How do websites know when you input a fake credit card number?

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How do websites know when you input a fake credit card number?

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6 Answers

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The very last digit of a card number is a checksum digit. Using the Luhn algorithm, the last digit should only have one single possible value for a given sequence before it. If that number doesn’t match what it should be, then there must have been some mistake entering the card number.

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