If you need to have an encrypted, private chat but don't have the tools handy to do so, you can just use Cryptocat. While anyone will be able to enter the chatroom if they know its name, nobody without your secret key will be able to see what you're saying.
To get started, you choose a chatroom name on the Cryptocat site and then share that name with anyone you want to talk to. You'll also need a secret key that both you and anyone else in the conversation knows. Cryptocat chatrooms feature a little blue bar at the bottom where you can type that key. If your key matches the key of someone else, your chat messages will be revealed to one another. If not, they'll just show up as "encrypted." Aside from its usefulness, everything is wonderfully 8-bit. You can even turn on chiptune chat alerts (although they only seem to work in Firefox and not Chrome). If you need to have a private, text-based conversation practically anywhere, Cryptocat makes the process really simple.