[FFmpeg-devel] [PATCH] mxf umid generation

Michael Niedermayer michaelni
Sun Mar 8 05:40:47 CET 2009


On Sat, Mar 07, 2009 at 08:05:41PM -0800, Baptiste Coudurier wrote:
> On 3/7/2009 7:52 PM, Michael Niedermayer wrote:
> > On Sat, Mar 07, 2009 at 07:25:54PM -0800, Baptiste Coudurier wrote:
> >> On 3/7/2009 7:16 PM, Michael Niedermayer wrote:
> >>> On Sat, Mar 07, 2009 at 06:31:53PM -0800, Baptiste Coudurier wrote:
> >>>> On 3/7/2009 5:23 PM, Michael Niedermayer wrote:
> >>>>> On Sat, Mar 07, 2009 at 04:14:19PM -0800, Baptiste Coudurier wrote:
> >>>>>> On 3/7/2009 3:36 PM, Michael Niedermayer wrote:
> >>>>>>> On Sat, Mar 07, 2009 at 02:48:49PM -0800, Baptiste Coudurier wrote:
> >>>>>>>> On 3/6/2009 7:44 PM, Michael Niedermayer wrote:
> >>>>>>>>> On Fri, Mar 06, 2009 at 07:28:55PM -0800, Baptiste Coudurier wrote:
> >>>>> [...]
> >>>>>>>> Property changes on: libavutil\random_seed.c
> >>>>>>>> ___________________________________________________________________
> >>>>>>>> Added: svn:eol-style
> >>>>>>>>    + LF
> >>>>>>> intended?
> >>>>>>>
> >>>>>>> except these ok
> >>>>>>>
> >>>>>> I'm not sure, I'm on windows because products I test with only works on
> >>>>>> windows, I set ending lines style to unix, but it keeps adding this...
> >>>>>>
> >>>>>> It should be ok I think.
> >>>>>>
> >>>>>> Btw, is lfg ok for this purpose or should I use something else ?
> >>>>> to generate these id numbers out of the seed?
> >>>>> id rather use
> >>>>> seed += 1LL<<32 :)
> >>>>>
> >>>>> lfg seens pointless complexity for this ...
> >>>>>
> >>>>> and patch ok
> >>>>>
> >>>> Well I'd like to use the defined methods for umid generation, see below:
> >>>>
> >>>> "A.3.2 Alternative masking methods
> >>>> The masked material number is an unpredictable number uniformly
> >>>> distributed over the range 0 thru 2^128-1. Its
> >>>> effectiveness as a unique identifier relies on this uniform random
> >>>> distribution, and the exact method of its generation is not
> >>>> important. Therefore, the use of the reference masking method is not
> >>>> normative, and any method providing an equivalent
> >>>> level of unpredictability and uniformity of distribution may be used
> >>>> with the ?masked method? value in the ?number
> >>>> generation method? field of the UMID universal label (reference table 1
> >>>> in 5.1.1)."
> >>>>
> >>>> And instance generation:
> >>>>
> >>>> "B.2 24-bit PRS generator (?2h?)
> >>>> Any suitable psuedo-random sequence (PRS) generator polynomial may be
> >>>> used provided it has a maximal length of
> >>>> 16,777,215 clock cycles. At the point of creating a new instance of the
> >>>> material, the 24-bits from the PRS generator are
> >>>> sampled to gain a new instance value.
> >>>> PRS generators shall not allow a zero value.
> >>> am i right in assuming that this "definition" is a 24bit LFSR?
> >>> if so, this is neither uniform over 2^128 nor unpredictable.
> >>> actually, its trivial to generate all future and past values
> >>> from just 2 24bit values even if the used polynomial is not known.
> >>>
> >>> also if my interrpretation of this "definition" is correct you can
> >>> expect 1 collision in ~4000 ids
> >> Well, "instance number" is 3 bytes and umid is 16 bytes, these are
> >> different numbers, this is what the code is trying to achieve, see the
> >> patch.
> >>
> >>>> NOTES
> >>>> 1 Any suitable seed may be used to start the pseudo-random sequence
> >>>> (PRS) 24-bit generator.
> >>>> 2 The PRS generator should use a free-running clock having no time
> >>>> relationship with the clock used to generate the sampling strobe.
> >>>> 3 The PRS generator clock frequency should be greater than 10 kHz.
> >>>> 4 The number of feedback taps resulting from the PRS generator
> >>>> polynomial should be between 8 and 16 to ensure the random nature
> >>>> of the sequence."
> >>>>
> >>>> What do you think ?
> >>> sounds like the spec is writen by some really incompetent people.
> >> Is it still true now you know that these numbers are different ?
> > 
> > a design that cannot be implemented in ANSI C or for the matter of fact
> > any deterministic language is broken
> > 
> > 
> >> Is the method ok at least for the "instance number" ?
> >>
> >>> [...]
> >>>
> >>> also you should set any bits left after the seed+counter to a random constant
> >>>
> >>> and if you have a 32bit seed you have 32bit of randomness and a PRNG making
> >>> 128 out of that still has just a randomness of 32, you could set 96 bits to
> >>> your pets name it wont make a difference.
> >>>
> >> So there is no way we could be able to generate the 128 bits umid
> >> according to the method ? Can I use the md5 of the 4 bytes of the seed ?
> > 
> > you can but it will have a collision in ~64k umids thats the same as if you
> > just take the 32bit seed + some constant like your name.
> > also it violates the spec because it is neither unpredictable not uniform.
> > 
> > If you want to follow the spec you need 128 strong random bits per umid.
> > a md5 of 32 LSB from the timer does not qualify ...
> > 
> 
> All right, what do you think about this ?

if /dev/random is available better if just a timer is available
you leak the cpu type & compiler version used.
also i think its missing a bitexact check for reg tests

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

There will always be a question for which you do not know the correct awnser.
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