Is it a pen, or a super secret way to pass messages to... umm... err... someone?
Yeah it's not encrypted, but who writes hand written notes anymore? That more than anything else provides security that no amount of electronic hacking can crack.
Step 1: Pick your weapon of mass secrecy!
Pick your favorite disposable ballpoint pen. I chose opaque. Translucent didn't seem right for a spy message pen, too revealing.
The only important thing is that you can separate the ballpoint head / ink assembly from the body of the pen. This will be your secret message compartment. Although if you couldn't get it open you could use some sort of top secret telepathy technology to transfer your message into the essence of the pen itself!