The THC Hack/Phreak Archives: CRDTCKSM.TXT (18 lines)
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Major credit card companies use the last digit in the card number as the
checksum. The actual algorithm runs thru the card number, left to right,
one   digit at a time. Each digit is multiplied by using the sequence
(2, 1, 2, 1,   2...), that is, the first digit is multiplied by 2,
second by 1, the third by  two and so on. If the result of the
multiplication is greater than 9, the      individual digits of the
result are added together (i.e. the result is 17, so  1 + 7 = 8). This
process is performed for each digit of the card. The result   of the
process for each digit is added together to give us a 'Total'. The
final calculation is...
Checksum = ((TotalFromAbove + 9) \ 10) * 10 - TotalFromAbove
This 'checksum' will match the last digit of the card i f the card is
valid. Charles
-- Maximus-CBCS v1.02
* Origin: Aronson Consulting: TIDMADT 703-370-7054, voice=x6508
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