HOW TO: Encrypt your instant messages with Gaim
You’re instant messaging your wife from the office and you’d rather the IT guys weren’t privy to the 
conversation. You’re IM’ing a potential investor about your stealth startup at the coffee shop. These are just two scenarios when you might want to keep your instant messages from prying eyes.
Using free, open source IM software, you can obscure the contents of your instant message conversations as they travel over the wire. Read on for the details on how to encrypt your sensitive IM’s with the excellent open source instant messenger client Gaim.
Here’s how this IM encryption setup works: Gaim uses public/private key pairs to scramble each message that it sends to your recipient, whose installation of Gaim de-scrambles that message and displays it to you. All this happens in the background with no intervention (except clicking on the button with the lock icon on it). Geek to Live: Encrypt your instant messages with Gaim - Lifehacker