I was wondering if there is a good secure chat server that I can install on my Raspberry Pi that will run on multiple platform (Windows, Mac OSX, Android and iOS).

I wonder when people will start using google.
It even has a "GUI".

Okay I had success installing prosody. It's plain and simple enough however the documentation is not well organized for newbies like me.

You are asking to install a "secure chat server" on a raberry pi device.
And I also believe you are asking for a cross-platform client interface to this "secure chat server" running on a rasperry pi?

How about using a client "service" already installed and accessible by everyone?

http://www.chatcrypt.com or
http://www.chatcrypt.com/for_your_site/ may work for your "server"...

Looks pretty slick too.
I did a while connected there and ran an obvious test
on a "password", it didn't transmit in the clear even though http://www.chatcrypt.com is NOT running over https.

Have fun.

This is cool this is the first thing I heard of in the web that its free. However, I still believe that building up your own and having it hosted on my PI in which is hosting only for internal is still a lot secure. My idea was that each clients will need to use OpenVPN to connect to internal network and thus connect to XMPP server on Pi then activate OTR.

I am kinda new to setting this up but I find Prosody to be one of the easiest way to configure. I did try ejabberd but it seems to be not so straightforward as Prosody. My other thoughts would be if I were to open port 5222 publicly, what potential vulnerabilities would I should be expecting. Is it safe to open this port in the Net? But I guess chatcrypt is really cool as well worth checking.