This specification defines the stream format for using ISO Base Media
File Format [ISOBMFF] content that uses the ISO Common Encryption
protection schemes [CENC] with the Encrypted Media Extensions
[ENCRYPTED-MEDIA].
Note
Although the ISO Base Media File Format [ISOBMFF] associated with
this format's MIME type/subtype strings supports multiple protection
schemes, when used with Encrypted Media Extensions, these strings refer
specifically to content encrypted and packaged using the 'cenc' or
'cbcs' protection schemes, as defined by section 4.2 of [CENC].
Status of This Document
This section describes the status of this
document at the time of its publication. 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.
Publication as an Editor's Draft does not
imply endorsement by W3C and its Members.
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.
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ISO Base Media File Format [ISOBMFF] content that is encrypted using
the ISO Common Encryption protection schemes [CENC] SHALL be
encrypted at the sample level with either the 'cenc' (AES-128 CTR) or
'cbcs' (AES-128 CBC) encryption schemes, as defined in section 4.2 of
[CENC]. These protection methods enable multiple Key Systems to
decrypt the same media content.
Each key is identified by a key ID and each encrypted
sample is associated with the key ID of the key needed to decrypt it.
This association is signaled either through the specification of a
default key ID in the track encryption box ('tenc') or by assigning the
sample to a Sample Group, the definition of which specifies a key ID.
Streams may contain a mixture of encrypted and unencrypted samples.
2. Detection
For a stream determined to be in the ISO Base Media File Format
[ISOBMFF], the ISO Common Encryption protection schemes may be
detected as follows.
Protection scheme signaling conforms with [ISOBMFF]. When protection
has been applied, the stream type will be transformed to 'encv' for
video or 'enca' for audio, with a Protection Scheme Information Box
('sinf') added to the sample entry in the Sample Description Box
('stsd'). The Protection Scheme Information Box ('sinf') will contain a
Scheme Type Box ('schm') with a scheme_type field set to a value of
'cenc' or 'cbcs' [CENC].
The encrypted block is a sample. Determining whether a sample is
encrypted depends on the corresponding Track Encryption Box ('tenc') and
the sample group with grouping type 'seig' (CencSampleEncryption group),
if any, associated with the sample. The default encryption state of a
sample is defined by the IsEncrypted flag in the associated track
encryption box ('tenc'). This default state may be modified by the
IsEncrypted flag in the SampleGroupDescriptionBox ('sgpd'), pointed to
by an index in the SampleToGroupBox ('sbgp').
Samples can be partially encrypted, specified by subsample information
referenced by SampleAuxiliaryInformationSizesBox ('saiz') and
SampleAuxiliaryInformationOffsetsBox ('saio') boxes.
Streams may contain one or more Protection System Specific Header
('pssh') boxes [CENC], each for a unique SystemID, at each location
where a 'pssh' box is necessary. Content using this stream format
SHOULD include a box containing the
Common SystemID and PSSH Box Format.
Each time one or more 'pssh' boxes are encountered, the
Initialization Data Enountered algorithm SHALL be invoked
with initDataType = "cenc"
[EME-INITDATA-REGISTRY] and initData = the 'pssh'
box(es). Multiple 'pssh' boxes MUST be provided together if and only if
they appear directly next to each other in the stream.
5. Conformance
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words MUST, SHALL, and SHOULD in this document
are to be interpreted as described in
BCP 14
[RFC2119] [RFC8174]
when, and only when, they appear in all capitals, as shown here.