WebM Stream Format

W3C Group Note

More details about this document
This version:
https://www.w3.org/TR/2024/NOTE-eme-stream-webm-20240718/
Latest published version:
https://www.w3.org/TR/eme-stream-webm/
Latest editor's draft:
https://w3c.github.io/encrypted-media/format-registry/stream/webm.html
History:
https://www.w3.org/standards/history/eme-stream-webm/
Commit history
Editors:
Joey Parrish (Google Inc.)
Greg Freedman (Netflix Inc.)
Former editors:
Mark Watson (Netflix Inc.) (Until September 2019)
David Dorwin (Google Inc.) (Until September 2017)
Jerry Smith (Microsoft Corporation) (Until September 2017)
Adrian Bateman (Microsoft Corporation) (Until May 2014)
Feedback:
GitHub w3c/encrypted-media (pull requests, new issue, open issues)
public-media-wg@w3.org with subject line [eme-stream-webm] … message topic … (archives)

Abstract

This specification defines the stream format for using WebM [WebM] content with the Encrypted Media Extensions [ENCRYPTED-MEDIA].

Status of This Document

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

This document was published by the Media Working Group as a Group Note using the Note track.

This Group Note is endorsed by the Media Working Group, but is not endorsed by W3C itself nor its Members.

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.

The W3C Patent Policy does not carry any licensing requirements or commitments on this document.

This document is governed by the 03 November 2023 W3C Process Document.

1. Stream Format

Encrypted WebM streams [WebM-Encryption] are encrypted at the block level with AES-128 CTR encryption. The container SHALL include appropriate values within the ContentEncryption [Matroska] element.

WebM streams may be partially encrypted, both at the Track level and the block level. In the former case, a subset of Tracks in the stream have a ContentEncryption element. In the latter case, a subset of the blocks within a Track containing a ContentEncryption element are marked as encrypted.

2. Detection

For a stream determined to be in the WebM format [WebM], when a Track [Matroska] is parsed, the presence of a ContentEncKeyID element indicates that blocks in the track may be encrypted.

3. Detecting Encrypted Blocks

For the purposes of the Encrypted Block Encountered algorithm, encrypted blocks are those marked encrypted by the Signal Byte [WebM-Encryption].

4. Initialization Data Extraction

Initialization Data is always a single key ID, as defined by the "webm" Initialization Data Format [EME-INITDATA-REGISTRY].

Each time a ContentEncKeyID [Matroska] element is encountered in a Track, the Initialization Data Encountered algorithm SHALL be invoked with initDataType = "webm" [EME-INITDATA-REGISTRY] and initData = the value in that element.

5. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key word SHALL in this document is to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

A. References

A.1 Normative references

[EME-INITDATA-REGISTRY]
Encrypted Media Extensions Initialization Data Format Registry. David Dorwin; Adrian Bateman; Mark Watson. W3C. 15 September 2016. W3C Working Group Note. URL: https://www.w3.org/TR/eme-initdata-registry/
[EME-INITDATA-WEBM]
"webm" Initialization Data Format. David Dorwin; Adrian Bateman; Mark Watson. W3C. 15 September 2016. W3C Working Group Note. URL: https://www.w3.org/TR/eme-initdata-webm/
[ENCRYPTED-MEDIA]
Encrypted Media Extensions. David Dorwin; Jerry Smith; Mark Watson; Adrian Bateman. W3C. 18 September 2017. W3C Recommendation. URL: https://www.w3.org/TR/encrypted-media/
[Matroska]
Matroska Specifications. Matroska. 9 January 2014. URL: https://matroska.org/technical/specs/index.html
[RFC2119]
Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc2119
[RFC8174]
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words. B. Leiba. IETF. May 2017. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc8174
[WebM]
WebM Container Guidelines. The WebM Project. 26 April 2016. URL: https://www.webmproject.org/docs/container/
[WebM-Encryption]
WebM Encryption. Frank Galligan. The WebM Project. 4 March 2015. URL: https://www.webmproject.org/docs/webm-encryption/