From: ghsvax!hal@uunet.UU.NET (Hal Finney)
Date: Thu, 3 Dec 92 18:36:43 PST
To: cypherpunks@toad.com
Subject: Re: digital banking
Message-ID: <9212040022.AA15172@nano.noname>
MIME-Version: 1.0
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I enjoyed seeing Karl's walk-through of Chaum's digital cash
algorithm.  The numbers looked right.  One point is that doing
different denominations isn't that much harder, you just need to have
more exponents.  As you generate your primes p and q, make sure that
p-1 and q-1 aren't divisible by any small primes (other than 2).  This
will ensure that phi = (p-1)(q-1) is not divisible by small primes, hence
that gcd(e, phi) will be 1 for those same small primes, in fact for
all the odd numbers.

Karl also quoted a comment from Ray Cromwell expressing concern over
proving deposits.  Ideally you'd get a signed receipt from the bank
for every deposit you make.  In the case of electronic deposits, there
exist protocols for "gradual secret exchange" that might be suitable
for this purpose.  What you'd like is to send the bank your deposit
"gradually", while the bank simultaneously gradually sends you a
digitally signed receipt.  I don't recall the details of these
protocols.  Gradual exchange would not be very convenient for email
because of the long turnaround times in mail systems, but if you had a
better connection it might be useful, especially for large deposits.

Another way to look at it is, what stops the local merchant from
taking your payment, putting it in his pocket, and then denying that
you've given him anything?  It would just be his word against yours.
One answer is, if he does that to multiple people, their multiple
stories would have more credibility than his denials.  Similarly, a
bank which made a habit of cheating its customers would find itself
publically exposed.  So it may be reasonable to trust the bank for
routine transactions.

Hal
74076.1041@compuserve.com