I am testing now to allow Credit Card Transactions on the field. I will use http://www.payjunction.com/ for the CC processing, I can also use http://www.firstdata.com/
All these APIs to do credit card transactions on the field use some form of ssl (secure socket layer) to interact with their https servers. Just the same way an end user login to his bank and do online payments. I can write extensively about the subject using openssl but believe me it is very complex.
Ok is in sourceforge, the last release with CC txns.