Zero knowledge( a>b )

cypherpunk cyphrpunk at
Mon May 9 12:35:25 PDT 2005

On 5/9/05, Sarad AV <jtrjtrjtr2001 at> wrote:
> If user A has the integer a and user B has the integer
> b, can a zero knowledge proof be developed to show
> that a>b,a<b or a=b.

You've got two different things mixed up here.  A zero knowledge proof
is normally used by one person to show that he knows a value
satisfying certain conditions, without revealing what the value is.
What you are asking for involves two people who want to compute a
function of their inputs, without revealing those inputs. That is
known as a multi party computation or MPC. As was pointed out,
Schneier has some good pointers on MPC calculations.

There is a program you can download called Fairplay which will perform
MPC calculations like this. One of them does exactly what you are
asking for. See


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