This is similar to an app I wrote for fun to learn some nodejs, http://www.selinked.com. I think it's pretty secure but I am no encryption expert. Main difference is that I store nothing but the encrypted messages and for the chat and group chat nothing at all. You can also change the type of encryption it uses, all js browser side. Group chat and chat are half baked, there are a still few bugs in there. Would love some feedback on it.