4 ms·
That's a weird way of doing checksums. Summing digits preserves residues mod 9, yet the final check is mod 10. Weird. (Now how do the 3 additional verificati
by Sniffnoy 16y ago
That's a weird way of doing checksums. Summing digits preserves residues mod 9, yet the final check is mod 10. Weird.
(Now how do the 3 additional verification digits on the back work?)
- gjm11 16y agoThey're generated independently of the card number and, in principle, known only to the card issuer and verifiable only by asking them. Merchants are supposed never to record them. (They don't really need to be independently generated for each card; instead they can be computed from the card number, expiry date, etc., using a secret key known only to the issuer and some cryptographic magic.)