[FFmpeg-devel] modification time in libavformat (.MOV files)

[Ayke van Laethem]

2014-02-15 21:02 GMT+01:00 Michael Niedermayer <michaelni at gmx.at>:
> On Fri, Feb 14, 2014 at 07:39:18PM -0800, Alex Sukhanov wrote:
>> On Fri, Feb 14, 2014 at 4:47 PM, Michael Niedermayer <michaelni at gmx.at>wrote:
>>
>> > On Fri, Feb 14, 2014 at 07:18:13PM +0100, Ayke van Laethem wrote:
>> > > Hi,
>> > >
>> > > I have been working on a patch to fix a bug I discovered in the saving
>> > > of .MOV files. In short, the creation time of individual streams isn't
>> > > copied as it says they will be (it is copied from the output .MOV file
>> > > instead, which is null if -map_metadata isn't used), and modification
>> > > time is set to the creation time.
>> > >
>> > > This SuperUser post is in part about this issue:
>> > >
>> > http://superuser.com/questions/510578/when-spliting-mp4s-with-ffmpeg-how-do-i-include-metadata
>> > > I can add a way to reproduce the issue if needed. I will certainly do
>> > > that before (or while) submitting the patch.
>> > >
>> > > The patch I wrote seems to work correctly, but fate fails. I assume
>> > > this is because the metadata is changed so the output file has a
>> > > different hash than what fate expects. Updating the hash so it
>> > > includes the correct creation_time should be easy, but the
>> > > modification time will be different each time.
>> > > So here is the question. Should the modification time be set at all? I
>> > > think it would be the more 'correct' way to go, but it makes testing
>> > > harder. If modification time will be set, how should testing be done?
>> >
>> > storing the current time in files without the users knowledge poses
>> > a risk to privacy and security.
>> >
>> >
>> > [...]
>> > --
>> > Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>> >
>> > it is not once nor twice but times without number that the same ideas make
>> > their appearance in the world. -- Aristotle
>> >
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>> >
>> >
>> Sorry for dumb question, but what is a risk to privacy and security?
>
> time and location allow indentfying a person in some/many cases
> so storing time without the person knowing gives half the information
> needed to identify the person. Now the location can possibly be
> identified from the video content.
>
> security wise, storing the time in files leaks information about
> the system clock. which could be helpfull to any attack that needs
> to be timed pricessely or could be feedback for an attack involving
> NTP. Iam no expert in this stuff iam sure there are others who could
> provide better examples
>

Okay, that makes sense. But I think in most cases, it is expected
ffmpeg will save the modification according to the specification (I
would expect that, at least).

What about making it a command line option whether to save
modification times as metadata?
Are there any other containers that save modification time, besides
the MOV/MP4 family? I tried to grep 'time', but I only found something
that seemed to be related to MPEG transport streams.

— Ayke

>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> You can kill me, but you cannot change the truth.
>
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