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*To*: [email protected]*Subject*: Re: digital banking*From*: [email protected] (Hal Finney)*Date*: Thu, 3 Dec 92 16:22:32 PST

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 [email protected]

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