[FFmpeg-devel] [PATCH] mxf umid generation

Baptiste Coudurier baptiste.coudurier
Sun Mar 8 05:05:41 CET 2009

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 ?

Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
Key fingerprint                 8D77134D20CC9220201FC5DB0AC9325C5C1ABAAA
checking for life_signs in -lkenny... no
FFmpeg maintainer                                  http://www.ffmpeg.org
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: mxf_umid3.patch
URL: <http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/attachments/20090307/a3069187/attachment.asc>

More information about the ffmpeg-devel mailing list