Just a little more off-topic ness
For RMXP, get both an hex and an octal editor. Then get the ZLib Ruby module and download ruby. The RGSS ad is actually compressed with a modified ZLIB inflate method. However, reversing the RGSSAD is not illegal if it wher meant to be on backup purpose, but I do recommend not to try. They even meant it in the license and help file.
Could you extract the encrypter from RMXP and reverse it?
Well, it is not like 'extract' it. RMXP uses part compiled, part interpreted scripts (you know the DLL, the program and the script editor) Now, the .exe was compiled, you would need to actually re-create it by decompiling to re-structuate the codes. Almost anyone knows that is like... not-easy.
On to the script. You used the DRE A with RMXP, but how exactly does it work? I mean, it randomly subsitutes characters? (With key offcourse, or you could not turn itback) or is it a bitwise conversion, with switching bits around?
I would appreaciate if you could tell me, but if you don't know, dont bother, ill look it up myself.