3 ms·

If you do things physically, you can do it easily. For example you could have a hat with pieces of paper in it each with a number. The numbers sum to 0 (some a

If you do things physically, you can do it easily. For example you could have a hat with pieces of paper in it each with a number. The numbers sum to 0 (some are positive, some are negative). Each person draws a piece of paper, adds his or her weight to it, then says that number. The numbers that each person says are summed and that's the total sum.

srfa 7y ago
Could use a similar technique remotely using a random number generator. Provided there are enough passengers the sum would approximately equal the true total weight. There might be behavioural issues here though - perhaps people would keep generating a random number until they are provided one that makes their weight look 'better' (and 'better' probably means 'smaller' for most people). Edit: comicjk made a great point about large numbers. Perhaps using large random numbers (eg a normal distribution with a mean of 4550 and a std deviation of 20-30) would help reduce the behavioural issue.
comicjk 7y ago
I like this better than my solution. The technique of adding random numbers has a more crypto feel to it. It would help to use big numbers, so that they would conceal the underlying number better (clearly single-digit numbers would be no help, might as well extend that and use 5-digit numbers).
sl8r 7y ago
Won't you possibly get some information about some passengers by doing this, if you know / can figure out the distribution of the random numbers? E.g., say that the random numbers are uniformly distributed between -200 and 200. If somebody says a number like 425, then I know they weigh at least 225 pounds. And the probability they weigh more than 225 - k pounds is 1 - k/400.
- skrtskrt 7y ago
  Yes absolutely, but you can just make the range wide enough to wash out the difference (like -2000 to 2000, summing to zero) Then you would probably get more information from looking at the person thank you could from hearing a weight number that could be +/-2000 pounds
  - XMPPwocky 7y ago
    You could also do it modulo some number, I think? Then knowing any individual number (weight + pad) mod n gives zero knowledge (I think. If all but one party reveal their pads, they can trivially determine the other's weight so it fails. Believe it to be secure if only 1 party is malicious, though. Cases for other numbers of bad actors are more complicated...)
emilfihlman 7y ago
This doesn't actually hide the information in edge cases.

srfa 7y ago

Could use a similar technique remotely using a random number generator. Provided there are enough passengers the sum would approximately equal the true total weight. There might be behavioural issues here though - perhaps people would keep generating a random number until they are provided one that makes their weight look 'better' (and 'better' probably means 'smaller' for most people). Edit: comicjk made a great point about large numbers. Perhaps using large random numbers (eg a normal distribution with a mean of 4550 and a std deviation of 20-30) would help reduce the behavioural issue.

comicjk 7y ago

I like this better than my solution. The technique of adding random numbers has a more crypto feel to it. It would help to use big numbers, so that they would conceal the underlying number better (clearly single-digit numbers would be no help, might as well extend that and use 5-digit numbers).

sl8r 7y ago

Won't you possibly get some information about some passengers by doing this, if you know / can figure out the distribution of the random numbers? E.g., say that the random numbers are uniformly distributed between -200 and 200. If somebody says a number like 425, then I know they weigh at least 225 pounds. And the probability they weigh more than 225 - k pounds is 1 - k/400.

skrtskrt 7y ago
Yes absolutely, but you can just make the range wide enough to wash out the difference (like -2000 to 2000, summing to zero) Then you would probably get more information from looking at the person thank you could from hearing a weight number that could be +/-2000 pounds
- XMPPwocky 7y ago
  You could also do it modulo some number, I think? Then knowing any individual number (weight + pad) mod n gives zero knowledge (I think. If all but one party reveal their pads, they can trivially determine the other's weight so it fails. Believe it to be secure if only 1 party is malicious, though. Cases for other numbers of bad actors are more complicated...)

emilfihlman 7y ago

This doesn't actually hide the information in edge cases.