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

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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

David Dorwin; Adrian Bateman; Mark Watson. W3C. Encrypted Media Extensions Initialization Data Format Registry. URL: https://www.w3.org/TR/eme-initdata-registry/
David Dorwin; Jerry Smith; Mark Watson; Adrian Bateman. W3C. Encrypted Media Extensions. 5 July 2016. W3C Candidate Recommendation. URL: https://www.w3.org/TR/encrypted-media/
Matroska. Matroska Specifications. 9 January 2014. URL: https://matroska.org/technical/specs/index.html
The WebM Project. WebM Container Guidelines. 26 April 2016. URL: https://www.webmproject.org/docs/container/
Frank Galligan. The WebM Project. WebM Encryption. 4 March 2015. URL: https://www.webmproject.org/docs/webm-encryption/