[FFmpeg-devel] [RFC] avcodec/avcodec.h: Add encryption info side data

[Michael Niedermayer]

On Mon, Dec 18, 2017 at 10:02:52PM +0100, wm4 wrote:
> On Mon, 18 Dec 2017 21:38:24 +0100
> Michael Niedermayer <michael at niedermayer.cc> wrote:
> 
> > On Mon, Dec 18, 2017 at 04:56:08PM -0300, James Almer wrote:
> > > On 12/18/2017 4:52 PM, wm4 wrote:  
> > > > On Fri, 15 Dec 2017 14:24:17 -0800
> > > > Jacob Trimble <modmaker-at-google.com at ffmpeg.org> wrote:
> > > >   
> > > >> From a1b2cbcb7da4da69685f8f1299b70b672ce448e3 Mon Sep 17 00:00:00 2001
> > > >> From: Jacob Trimble <modmaker at google.com>
> > > >> Date: Tue, 5 Dec 2017 14:52:22 -0800
> > > >> Subject: [PATCH] avcodec/avcodec.h: Add encryption info side data.
> > > >>
> > > >> This new side-data will contain info on how a packet is encrypted.
> > > >> This allows the app to handle packet decryption.  To allow for a
> > > >> variable number of subsamples, the buffer for the side-data will be
> > > >> allocated to hold both the structure and the array of subsamples.  So
> > > >> the |subsamples| member will point to right after the struct.
> > > >>
> > > >> Signed-off-by: Jacob Trimble <modmaker at google.com>
> > > >> ---
> > > >>  libavcodec/avcodec.h | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++
> > > >>  1 file changed, 70 insertions(+)
> > > >>
> > > >> diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> > > >> index 5db6a81320..ccc89345e8 100644
> > > >> --- a/libavcodec/avcodec.h
> > > >> +++ b/libavcodec/avcodec.h
> > > >> @@ -1112,6 +1112,63 @@ typedef struct AVCPBProperties {
> > > >>      uint64_t vbv_delay;
> > > >>  } AVCPBProperties;
> > > >>  
> > > >> +typedef struct AVPacketSubsampleEncryptionInfo {
> > > >> +    /** The number of bytes that are clear. */
> > > >> +    unsigned int bytes_of_clear_data;
> > > >> +
> > > >> +    /**
> > > >> +     * The number of bytes that are protected.  If using pattern encryption,
> > > >> +     * the pattern applies to only the protected bytes; if not using pattern
> > > >> +     * encryption, all these bytes are encrypted.
> > > >> +     */
> > > >> +    unsigned int bytes_of_protected_data;
> > > >> +} AVPacketSubsampleEncryptionInfo;
> > > >> +
> > > >> +/**
> > > >> + * This describes encryption info for a packet.  This contains frame-specific
> > > >> + * info for how to decrypt the packet before passing it to the decoder.  If this
> > > >> + * side-data is present, then the packet is encrypted.
> > > >> + */
> > > >> +typedef struct AVPacketEncryptionInfo {
> > > >> +    /** The fourcc encryption scheme. */
> > > >> +    uint32_t scheme;
> > > >> +
> > > >> +    /** The ID of the key used to encrypt the packet. */
> > > >> +    uint8_t key_id[16];
> > > >> +
> > > >> +    /** The initialization vector. */
> > > >> +    uint8_t iv[16];
> > > >> +
> > > >> +    /**
> > > >> +     * Only used for pattern encryption.  This is the number of 16-byte blocks
> > > >> +     * that are encrypted.
> > > >> +     */
> > > >> +    unsigned int crypt_byte_block;
> > > >> +
> > > >> +    /**
> > > >> +     * Only used for pattern encryption.  This is the number of 16-byte blocks
> > > >> +     * that are clear.
> > > >> +     */
> > > >> +    unsigned int skip_byte_block;
> > > >> +
> > > >> +    /**
> > > >> +     * The number of sub-samples in this packet.  If 0, then the whole sample
> > > >> +     * is encrypted.
> > > >> +     */
> > > >> +    unsigned int subsample_count;
> > > >> +
> > > >> +    /** The subsample encryption info. */
> > > >> +    AVPacketSubsampleEncryptionInfo *subsamples;  
> > > > 
> > > > I don't think this is sane. So far, side data could simply be copied
> > > > with memcpy, and having pointers to non-static data in side data would
> > > > break this completely.  
> > > 
> > > Even more reasons to ditch the current side data API and come up with a
> > > better designed one that can also be reused for packet, frame and
> > > container needs.  
> > 
> > yes, 
> > also if its redesigned, it should become possible to pass it over the
> > network. Currently all side data depends on the platform ABI, so its
> > only valid on the platform its created on. That also makes it different
> > from all other data, AVPacket.data and extradata are all
> > platform independant. And there is no API to
> > convert it into a platform ABI independant form. 
> 
> Why should it be transferable over network? That makes no sense. We
> don't have the ability to transfer AVFrame, AVPacket, or AVCodecContext
> over network either.

If you have an application that streams video, and on a different computer
an application which plays that video you need to transfer the compressed
data (that is AVPackets and their side data) over the network.

You can use RT*P/hls and others but they lack the concept of our side data
so many cases will not work over such system, especially not if there is
no platform independant representation of the data which you have to
transmit between platforms.
At some point you have to get the side data from one platforms format
to another.

ATM this is only possible by storing it in exactly the file format which
supports the exact set of side data used transfer that file to the other
platform and then read that in again. Thats a very heavyweight approuch
that does alot of steps which are not really needed.
For low latency lightweight streaming a more direct transfer of serialized
AVPackets and their side data over the network should work much better


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Many that live deserve death. And some that die deserve life. Can you give
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even the very wise cannot see all ends. -- Gandalf
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