Boston Globe Online has a very nice article on a payment technique that is based on statistical characteristics (and thus requires a lot of payments to work). It is interesting enough to quote and let your mind wander...
The service will be free to consumers, who sign up with Peppercoin and provide a credit card number. Now the user can go to any Peppercoin retailer and purchase a single, very cheap item -- an MP3 song priced at 50 cents, for instance. By clicking on a link, the music gets downloaded to the customer's computer. The merchant gets a Peppercoin -- a sort of electronic token that's got the customer's digital signature embedded in it.
What's the token worth to the merchant? It depends. Peppercoin uses an algorithm that assigns a value to the token. Actually it assigns one of two values. Either the token is worth some preset amount -- say, $10 -- or it's worth nothing at all. When the token is worthless, the merchant throws it away. When it's not, the merchant collects $10 from Peppercoin, even if the customer only spent 50 cents.
It seems utterly nutty until you apply this method to millions of 50-cent transactions every month. Maybe 5 percent of these transactions will be sent to Peppercoin, which processes them through the credit card system. The rest are thrown away. This keeps transaction costs way low. And the transactions that are processed have a value of $10 apiece, which brings in cash to make up for the 95 percent that were thrown away. Spread over millions of purchases, it all averages out
For those interested in the original sources:
-the presentation by Rivest at RSA 2002,
-the technical paper (math!).