Credit card validation

We’ve been updating the e-commerce side of the website to work with Maestro cards in the near future. As a result, I’ve had my first delve into card processing and some of the simple techniques that you can use to validate a card before submitting details to a bank / online card processing service.

The strange sounding Luhn Algorithm is used as a quick checksum of the credit card digits (and a few other things) and will detect most transposed digits.

The code to check if the credit card number is valid used to check if the card number was less than or equal to 16 digits long. However,we’ve found that the PAN can be between 12 – 19 digits (International Maestro cards can be 12). So after a bit of testing we discovered that the Luhn algorithm works regardless as long as the card number is valid.

public override bool Validate( string number )
{
valid = false;
if (number != null)
{
int sum = 0;
int len = number.Length;
for (int i = len - 1; i >= 0; i--)
{
byte b = Byte.Parse(number[i].ToString());
if ((i % 2) == len % 2)
{
int n = b * 2;
sum += (n / 10) + (n % 10);
}
else
{
sum += b;
}
}
valid = (sum % 10 == 0);
if (!IsValid)
{
error = "The card number is invalid";
}
}
return valid;
}

To test your validation checks, there are several ‘safe’ credit card numbers which are defined as test cards and won’t charge anyone for your attempts.
Here are a few I’ve collected in one place:

The usual disclaimers about checking that your card processing service does handle you using test numbers, and that this post is for information only and no liability is accepted if you choose to not double check the details before using them and incurr any charges or damages.