I doubt there is such a thing as "numbers which cannot be hacked", unless you go the cryptographic route.

You start with a UUID and a checksumm over it and then code both with a secure cryptographic method of which only you know the key.

It would be next to impossible to change a few numbers and end up with a valid UUID and checksum.

If you combine that with a maximum number of tries before your log-in is locked-out and you have a pretty secure way of providing such "cards".

CountZero

"If you have four groups working on a compiler, you'll get a 4-pass compiler." - Conway's Law

In reply to Re^2: Prepaid Cards by CountZero
in thread Prepaid Cards by Anonymous Monk

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