PDF - Apple Developer

MPEG-2 Stream
Encryption Format for
HTTP Live Streaming

Contents
1.0 Introduction 4
2.0 Encryption 5
2.1 Encryption Overview 5
2.2 H.264 Video Streams 5
2.3 Audio Streams 6
2.3.1 General 6
2.3.2 AAC Audio 6
2.3.3 AC-3 Audio 7
2.3.4 Audio Setup Information 8
2.4 Other Stream Types 9
3.0 Transport Stream Signaling 10
4.0 References 11
ISO/IEC 13818-1 11
HTTP Live Streaming 11
ISO/IEC 14496-10 11
ISO/IEC 14496-3 11
ETSI TS 102 366 v1.2.1 11
ID3 tag version 2.4.0 11
NIST Special Publication 800-38A 12
Document Revision History 13
2012-09-26 | © 2012 Apple Inc. All Rights Reserved.
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Listings
2.0 Encryption 5
Listing 1-1 Encryption of NAL Units 5
Listing 1-2 Encryption of AAC Audio Frames 6
Listing 1-3 Encryption of AC-3 Audio Frames 7
Listing 1-4 Setup Information Format 8
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1.0 Introduction
This document describes a sample-level encryption format for several types of elementary streams that can
be carried in MPEG-2 Transport Streams “ISO/IEC 13818-1” (page 11) and MPEG Elementary Audio streams.
This format is used by the HTTP Live Streaming “HTTP Live Streaming” (page 11) protocol.
This document specifies:
● How the media data of audio and video streams are encrypted.
● How encrypted streams are identified in the transport stream.
● How unencrypted audio setup information is carried.
2012-09-26 | © 2012 Apple Inc. All Rights Reserved.
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2.0 Encryption
2.1 Encryption Overview
For each encrypted stream type a protected block is identified, over which the protection process is performed.
An audio stream protected block is typically a frame of audio; H.264 video protected blocks are the body of
specific types of Network Adaptation Layer (NAL) Units. The encryption method defined by this specification
protects certain contiguous sections of the audio or video stream within the protected blocks.
Each section contains an integer number of 16-byte blocks that are encrypted using AES-128 Cipher Block
Chaining (CBC) mode as specified in “NIST Special Publication 800-38A” (page 12). Cipher block chaining occurs
within each protected block, and the initialization vector must be reset to its original value at the start of each
new protected block.
In video data, the first 16-byte block of the section and every tenth block thereafter must be encrypted.
In audio data, all the 16-byte blocks must be encrypted.
2.2 H.264 Video Streams
H.264 (AVC) video encoding “ISO/IEC 14496-10” (page 11) must be used for video when this specification is
in operation. Stream encryption is performed within each NAL unit, in byte-stream form using start codes, as
detailed in Annex B of “ISO/IEC 14496-10” (page 11).
NAL units of type 1 and type 5 must be encrypted to this specification; other NAL unit types must not be
encrypted. Listing 1-1 shows the format of a NAL unit that contains encrypted data.
Listing 1-1 Encryption of NAL Units
Encrypted_NAL_Unit () {
NAL_unit_type_byte // 1 byte
unencrypted_leader // 31 bytes
while (bytes_remaining() > 16) {
}
protected_block_one_in_ten // 16 bytes
unencrypted_trailer // 1-16 bytes
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2.0 Encryption
2.3 Audio Streams
}
Each NAL unit is formed with start code emulation prevention applied. The preceding start code is not part of
the protected block and is not encrypted.
The byte containing the nal_unit_type value, plus the 31 bytes that follow, are unencrypted. The next
contiguous data section is protected. The size, in bytes, of the protected section must be a multiple of 16 and
may be 0; therefore if a NAL unit has 48 or fewer bytes, that NAL unit is completely unencrypted.
The protected section uses 10% skip encryption. Each 16-byte block of encrypted data is followed by nine
16-byte blocks of unencrypted data. At the end of the NAL unit, there are between 1 and 16 unencrypted
trailing bytes, inclusive. If any block is encrypted (because the NAL Unit’s length is 48 bytes or more), start code
emulation prevention must again be applied over the entire NAL Unit, including the unencrypted sections.
To encrypt an H.264 stream, first start with a byte stream that has had start code emulation prevention applied.
NAL types 1 and 5 that have a length greater than 48 bytes must be protected as defined above, and then for
those NAL Units only, start code emulation prevention must be re-applied over the entire NAL Unit.
To decrypt an H.264 stream, NAL units of type 1 and type 5 must be identified and unprotected. For each NAL
unit of either type, start code emulation prevention must be removed unless the NAL Unit’s length is 48 bytes
or less. Then the NAL Unit’s encrypted section must be located and the data in that section must be decrypted.
(The resulting bitstream can then be processed by a standard H.264 decoder.)
2.3 Audio Streams
2.3.1 General
The encryption technology defined by this specification supports two audio formats: Advanced Audio Coding
(AAC) “ISO/IEC 14496-3” (page 11) and AC-3 audio (formerly Dolby Digital) “ETSI TS 102 366 v1.2.1” (page 11).
2.3.2 AAC Audio
An AAC protected block is an audio frame that includes an Audio Data Transport Stream (ADTS) header, as
shown in Listing 1-2.
Listing 1-2 Encryption of AAC Audio Frames
Encrypted_AAC_Frame () {
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2.0 Encryption
2.3 Audio Streams
}
ADTS_Header // 7 or 9 bytes
unencrypted_leader // 16 bytes
while (bytes_remaining() >= 16) {
}
protected_block // 16 bytes
unencrypted_trailer // 0-15 bytes
The ADTS header, which can be 7 or 9 bytes long, plus the first 16 bytes of the frame after it, are unencrypted.
The contiguous data section that follows is encrypted. The size, in bytes, of the encrypted section must be an
integer multiple of 16 and is possibly zero. The AAC frame ends with 0 to 15 unencrypted bytes. Start code
emulation prevention is not performed on the encrypted frame.
2.3.3 AC-3 Audio
An AC-3 protected block is the full audio frame (a syncframe() as defined in “ETSI TS 102 366 v1.2.1” (page
11)), as shown in Listing 1-3.
Listing 1-3 Encryption of AC-3 Audio Frames
Encrypted_AC3_Frame () {
}
unencrypted_leader // 16 bytes
while (bytes_remaining() >= 16) {
}
protected_block // 16 bytes
unencrypted_trailer // 0-15 bytes
The first 16 bytes, starting with the syncframe() header, are not encrypted. The contiguous data section that
follows is encrypted. The AC-3 frame ends with 0 to 15 unencrypted bytes. Start code emulation prevention
is not performed on the encrypted part of the frame.
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2.0 Encryption
2.3 Audio Streams
2.3.4 Audio Setup Information
2.3.4.1 Introduction
Unencrypted audio setup information must be supplied when a stream is encrypted in conformance with this
specification. The big-endian setup information format is shown in Listing 1-4.
Listing 1-4 Setup Information Format
audio_setup_information() {
}
audio_type // 4 bytes
priming // 2 bytes
version // 1 byte
setup_data_length // 1 byte
setup_data // setup_data_length
The first field is a 32-bit format identifier, followed by a 16-bit priming field and an 8-bit version field. This is
followed by format-specific data: first an 8-bit value containing the length, in bytes, of the format-specific data
and then the format-specific data itself in an array of bytes. The setup information must be packed, with no
alignment padding. The size of the setup information is 8 bytes plus the size of the format-specific data.
The field’s values are:
● audio_type—as defined in the following sections; identifies the type of setup data carried.
● priming—set to 0x0000 for AC-3. For AAC retrieve this value from the encoder, using the Apple encoding
API. If a non-Apple encoder is used and does not provide a priming value, set to 0x0000. (This may lead
to incorrect audio/video synchronization if the encoder has a different priming value than the value
provided to the AAC decoder when the content is rendered.)
● version—set to 0x01.
● setup_data_length—the number of bytes in the following setup data.
● setup_data—format-specific information, as defined in the following sections.
2.4.3.2 AAC Setup
Format identifiers:
AAC-LC
‘zaac’
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2.0 Encryption
2.4 Other Stream Types
AAC-HEv1
AAC-HEv2
‘zach’
‘zacp’
The AAC format-specific setup information is the AudioSpecificConfig() value, as defined in Section
1.6.2.1 of “ISO/IEC 14496-3” (page 11). (Note that this value is called DecoderSpecificInfo in MPEG-4).
2.4.3.3 AC-3 Setup
Format identifier: ‘zac3’
The AC-3 format-specific setup information is the first 10 bytes of the audio data (the syncframe()). This
comprises the syncinfo() structure and the initial part of the bsi() structure, as defined in 5.3.1 and 5.3.2
of “ETSI TS 102 366 v1.2.1” (page 11).
2.4.3.4 Elementary Stream Setup
Format identifier: ‘PRIV’
In elementary streams the audio setup information is carried inside an ID3 Private Frame, as defined in “ID3
tag version 2.4.0” (page 11). The owner identifier is com.apple.streaming.audioDescription.
2.4.3.5 Transport Stream Setup
Format identifier: ‘apad’
In transport streams, the audio setup information is carried in a registration_descriptor(), as defined
in “ISO/IEC 13818-1” (page 11), sections 2.6.8 and 2.6.9 and Table 2-45.
2.4 Other Stream Types
Stream types other than audio or video are not encrypted.
2012-09-26 | © 2012 Apple Inc. All Rights Reserved.
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3.0 Transport Stream Signaling
Transport stream encryption must be signaled in the stream’s Program Map Table (PMT). The PMT must fit
into a single transport stream packet; it must not be fragmented. The PMT is defined in “ISO/IEC 13818-1” (page
11).
To avoid colliding with existing names, new stream_type values have been defined for each stream type
covered by this specification:
H.264 Video
AAC Audio
AC-3 Audio
The stream_type value in the PMT must be set to 0xdb. In the descriptor loop following
the ES_info_length field, a private data indicator descriptor must be added with its
big-endian private_data_indicator value set to 'zavc'.
The stream_type value in the PMT must be set to 0xcf. In the descriptor loop following
the ES_info_length field, a private data indicator descriptor must be added with its
big-endian private_data_indicator set to 'aacd'. The AAC setup information listed
in “2.3.4 Audio Setup Information” (page 8)) must be supplied in a Registration
Descriptor.
The stream_type value in the PMT must be set to 0xc1. In the descriptor loop following
the ES_info_length field, a private data indicator descriptor must be added with its
big-endian private_data_indicator set to 'ac3d'. The AC-3 setup information listed
in “2.3.4 Audio Setup Information” (page 8)) must be supplied in a Registration
Descriptor.
The private data indicator descriptors listed above must be signaled in the descriptor loop following the
ES_info_length field of the PMT, as defined in “ISO/IEC 13818-1” (page 11), section 2.6.28, table 2-58.
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4.0 References
ISO/IEC 13818-1
ISO/IEC 13818-1:2007 Information technology – Generic coding of moving pictures and associated audio infor-
mation: Systems
HTTP Live Streaming
IETF Internet Draft draft-pantos-http-live-streaming “HTTP Live Streaming”
ISO/IEC 14496-10
ISO/IEC 14496-10:2012, Information technology – Coding of audio-visual objects – Part 10: Advanced Video
Coding
ISO/IEC 14496-3
ISO/IEC 14496-3:2009 Information technology – Coding of audio-visual objects – Part 3: Audio
ETSI TS 102 366 v1.2.1
ETSI TS 102 366 v1.2.1 – Digital Audio Compression (AC-3, Enhanced AC-3) Standard
ID3 tag version 2.4.0
ID3 tag version 2.4.0 — Native Frames
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4.0 References
NIST Special Publication 800-38A
NIST Special Publication 800-38A
Recommendation of Block Cipher Modes of Operation, NIST, NIST Special Publication 800-38A
2012-09-26 | © 2012 Apple Inc. All Rights Reserved.
12

Document Revision History
This table describes the changes to MPEG-2 Stream Encryption Format for HTTP Live Streaming .
Date
2012-09-26
Notes
New document describing encryption format into MPEG-2 transport
streams as used in HTTP Live Streaming.
2012-09-26 | © 2012 Apple Inc. All Rights Reserved.
13

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