How to flip a coin over e-mail?

From: Jon Haugsand (jonhaug_at_ifi.uio.no)
Date: 06/17/04 

Date: 17 Jun 2004 07:32:02 +0200

I have a real practical problem where I am going to place several
people in the same number of rooms. However, the rooms are of
different quality so the actual room any person get is done by
random. There are som preconditions, so I can list all acceptable
outcomes and the problem is to find some way to choose randomly
between these outcomes.

How can I do this when all I have is e-mail communication between the
participants?

My own solution is to make a file where I denumerate each outcome and
shuffle them randomly. I pad this file with some secret random
numbers at the end, and I send a cryptographic hash (md5) to all
participants. I then ask the others for a number between 1 and N and
this number will point to the actual outcome. I then send them the
file such that all of them can check that I did not make this file
after I got the number drawn.

However, this method requires that my friends have trust in the md5
cryptographic hash. Of course, they should, but I cannot demand such
knowledge.

Any other method? 

-- 
Jon Haugsand
  Dept. of Informatics, Univ. of Oslo, Norway, mailto:jonhaug@ifi.uio.no
  http://www.ifi.uio.no/~jonhaug/, Phone: +47 22 85 24 92