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    • How to Determine if a Credit Card Number is Valid

      How to Determine if a Credit Card Number is Valid

      Credit cards use the Luhn Check Digit Algorithm. The main purpose of the Luhn Check Digit Algorithm is to catch data entry errors, but it does double duty here as a weak security tool. The Luhn Check Digit Algorithm For a card with an even number of digits, double every odd numbered digit (1st digit,

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