The Millionaire’s Problem is the archetypal example used to explain secure computation: how can two millionaires know who is richer without revealing their actual wealth?
On one side, the first party inputs his wealth:
And on the other side, the second party inputs his wealth:
After the secure computation is done, both parties know who is richer without disclosing their actual wealth.
NOTE: for the converse problem (i.e., the problem of the debtor) please read the page about Secure Debt Settlements.