Last time I looked at card processor you had to echo back the approval numbers with the other information when you did your batch close transaction, so the card

Last time I looked at card processor you had to echo back the approval numbers with the other information when you did your batch close transaction, so the card processor should be able to reconcile the batch record with the original approval record and stop arbitrary new data being introduced. Plus not every card processor I looked at used batches. At one point the card processors had a very nice crypto protocol called SET that kept the card numbers out of everyone’s hands. At the time the crypto was really overwhelming and it would take like twenty seconds to do all the math on a high end (for the time) processor. The other problem was that the reference implementation was done by an absolute c++ zealot who was furious he had to write the code in C so tens of thousands of lines of code was dedicated to implementing all the c++ features in C which pretty much made the reference useless except for interoperability testing. Unfortunately the standard never took off and you can’t find it anymore.