Cracking the credit card code

...there's nothing to crack here, so I find the title misleading. I would find articles such as "Cracking the ISBN code" or "Cracking the CRC32 code" equally misleading.

To me, cracking implies breaking through a barrier intended to be opaque. Hence, Ross Anderson's paper on an attack on the hardware security modules in ATM machines to brute force a PIN for a given account in less than the full PIN space is, truly, an example of cracking:

http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-560.pdf

Original link/source: http://www.mint.com/blog/trends/credit-card-code-01202011

Back in the day, when processing payments for online transactions cost the earth and pre-auth was but a twinkle in the eye of the bank API developers, knowledge of the Luhn algorithm allowed one to test services via their free trials without having to give up one's credit card number. These days it's only useful as a pre-pre-auth sanity check.

The article mentions and informally describes the Luhn algorithm which is fairly interesting.

http://en.wikipedia.org/wiki/Luhn_algorithm

Why should anyone do this it is against the law.