[FFmpeg-devel] [PATCH] mxf umid generation

Baptiste Coudurier baptiste.coudurier
Sun Mar 8 23:00:18 CET 2009

On 3/8/2009 2:08 PM, Michael Niedermayer wrote:
> On Sun, Mar 08, 2009 at 01:05:55PM -0700, Baptiste Coudurier wrote:
>> On 3/8/2009 7:10 AM, Michael Niedermayer wrote:
>>> On Sat, Mar 07, 2009 at 08:53:03PM -0800, Baptiste Coudurier wrote:
>>>> 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 :/
>>> I didnt suggest to use more than 1 value from it
>> You mean using more than once "seed" ?
> yes, using the lsb of the timer twice stores the number of clock ticks
> between the 2 calls, that is strongly dependant on cpu & compiler thus
> leaks this information while not doing much else
>> Well I'm following your suggestion of using only seed.
>> Can I use LFG like I did in the first place ? I'd like to have this
>> problem fixed.
> you are the maintainer of the code, you can use what you prefer
> If you want to conform to the spec you need 16(+3)bytes
> from /dev/random or if that is unavailable gather bits by user interaction
> and its timing like some other tools do.
> And you have to redo this for each mxf file muxed.
> if you dont care about the spec and just want to minimize collisions
> the 32bit seed alone will be as good as filling all 16 bytes by lfg

I'll use the "undefined method" value I think.

Ok, what if I use seed only once like this ?
I prefer having your opinion on randomness and security matter.

> and of course you ca just set the thng to 0 and tell users to use a
> container that is better designed (that is what i would do)

I wish I could :>

Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
Key fingerprint                 8D77134D20CC9220201FC5DB0AC9325C5C1ABAAA
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FFmpeg maintainer                                  http://www.ffmpeg.org
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