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[Bug 17673] Define Initialization Data for implementations that choose to support the ISO Base Media File Format

From: <bugzilla@jessica.w3.org>
Date: Tue, 02 Oct 2012 14:22:55 +0000
Message-Id: <E1TJ3N9-0002PU-VB@jessica.w3.org>
To: public-html-bugzilla@w3.org

--- Comment #4 from johnsim@microsoft.com 2012-10-02 14:22:55 UTC ---
7.2 ISOBMFF Media using the “cenc” protection scheme
This section defines the stream format and initialization data for
implementations that choose to support Common Encryption (“cenc”) protected ISO
Base Media File Format (ISOBMFF) content.

7.2.1 Stream format
Under the “cenc” protection scheme, ISOBMFF content is encrypted at the sample
level with AES-128 CTR encryption, according to ISO/IEC 23001-7:2012,
“Information technology - MPEG system technologies - Part 7: Common encryption
in ISO base media file format files”. This protection method enables multiple
Key Systems to decrypt the same media content.

7.2.2 Detecting Encryption
Protection scheme signaling conforms with ISO/IEC 14496-12. When protection has
been applied, the stream type will be transformed to ‘encv’ for video or ‘enca’
for audio, with a scheme information box (‘sinf’) added to the sample entry in
the sample description box (‘stsd’). The scheme information box (‘sinf’) will
contain a scheme type box (‘schm’) with a scheme_type field set to a value of
“cenc” (Common Encryption).

With ISOBMFF common encryption, the “encrypted block” is a sample. Determining
whether a sample is encrypted depends on the corresponding track encryption box
(‘tenc’) and the sample group 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 Sample Group Description Box (‘sgpd’),
pointed to by an index in the Sample to Group Box (‘sbgp’).

For complete information see ISO/IEC 23001-7:2012. 

7.2.3 Initialization Data and Events
Under the “cenc” protection scheme, ISOBFF media content can be decrypted by
multiple Key Systems. The file may contain protection system specific header
('pssh') boxes for one or more Key Systems, each containing a SystemID
identifying the Key System. These multiple ‘pssh’ boxes are concatenated into a
single Initialization Data and returned with the needkey event. 

In a file encrypted with Common Encryption, 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. Common
Encryption files may contain a mixture of encrypted and unencrypted samples.
Playback of unencrypted samples should not be impeded by unavailability of the
keys needed to decrypt other samples in the same file or track.

Note that if there is already an active Key System CDM and the key storage for
that Key System already contains the key associated with the Key ID, there is
no need to generate a needkey event.

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Received on Tuesday, 2 October 2012 14:23:00 UTC

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