rc4 decrypt class interaction with streaming data

[TacTiXs]

Hi everyone,
yes i'm a noob in php who needs real help here

Here's my prob; i have installed a php-nuke cms website, not the latest version but a quite advanced one already (6.9).
Now the registration procedure for that cms is quite simple and i want to use a more secure and unique method for registering users.
A friend is creating an application that will be client side.
The application is used to replace the registering procedure, which will work as a normal registering form and will send the user data straight to the registering page on the php-nuke cms. This to prevent users to register directly on the cms.
The reason why i want to use a client side app is because i need a guid to make all my users unique.

First hick-up here is, the app is gonna encrypt the data with rc4 encryption and send details over internet in encrypted form. Since the cms doesnt support rc4 i need to embed a decrypt class in the registering page that will interact with the sent data from the app and decrypt it before it hits the input fields from the registering page on the cms itself.

Now, i have the rc4 encrypt/decrypt code but i don't have a clue how to put this code on the registering page and make sure the sent data is first of all decrypted and after it's been decrypted it's being forwarded to the input fields of the regisitering form on the cms.

i Have attached the registering page to this post as well as the rc4 crypt class.
Thx forward to anyone that is willing to help me on this.