[FFmpeg-soc] [soc]: r532 - matroska/matroskaenc.c

[Michael Niedermayer]

Hi

On Sat, Jul 28, 2007 at 11:48:25AM +0200, Luca Barbato wrote:
> Michael Niedermayer wrote:
> > Hi
> > 
> > On Sat, Jul 28, 2007 at 02:47:11AM -0400, David Conrad wrote:
> >> On Jul 25, 2007, at 8:48 PM, Michael Niedermayer wrote:
> >>
> >>> Hi
> >>>
> >>> On Thu, Jul 26, 2007 at 02:05:33AM +0200, conrad wrote:
> >>>> Author: conrad
> >>>> Date: Thu Jul 26 02:05:32 2007
> >>>> New Revision: 532
> >>>>
> >>>> Log:
> >>>> Write segment UID
> >>> [...]
> >>>> @@ -507,6 +510,9 @@ static int mkv_write_header(AVFormatCont
> >>>>      MatroskaMuxContext *mkv = s->priv_data;
> >>>>      ByteIOContext *pb = &s->pb;
> >>>>      offset_t ebml_header, segment_info;
> >>>> +    int i;
> >>>> +
> >>>> +    av_init_random(av_gettime(), &mkv->rand_state);
> >>> leaks current time (=security risk ...)
> >> What would be best way to seed the random number generator? All the  
> >> other uses of av_init_random() use a constant, but the purpose of the  
> >> segment UID is to be unique among segments, so that other files can  
> >> refer to it. 
> > 
> > well, if so using the current time is not a good idea either, if 2
> > people start encoding at the same time ...
> > 
> > 
> >> Another idea might be to use a SHA-1 hash of the frame  
> >> contents, but that seems like it would slow down muxing a fair bit.
> > 
> > i dont know a solution, finding a good and secure random number is hard
> > you could use the sha1/crc of just the first frame, but that might be a 
> > all black frame and thus has a fair chance of being identical between
> > 2 movies (if used quantizer and other encoding options match)
> > 
> 
> given a collision is always possible:
> 
> sum the avformat struct and the current time and feed everything to the
> mersenne twister.
> 
> lu - too much simple?

most stupid idea of all
the struct contains pointers, so you now leak very usefull information for
an exploit

... as i said getting a good and secure random number is not easy, you 
cannot use anything that could be usefull for an expoit or anything that
could by used to identify time or place where encoding happened,
not the current time, no pointers, ...
you are left with the video/audio you are going to encode, and some stuff
we cant easily use like the least significant bits of the time of some
interrupts

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

In a rich man's house there is no place to spit but his face.
-- Diogenes of Sinope
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