's mediaKeys attribute is not null, run the following steps:

   1. Let media keys be the MediaKeys object referenced by that atribute.

   2. Let cdm be the cdm loaded during the initialization of the media keys.

   3. If at least one MediaKeySession created by the media keys is in the PENDING or READY state, run the following steps:

      This check ensures the cdm has finished loading and is a prequisite for a matching key being available.

      1. Let the block key ID be the key ID of the current block (as specified by the container).

      2. Use the cdm to execute the following steps:

         1. Let available keys be the union of keys in sessions that were created by the media keys.

         2. Follow the steps for the first matching condition from the following list:

            In the following steps, a key is considered usable if it is valid as determined by the CDM. For example, a key is not usable if its license has expired.

            If any of the available keys corresponds to the block key ID and is usable

            Run the following steps:

            1. Let block key be the matching key.

               Note: If multiple sessions contain a usable key for the block key ID, which key to use is Key System-dependent.

            2. Use the cdm to decrypt the block using block key.

            3. Follow the steps for the first matching condition from the following list:

               If decryption fails

               Abort the media element's resource fetch algorithm, run the steps to report a MEDIA_ERR_DECODE error, and abort these steps.

               Otherwise

               Abort these steps and process the decrypted block as normal. (Decode the block.)

               Note: Not all decryption problems (i.e. using the wrong key) will result in a decryption failure. In such cases, no error is fired here but one may be fired during decode.

            If any of the available keys corresponds to the block key ID and is unusable

            Run the following steps:

            1. Let session be the MediaKeySession object associated with that session.

            2. Run the Queue an "error" Event algorithm on the session, providing the appropriate error name and system code value, if provided, and 0 otherwise.

            3. Abort these steps.

            Otherwise (there is no key for the block key ID in any session)

            Continue.

2. Abort these steps and wait for a signal to resume playback.

   There is no usable key for the block.

   If playback stops because the stream cannot be decrypted when the media element is potentially playing, the media element is said to be waiting for a key.

The following paragraph is added to Playing the media resource.

• A media element is said to be waiting for a key when it would be potentially playing but the user agent has reached a point in the media resource that must be decrypted for the resource to continue and the CDM does not have the necessary key.
• The media element leaves this state when seeking but could re-enter it if the same conditions exist.

• It is possible for a media element to be playing media and encounter content that requires decryption keys. To accommodate changes caused by key status, apply the first appropriate substeps from the following list:

  If media element was previously playing and had a waitingFor value of "none":

  If a readyState change queues a task to fire a waiting event, the user agent must also set the waitingFor attribute on the Media Element to "data".
  If decryption key(s) are needed to continue playback for any selected video track(s), enabled audio track(s), or "showing" or "hidden" text track(s), and the element has not ended playback, the user agent must set the waitingFor attribute on the Media Element to "key", queue a task to fire a simple event named timeupdate at the element, and queue a task to fire a simple event named waiting at the element.

  If media element was previously waiting and had a waitingFor value of "data":

  If a readyState change queues a task to fire a canplay event, the user agent must also set the waitingFor attribute on the Media Element to "none".

  If media element was previously waiting and had a waitingFor value of "key":

  If decryption key(s) become available that allow playback to proceed, the user agent must set the waitingFor attribute on the Media Element to "none", queue a task to fire a simple event named timeupdate at the element, and queue a task to fire a simple event named canplay at the element.

4.3. Queue a "message" Event

The Queue a "message" Event algorithm is run when the CDM needs to queue a message event to a MediaKeySession object. Requests to run this algorithm include a target MediaKeySession object, a request, and a destination URL.

The following steps are run:

1. Let the session be the specified MediaKeySession object.

2. Let the state of the session be PENDING.

3. Queue a task to fire a simple event named message at the session.

   The event is of type MediaKeyMessageEvent and has:

   • message = the specified request
     destinationURL = the specified destination URL

4.4. Queue an "error" Event

The Queue an "error" Event algorithm is run when the CDM needs to queue an error event to a MediaKeySession object. Requests to run this algorithm include a target MediaKeySession object, an error name, and a system code.

The following steps are run:

1. Let the session be the specified MediaKeySession object.

2. Create a new MediaKeyError object with the following attributes:

   • name = error name
     systemCode = system code

3. Set the session's error attribute to the error object created in the previous step.

4. Let the state of the session be ERROR.

5. Queue a task to fire a simple event named error at the session.

4.5. Session Close

The Session Close algorithm is run when the CDM closes the session associated with a MediaKeySession object.

The CDM may close a session at any point, such as in response to a release() call, when the session is no longer needed, or when resources are lost. Keys in other sessions should be unaffected, even if they have overlapping key IDs.

The following steps are run:

1. Let the session be the associated MediaKeySession object.

2. Let the state of the session be CLOSED.

3. Queue a task to fire a simple event named close at the session.

5. Simple Decryption

All user agents must support the simple decryption capabilities described in this section regardless of whether they support a more advanced CDM. This ensures that there is a common baseline level of protection that is guaranteed to be supported in all user agents, including those that are entirely open source. Thus, content providers that need only basic protection can build simple applications that will work on all platforms without needing to work with any content protection providers.

5.1. Clear Key

The "org.w3.clearkey" Key System indicates a plain-text clear (unencrypted) key will be used to decrypt the source. No additional client-side content protection is required. Use of this Key System is described below.

The keySystem parameter and keySystem attributes are always "org.w3.clearkey". The sessionId string is numerical.

The initData attribute of the needkey event and the initData parameter of createSession() are the same container-specific Initialization Data format and values. If supported, these values should provide some type of identification of the content or key that could be used to look up the key (since there is no defined logic for parsing it). For containers that support a simple key ID, it should be a binary array containing the raw key ID. For other containers, it may be some other opaque blob or null.

The MediaKeyMessageEvent generated by createSession() has:

• message = a container-specific value extracted from the initData parameter of createSession(). null if initData was null or a value could not be extracted.
  destinationURL = value of the default URL if present in the media data and null otherwise.

The response parameter of update() should be a JSON Web Key (JWK) representation of the symmetric key to be used for decryption, as defined in the IETF Internet-draft JSON Web Key (JWK) specification. The JSON string is encoded into the Uint8Array parameter using ASCII-compatible character encoding.

When the JWK 'key type' ("kty") member value is 'octet sequence' ("oct"), the 'key value' ("k") member will be a base64 encoding of the octet sequence containing the symmetric key value.

For example, the following contains a single symmetric key represented as a JWK, designated as being for use with the AES Key Wrap algorithm (line breaks for readability, only).

{
  "keys": 
    [{
      "kty":"oct",
      "alg":"A128KW",
      "kid":"67ef0gd8pvfd0=",
      "k":"GawgguFyGrWKav7AX4VKUg"
    }]
}

6. Security Considerations

7. Privacy Considerations

8. Examples

This section and its subsections are non-normative.

This section contains example solutions for various use cases using the proposed extensions. These are not the only solutions to these use cases. Video elements are used in the examples, but the same would apply to all media elements. In some cases, such as using synchronous XHR, the examples are simplified to keep the focus on the extensions.

8.1. Source and Key Known at Page Load (Clear Key)

In this simple example, the source file and clear-text key are hard-coded in the page.

This example is very simple because it does not care when the key has been added or associating that event with the update() call. It also does not handle errors.

<script>
  function load() {
    var video = document.getElementById("video");

    if (!video.mediaKeys)
      video.setMediaKeys(new MediaKeys("org.w3.clearkey"));
    if (!video.mediaKeys)
      throw "Could not create MediaKeys";

    var keySession = video.mediaKeys.createSession();
    if (!keySession)
      throw "Could not create key session";

    keySession.addEventListener("message", handleMessage, false);
  }

  function handleMessage(event) {
    var keySession = event.target;

    var key = new Uint8Array([ ... ]);
    keySession.update(key);
  }
</script>

<body onload="load()">
  <video src="foo.webm" autoplay id="video"></video>
</body>

8.2. Source Known but Key Not Known at Page Load

In this case, the Initialization Data is contained in the media data. If this was not the case, handleKeyNeeded() could obtain and provide it instead of getting it from the event.

8.2.1. Clear Key

This solution uses the Clear Key Simple Decryption.

As with the previous example, this one is very simple because it does not care when the key has been added or handle errors.

<script>
  function handleKeyNeeded(event) {
    var video = event.target;

    if (!video.mediaKeys)
      video.setMediaKeys(new MediaKeys("org.w3.clearkey"));
    if (!video.mediaKeys)
      throw "Could not create MediaKeys";

    var keySession = video.mediaKeys.createSession(event.initDataType, event.initData);
    if (!keySession)
      throw "Could not create key session";

    keySession.addEventListener("message", handleMessage, false);
  }

  function handleMessage(event) {
    var keySession = event.target;
    var message = event.message;

    var xmlhttp = new XMLHttpRequest();
    xmlhttp.open("POST", "http://.../getkey");
    xmlhttp.onreadystatechange = function() {
      if(xmlhttp.readyState==4) {
        var key = new Uint8Array(xmlhttp.response);
        keySession.update(key);
      }
    }
    xmlhttp.send(message);
  }
</script>

<video src="foo.webm" autoplay onneedkey="handleKeyNeeded(event)"></video>

8.2.2. Other Key System

This solution uses more advanced decryption from a fictitious content decryption module called Some System.

<script>
  function handleKeyNeeded(event) {
    var video = event.target;

    if (!video.mediaKeys)
      video.setMediaKeys(new MediaKeys("com.example.somesystem.1_0"));
    if (!video.mediaKeys)
      throw "Could not create MediaKeys";

    var keySession = video.mediaKeys.createSession(event.initDataType, event.initData);
    if (!keySession)
      throw "Could not create key session";

    keySession.addEventListener("message", licenseRequestReady, false);
  }

  function licenseRequestReady(event) {
    var keySession = event.target;
    var request = event.message;
    if (!request)
      throw "Could not create license request";

    var xmlhttp = new XMLHttpRequest();
    xmlhttp.open("POST", "http://.../getkey");
    xmlhttp.onreadystatechange = function() {
      if(xmlhttp.readyState==4) {
        var license = new Uint8Array(xmlhttp.response);
        keySession.update(license);
      }
    }
    xmlhttp.send(request);
  }
</script>

<video src="foo.webm" autoplay onneedkey="handleKeyNeeded(event)"></video>

8.3. Selecting a Supported Key System

Below is an example of detecting supported Key System using the isTypeSupported() and selecting one.

<script>
  var keySystem;
  var licenseUrl;

  function selectKeySystem() {
    if (MediaKeys.isTypeSupported("com.example.somesystem", "video/webm; codecs='vp8, vorbis'")) {
      licenseUrl = "https://license.example.com/getkey"; // OR "https://example.<My Video Site domain>"
      keySystem = "com.example.somesystem";
    } else if (MediaKeys.isTypeSupported("com.foobar", "video/webm; codecs='vp8, vorbis'")) {
      licenseUrl = "https://license.foobar.com/request";
      keySystem = "com.foobar";
    } else {
      throw "Key System not supported";
    }
  }

  function handleKeyNeeded(event) {
    var video = event.target;

    if (!video.mediaKeys) {
      selectKeySystem();
      video.setMediaKeys(new MediaKeys(keySystem));
    }
    if (!video.mediaKeys)
      throw "Could not create MediaKeys";

    var keySession = video.mediaKeys.createSession(event.initDataType, event.initData);
    if (!keySession)
      throw "Could not create key session";

    keySession.addEventListener("message", licenseRequestReady, false);
  }
  
  function licenseRequestReady(event) {
    var keySession = event.target;
    var request = event.message;
    if (!request)
      throw "Could not create license request";

    var xmlhttp = new XMLHttpRequest();
    xmlhttp.open("POST", licenseUrl);
    xmlhttp.onreadystatechange = function() {
      if(xmlhttp.readyState==4) {
        var license = new Uint8Array(xmlhttp.response);
        keySession.update(license);
      }
    }
    xmlhttp.send(request);
  }
</script>

<video src="foo.webm" autoplay onneedkey="handleKeyNeeded(event)"></video>

8.4. Using All Events

This is a more complete example showing all events being used.

Note that handleMessage() could be called multiple times, including in response to the update() call if multiple round trips are required and for any other reason the Key System might need to send a message.

<script>
  var keySystem;
  var licenseUrl;

  function handleMessageResponse() {
    var license = new Uint8Array(xmlhttp.response);
    this.keySession.update(license);
  }
  
  function sendMessage(message, keySession) {
    xmlhttp = new XMLHttpRequest();
    xmlhttp.keySession = keySession;
    xmlhttp.onreadystatechange = handleMessageResponse;
    xmlhttp.open("POST", licenseUrl);
    xmlhttp.send(message);
  }

  function handleMessage(event) {
    var keySession = event.target;
    var message = event.message;
    if (!message)
      throw "Invalid key message";
  
    sendMessage(message, keySession);
  }

  function handleReady(event) {
    // Do some bookkeeping with event.target.sessionId if necessary.
  }

  function handleError(event) {
    // Report event.target.error.name and event.target.error.systemCode,
    // and do some bookkeeping with event.target.sessionId if necessary.
  }

  function handleKeyNeeded(event) {
    var video = event.target;

    if (!video.mediaKeys) {
      selectKeySystem();  // See previous example for implementation.
      video.setMediaKeys(new MediaKeys(keySystem));
    }
    if (!video.mediaKeys)
      throw "Could not create MediaKeys";

    var keySession = video.mediaKeys.createSession(event.initDataType, event.initData);
    if (!keySession)
      throw "Could not create key session";

    keySession.addEventListener("message", handleMessage, false);
    keySession.addEventListener("ready", handleReady, false);
    keySession.addEventListener("error", handleError, false);
  }
</script>

<video src="foo.webm" autoplay onneedkey="handleKeyNeeded(event)"></video>

9. Revision History