A US Navy submarine $($hoo$-$yah!$)$ has received an order to launch a missile, but they need to be absolutely certain the order came from the President and not someone else trying to start a war before they launch. In order to guarantee the authenticity of the message, the President should have encrypted it with choose your answer...

If the submarine can decrypt it witbe choose your answer... $,$ they know it must have come from the President.