Two9A

For real though, the shortest license is probably the WTFPL:

0. You just DO WHAT THE FUCK YOU WANT TO.

Might’ve used it a couple of times myself.

“I do not know with what weapons World War III will be fought, but World War IV will be fought with sticks and stones.” - Albert Einstein.

Snopes says this sentiment was actually expressed by Einstein, though perhaps not in this exact phrasing.