WebM Stream Format

W3C Editor's Draft

This version:
Latest published version:
Latest editor's draft:
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)


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. 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 https://www.w3.org/TR/.

This document was published by the Media Working Group as an Editor's Draft.

Comments regarding this document are welcome. Please send them to public-media-wg@w3.org (archives).

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.

This document was produced by a group operating under the 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.

This document is governed by the 1 March 2019 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, 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.

A. References

A.1 Normative references

Encrypted Media Extensions Initialization Data Format Registry. David Dorwin; Adrian Bateman; Mark Watson. W3C. URL: ../initdata/index.html
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 Specifications. Matroska. 9 January 2014. URL: https://matroska.org/technical/specs/index.html
WebM Container Guidelines. The WebM Project. 26 April 2016. URL: https://www.webmproject.org/docs/container/
WebM Encryption. Frank Galligan. The WebM Project. 4 March 2015. URL: https://www.webmproject.org/docs/webm-encryption/