Quoting Robert Hettinga (rah@shipwright.com):

I wonder if these guys have heard of double spending?

Does anyone know whether this is some kind of 2-d barcode (it must be) or is it something else?

Cheers, Robert Hettinga

It's a 2-d barcode, with a bunch of info encoded into it.. Short answer: they use statistical security measures to prevent double spending, but they have a *scary* reputation and limitless resources to pursue fraud, something no one else really has. I would not be brave enough to double spend with this system from my home state, if I were in the US. Longer rambling answer: The E-stamp folks are pretty hardcore as far as security goes, I recall. They use a Dallas Semiconductor iButton (plug: check out the ibuttonpunks mailing list....) as a local value store, after paying for postage at a central location, and do other security measures I don't quite recall -- perhaps encoding your local post office, such that the local post office could keep its own database of double spenders, or having a global double spending database. The post office is pretty much protected from fraud, in any case -- much more than existing letters (which can be forged against high-speed equipment using a phosophor pen). I'm not sure how E-stamp protects itself from fraud, or protects its customers from E-stamp committing fraud, but I'm fairly convinced there is at least statistical security for the USPS against users. I believe the USPS published a standard during the search for a new postage system -- they did a pretty good job of it, I just never bothered to buy a copy. If someone bought a copy and mailed it to jya, it would be doing everyone a service. (Even if you have the nerve to commit bank wire fraud, check fraud, armed robbery, espionage, sedition, etc., you probably still aren't brave/reckless enough to commit postal fraud, though -- USPS inspectors make IRS auditors look passive) Cheers, Ryan ryan@venona.com