I already found a protocol to find out who is richer (older) between two parties, but Is there any protocol to find the youngest person among 3 parties, without revealing actual ages?
There might be better ways to do this, but I wanted to do it with only primitives found in VIFF (why? because it is the MPC framework I am most familiar with). There could be specialized protocols which are better.
In VIFF, we have access a primitive >= which returns 0 or 1 (false or true). We can do the comparison you seek using that plus some simple arithmetic on secret shared values. I think these are all passive protocols and do not guarantee fairness (if fairness even makes sense in the passive world).
Let the three parties be alice, bob and charlie. They have each secret shared their ages with the others. We'll let a,b,c be the variables that represent the secret shared ages.
We can compute who is the youngest as follows:
This computation only reveals who is the youngest. It does not reveal ages or the overall order. It requires 6 calls to >=. This will also only return 1 person as youngest. To get all of the youngest, you would replace the 2nd, 4th, and 6th lines with another >= instead of a 1-XX.