Re: Will this scheme work ...

From: markt@harlqn.co.uk (Mark Tillotson)

Date: Mon, 11 Oct 1993 22:17:43 +0000

> >>>The Problem:
> >>>A group of people are playing a game over e-mail. They need to throw dice
> >>>every turn. Unfortunately, they can't find somebody ``trustworthy'' to
> >>>do it for them. So, how can they throw dice
> >>>
> >>
>
> How about this:  Each player generates a large random number, computes
> the MD5 hash, and then discloses the hash to the other players.
> Each player is now committed to his number, since no ther number will
> produce the same hash.  Then each player reveals his number to the others,
> the result is summed (and optionally hashed again, if you like) and the
> result mod 6 is the die roll.

But this is relying on no-one guessing the random number algorithms
used by the other players, and also requires that the random number
algorithms be such that their sum is truly random---how can each
player trust the other players' RNGs without knowing how they work,
and if they know how they work they can predict them, and thus break
the scheme...

I don't see how the above is any different from just disclosing the
random numbers themselves without bothering to go through the MD5 hash
stage?

I would suggest taking all the numbers (preferrably verified by a
public key authenication step, and hashing them _all_ together (don't
sum them first!), and then deriving the die number from that.

You also need to beware of someone faking _all_ the die-roll messages,
so a public-key protocol where each message has a time stamp and
die-roll number encrypted in would be needed (however if the
participants cannot securely exchange authenicated public keys, that
doesn't really work!)

======================================================
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| \/ |  /\| |/\ |<   markt@uk.co.harlqn  Barrington Hall,\
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I came, I saw, I core-dumped...            Cambridge CB2 5RG \

References

Re: Will this scheme work ...
TSOS@uni-duesseldorf.de (Detlef Lannert)
Thu, 07 Oct 1993 10:01:49 +0000
Re: Will this scheme work ...
payson@dr-hibbert.cs.wisc.edu ( Payson)
Thu, 07 Oct 1993 16:51:59 +0000
Re: Will this scheme work ...
anspring@news.delphi.com (ANSPRING@DELPHI.COM)
Fri, 08 Oct 1993 01:41:50 +0000
Will this scheme work ...
moudgill@cs.cornell.edu ( Mayan Moudgill)
Sat, 25 Sep 1993 05:56:07 +0000

Referenced By

Re: Will this scheme work ...
TSOS@uni-duesseldorf.de (Detlef Lannert)
Thu, 07 Oct 1993 10:01:49 +0000
Re: Will this scheme work ...
payson@dr-hibbert.cs.wisc.edu ( Payson)
Thu, 07 Oct 1993 16:51:59 +0000
Re: Will this scheme work ...
anspring@news.delphi.com (ANSPRING@DELPHI.COM)
Fri, 08 Oct 1993 01:41:50 +0000
Will this scheme work ...
moudgill@cs.cornell.edu ( Mayan Moudgill)
Sat, 25 Sep 1993 05:56:07 +0000