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This specification defines the stream format and initialization data for using WebM with the Encrypted Media Extensions.
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Encrypted WebM streams are encrypted at the block level with AES-128 CTR encryption. The container shall include appropriate values within the ContentEncryption 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.
When a WebM Track 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.
Initialization Data is always a single key ID, as defined by the ContentEncKeyID element of the Track.
Each time a ContentEncKeyID element is encountered, the Initialization Data encountered algorithm shall be invoked with initDataType = "webm" and initData = the value in that element.