You should be able to solve this.

A group of jealous professors is locked up in
a room. There is nothing else in the room but pencils
and one tiny scrap of paper per person. The profes-
sors want to determine their average (mean, not me-
dian) salary so that each one can gloat or grieve over
his or her personal situation compared to their peers.
However, they are secretive people, and do not want to
give away any personal salary information to anyone
else. Can they determine the average salary in such a
way that no professor can discover any fact about the
salary of anyone but herself? For example, even facts
such as "three people earn more than $40,000" or "no
one earns more than $90,000" are not allowed.

>>12
Oh crap, I forgot that they can collude to learn about the other profs.  I assumed that all the profs were reclusive, and would use only what they know to learn about the other profs.  I guess my plan fails.

>>13
In a semirealistic scenario, you still have the problems
a) (authentication?) are you sure it was your friend's lock on that diamond, and not somebody who intercepted the box, and guessed your algorithm?
b) (corruption) malicious 3rd parties can add their own locks for the shits & giggles, thus depriving everybody of the diamond