I've scoured the web; other people have had this problems but all the forums peeter out with no solution. I believe the problem is not in the authorizenet_aim.php file, has to do with the hosting company, SSL, or database. I believe it has to do with the settings in the admin panel.
The reason being is I hadn't previously touched the actual payment php files (neither had anyone else) and people have had this problem across different hosting companies. Also I found out my site was hacked; I have Header Tags SEO installed and some of the page titles were amended with xxx, porn, etc. I use 1password and have different logins/pw for all my various accounts. It seems that the damage thus far has been caused solely in what can be accessed from the oscommerce admin section.
Regarding the Authorize.net AIM:
When I select "update" there are three fill-forms for "Login Username" but the second two are blank and also display blank when confirmed. I assume because that is for sensitive information like the transaction key and whatnot. Just thought I'd mention it. I've also tried uninstalling, changing keys, etc suggested in other places. Nothing worked so at this point I'm grasping at straws.
The specific error I am getting is:
"This transaction cannot be accepted. - Your credit card could not be authorized for this reason. Please check your billing address and try again."
The actual orders aren't even being sent to Auhtorize.net because there are no unsettled or declined transactions (Thank God!), so whatever is broken is gets broken inside of oscommerce.