Looking for directions. Unity Web and Android Login system

Hello all!. I've read almost everything available on how Unity comunicates with other sources, like gameservers and whatnot, but im kida lost in how to implement a multiplatform login system. Its really simple actually, my game, running in Web Browsers and Android/iOS, cant be played unless you login first. When you login, the game pulls an XML with your data to display some prefabs on the screen.

What i dont know is what should i be looking for. At first, it seemed that ExternalCall was the choice, but it doesnt work on Android, at least what i found on Unity Answers. Then, using the WWW class, but thats just for pulling assets, not login. Some people also post that when you use files that are not hosted on the same server as the WebPlayer build, the server needs some kind of custom permisology.

Can someone point me in the right direction? As i explained before, its rather simple, Login and pull and XML, or Object, or List, or JSon or whatever, a TXT. The system needs to be the same for WebPlayer Builds and Mobile Builds. Is this possible? What do i use?

Thanks in advance, and excuse me if i sound a bit desperate.

 

Basically if my post is not 4.3 Related i can basically go **** myself

 

why not socket?