[viff-devel] Player thresholds

[Mikkel Krøigård]

Citat af Håvard Vegge <havardv at stud.ntnu.no>:

> Thanks for clarifying!
>
> Guess there are a lot of different variations of the definitions,  
> notions, etc. regarding secret sharing and MPC...
> One of my problems is that when running the included sort.py  
> protocol with n=5 and t=2, the sorting of the array fails:
>
> Original array: [{33}, {61}, {95}, {67}, {37}]
> Sorted array:   [{6692966529242218069}, {35128728804386641877},  
> {6621921405115695795}, {27880759555216652877}, {34356088148296101764}]
> Made 9 comparisons
>
> Works with n=5 and t=1 though...

That definitely looks like a bug to me. It has been about forever  
since I looked at the VIFF code, but I think I'll try this out, see if  
I can get the bug as well.

> Mikkel Krøigård wrote:
>> Citat af Håvard Vegge <havardv at stud.ntnu.no>:
>>
>>> Hi!
>>>
>>> I'm trying to do some basic benchmarks with different number of  
>>> players. A few questions:
>>>
>>> 1. Why have VIFF defined the threshold t to be the number of  
>>> corrupt players, while the classical literature defines it as "t  
>>> participants can reconstruct the message"?
>> Well, in what I have read, it is most often referred to as the  
>> threshold for the number of tolerated corruptions, though yes I  
>> have also seen it described as the number of shares needed to  
>> reconstruct. I don't think there's anything wrong with the way we  
>> do it, as long as it is clearly defined.
>>
>>> 2. Say I create the config files this way:
>>> python generate-config-files.py -n 5 -t 2 localhost:9001  
>>> localhost:9002 localhost:9003 localhost:9004 localhost:9005
>>>
>>> Would this indicate that the polynomial is quadratic (of degree 2)  
>>> and that 3 players in theory could reconstruct some secret?
>> Yes.
>>
>>
>>> 3. Assume three of these five players provide input, while the  
>>> last two players do not. Will all players still participate in the  
>>> actual computation, or are the last two just passive spectators  
>>> and receives only the output?
>> -n 5 indicates that 5 servers will participate, regardless of how  
>> many will provide input. Think of it from the perspective of  
>> security. If the input was shared to 3 servers, we could not  
>> tolerate 2 corruptions.
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