I bought this book, "Identification Numbers and Check Digit Schemes" in the hopes that it would describe the ID number / checksum / hashing schemes actually used in some real applications. I'd like to give a talk on the subject to a general audience. http://www.amazon.com/gp/product/0883857200/104-8328046-6239112?v=glance&n=2... However, this book left me unsatisfied. Ideally, I'd like to know, for example, how to distinguish between a valid UPS tracking number and an invalid one, or (say) a valid US twenty dollar bill serial number and an invalid one. UPS tracking#: 1Z9999999999999999 US $20 bill, Series 2004: EB 42994244 D Such schemes are presumably often proprietary / unpublished, for security reasons, although the "there is no security through obscurity" maxim would seem to apply. Googling hasn't turned up much, except in a few unsavory hacking resources (mostly how to forge a valid credit card number) Anyone know a good resource? -- Thane Plambeck http://www.plambeck.org/ehome.htm