[FFmpeg-devel] [PATCH] mxf umid generation

[Baptiste Coudurier]

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 ?

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 ?

-- 
Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
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