Convert .NET "Rijndael algorithm" script into compatible PHP or PHP understadable script

I have a joomla 2.5 Virtuemart 2 website. The payment gateway is a private gateway written in .NET with "Rijndael" algorithm which I do not understand. I need to be able to accept the encripted data passed over to my virtuemart and decript it to obtain the parameters and payment data. My problem is I tried various Rijndael script written in PHP but I can't get the same answer from the Rijndael script from .NET. There are 2 solutions you might provide :- Either 1) Try to understand the existing .NET script containing the Rijndael algo and re-write it in PHP as compatible script. I'll then copy the function into a new Virtuemart Payment plugin and decrypt the data. OR 2) I can host my Virtuemart on a .NET server, you job is to place the existing .NET rijndael file in it, and make it communicates with my Virtuemart payment plugin. Since I do not know how to make PHP understand the .NET thing... I just need to decrypt the encryted data passed over to me from the payment gateway. I prefer method no.1 above because I can host in Linux instead of .NET server.