

Computing the maximum salary - J3L2404
http://mathoverflow.net/questions/6217/computing-the-maximum-salary

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lmkg
I was reading over the first response, and the details of the (second,
correct) n-private sum computation, and I noticed they're omitting a small but
important detail. The protocol has you distribute various numbers, to various
people, the sum of which is your salary, and claims that no one gains any
information about your salary. If the numbers you distribute are all positive,
then when you give someone the number X they gain the information that your
salary is greater than X, so the claim is incorrect. This problem can be
removed by allowing the numbers you distribute to be negative, because then
you could distribute a number higher than your salary. The ability to
distribute negative numbers is critical to the n-private property of the
protocol.

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trjordan
In the first version, the first person adds their salary, modulo 10 _N_ B, so
you don't get any information.

In the second version, you are correct, with a sign error. You are presenting
an _upper_ bound on your salary, which is probably not a serious problem. That
said, you do need negative numbers to truly conceal all the information about
the initiator's salary.

