[FFmpeg-devel] [PATCH] mxf umid generation

[Baptiste Coudurier]

On 3/7/2009 8:40 PM, Michael Niedermayer wrote:
> 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.

Well, I added ff_random_get_seed according to your indication :/

> also i think its missing a bitexact check for reg tests

Added.

-- 
Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
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FFmpeg maintainer                                  http://www.ffmpeg.org
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