regardingg the XXXing on receipts it turns out that things aren't
as grim as i thought.  i anlayzed the checksum algorithm and if
you are missing n digits there are 10^(n-1) clashes.

i verified this with a brute force program.

but in the "photograph the card" scenario ... if one digit is
blurry then you still win because 10^(n-1) is 1.

if two are unknown then mr nasty could try buying stuff from
10 diferent sites.


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