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The Computer Corner


By Charles Miller

Have you ever had the experience of shopping online and in the process having your credit card rejected? It happens sometimes you enter all your information, click on Next, the web site churns for half a minute then says the zip code you entered does not match your account. Then another time you enter your information, click on Next, and instantly without a millisecond delay the site says your credit card number is wrong. When you look at what you typed you see that one digit was entered incorrectly. Why would it take 30 seconds to check a zip code but checking the credit card number is absolutely instantaneous? The answer is what is called a checksum.

A checksum is what makes it possible to simply look at a number and immediately see if it is right or wrong. So how is that possible? To explain in the space available in this column I need you to pretend, for the purposes of this explanation, that a credit card number is only five digits long.

To take a five-digit credit card number 36354 and compute its checksum, you could do some mathematical gymnastics such as take the first number, multiply it by the second number, subtract the third number, divide by the fourth number and finally add the fifth number. This could be a checksum. So you would have 3 x 6 -3 ÷ 5 + 4 = 7. Then your credit card number would be written as 363547 so that anyone could do the math on the first five digits and instantly tell if the last digit, the checksum digit, was correct without calling the bank. Even this crude example is 90% accurate.

Calculation of real checksums is much more accurate and more complex. The Luhn algorithm is a very simple one: “Double the value of alternate digits of the account number beginning with the second digit from the right. Add the individual digits comprising the products to each of the unaffected digits in the original number. The total obtained must be a number ending in zero (30, 40, 50, etc.) for the account number to be valid.” Simple, right?

This process actually is simple for computers, and checksums are one of the reasons computerized systems are as accurate as they are today. When your computer reads a file from your hard disk, that process includes checksums to verify what was read is accurate. When you send an email, that process includes checksums so that the receiving computer can instantly know if the checksum does not compute that there was an error somewhere in your email. A checksum is one of the things that help guarantee that the bank statement you downloaded is exactly letter-for-letter the same bank statement the bank has.

By themselves, checksums are used to verify data integrity but not authenticity. To put it another way, checksums can verify that there is or is not an error in data, but a checksum does not tell what the error is. Once an error is known to exist it is up to other systems to find and correct it, and that is for another column.

Charles Miller is a freelance computer consultant, a frequent visitor to San Miguel since 1981 and now practically a full-time resident. He may be contacted at 044 415 101 8528 or email FAQ8 (at)


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