A little puzzle

[Robbie Bow]

Here's the scenario: I am playing a game of Diplomacy with online
friends. One guy (who isn't a player) is the games master; receives our
orders by email and when the deadline for that turn is up, processes the
orders and posts the results on a forum for us all to read. Now it
happens that the GM is off on holiday for 2 weeks, so we have a problem:
how to process the orders without a GM.

I was thinking along the lines of having a script that we could all
submit orders which it duly encrypts. The tricky bit (for me) is how to
make a system such that to decrypt the data it would require all 4
players to process it in some way (thus none of us can peek at anyone
else's orders before everyone is satisfied with their orders) - this
also means the system needs to be able to replace orders from any player
up until the point we all do our bit to decrypt it, and seeing as I will
probably be hosting the script, will need to be able to set the other
players up with their keys such that I cannot easily find what those
keys are.

Hmm. Any tips gratefully received.