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This specification defines the stream format for using WebM [WebM] content with the Encrypted Media Extensions [ENCRYPTED-MEDIA].
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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.
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.
For the purposes of the Encrypted Block Encountered, encrypted blocks are those marked encrypted by the Signal Byte. [WebM-Encryption]
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.