From: Hal <hfinney@shell.portal.com>
Date: Fri, 8 Mar 1996 05:42:12 +0800
To: cypherpunks@toad.com
Subject: Re: Square pegs in round holes, matchmaking, corporate mailservers
Message-ID: <199603071812.KAA16510@jobe.shell.portal.com>
MIME-Version: 1.0
Content-Type: text/plain


From: Bill Stewart <stewarts@ix.netcom.com>
> [>>Dimitris Tsapakidis <dimitrt@dcs.rhbnc.ac.uk> wrote:]
> >  >Bob must find out whether Alice has declared (commited) her interest
> >  >in him, if and only if he has declared (commited) his interest in her.
> >  >Before he does so, he can at most know that a girl is interested in him.
> >  >Another description: Bob and Alice can have a date if they both commit
> >  >to each other. If only one commits, nobody will ever find out about it.
> >  >- T is the trusted third party.
> [Padgett contribution elided]
> Oh, that would work fine.  Let a, b, and t be Alice, Bob, and Trent's secret DH
> keys, and g and p be the generator and prime (all math below is mod p.)
> If Bob wants to talk to Alice, he sends Trent B = g**b, marked "For Alice",
> optionally anonymously.  Trent calculates X = B**t == g**bt, and sends it to
> Alice.
> Alice calculates K = X**a == g**bat, calculates H = Hash(K) and 
> posts it anonymously, or sends it to Trent to post or mail to Bob. 
> If Alice wants to talk to Bob, she calculates A = g**a mod p,
> sends it to Trent, optionally anonymously, marked "For Bob".
> Trent calculates Y = A**t == g**at , and sends it to Bob.  
> Bob calculates K' = Y**b == g**abt, calculates H' = Hash(K') and
> notices that it's the same as the H he pulled off the net earlier.
> Bob says "Oh, wow!  Alice wants to talk to me!", encrypts some lame drivel 
> of a message M with key K'==K, and mails it to Alice if he knows her address
> or posts it with Subject: H', which Alice receives.

I don't think this satisfies the requirements.  Once Bob calculates H'
and sees that it matches H, he knows that Alice likes him, but Alice
doesn't know that he likes her.  The whole point of the protocol was to
be fair.  Bob must only learn that Alice likes him if Alice is guaranteed
to learn that he likes her.

I have posted an alternate solution in another message.

Hal