Hi there,
I'll try to keep it as short as I can. I have 14+ years of experience in Web development and after reading your requirements I'm 101% confident I can help you with your task. Just to prove my coding skills, you can test me in any way you want, be it by solving a short but complicated task or answering questions. Moreover, my english is great so you can be sure that there will be no language barriers.
Regarding your project, I'll need access to the structure of your DB, especially to user tables. The best way to create this is with MongoDB, WebSockets (the server will be created with PHP) with a fallback to long pooling Ajax in case the user is using an old version of IE. On top of this, all messages will be encrypted with public private key. For key exchange we'll use Diffie–Hellman.
When you're available, ping me with more details so that we can take it further and get this done ASAP.
Looking forward to helping your reach your goals and build a great business relationship.
Regards,
Kalin Ross