As stevesh is indicating the security issues aren't with the transmission but with the person who receives the information.
Indeed it's long been a mystery to me why some people feel that it's more risky to send cc details over an encryted line to a secured server than to dictated them to somebody being paid minimum wage in a call center located in a "low rent" district or 3rd world country. But hey ho, each to their own.
Suffice to say that that the card companies are making offline processing more difficult because it reduces their exposure to fraud (and hopefully keeps their costs down). So it may be inconvenient, but it's not necessarily bad.
If offline processing really is necessary for you, then the code can be lifted from Zen Cart 1.3.8 or better still look for Ceon's offline processing mod. But be sure that you're not contravening the terms of your merchant agreement.