Detecting Credit Card Numbers in Network Traffic / Web Security Blog, 1 January 2008
http://www.modsecurity.org/blog/archives/2008/01/detecting_credi.html
"Detecting theft of credit card numbers by monitoring network traffic is very difficult, but such monitoring can be useful for detecting unintentional leakage of credit card numbers. In order to do so the monitoring system has to be application and protocol aware so that it can both compensate for encoding and encryption applied to the data as well as provide a tool for creating exceptions for valid credit card numbers or other information detected as credit card numbers."