This specification defines the stream format and initialization data for using WebM with the Encrypted Media Extensions.

The Working Group maintains a list of all bug reports that the editors have not yet tried to address.

Implementors should be aware that this specification is not stable. Implementors who are not taking part in the discussions are likely to find the specification changing out from under them in incompatible ways. Vendors interested in implementing this specification before it eventually reaches the Candidate Recommendation stage should join the mailing list mentioned below and take part in the discussions.

Stream Format

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.

Detecting Encryption

When a WebM Track is parsed, the presence of a ContentEncKeyID element indicates that blocks in the track may be encrypted.

Encrypted blocks are those marked encrypted by the Signal Byte.

Initialization Data and Events

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.