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The process of encrypting and decrypting MPEG-2 elementary stream (ES) content follows the procedure that is defined here. The data segment corresponds to a single MPEG-2 ES media access unit (MAU) as defined here.
Optionally, the transmitter MAY also leave portions of the data in the clear to facilitate the analysis of the content by the receiver. For a portion to be left in the clear, it MUST contain one of the following stream marks.
|
Stream mark |
Start code |
Byte sequence |
Maximum data payload length |
|---|---|---|---|
|
Sequence header |
B3 |
00 00 01 B3 |
12 bytes |
|
GOP header |
B8 |
00 00 01 B8 |
8 bytes |
|
Picture header |
00 |
00 00 01 00 |
6 bytes |
|
Private data |
B2 |
00 00 01 B2 |
107 bytes |
The amount of data to be left in the clear corresponds, in the table above, to the length of the stream mark plus the maximum data payload length. Notice, however, that the clear section MAY start prior to the stream mark and end after the combined length of the stream mark and maximum data payload length. This MAY occur as long as the combined length does not exceed 368 bytes (equivalent to the length of two consecutive MPEG-2 TS packet payloads).
For example, a transmitter MAY leave between 16 and 368 bytes in the clear for a stream mark that denotes a sequence header (4 bytes for the stream mark plus 12 bytes for the maximum data payload length).
It is also possible to have some amount of data from the previous MAU left in the clear, in case the stream mark appears near the beginning of the current MAU. That is allowed, as long as the length of the clear section does not exceed 368 bytes, as previously indicated.