Actually 00000000, according to Bruce Blair's first-hand account. He has been one of the go-to authors on nuclear weapons control issues for many years. (Received a MacArthur Prize for it, in fact.)<p>See <a href="http://www.cdi.org/blair/permissive-action-links.cfm" rel="nofollow">http://www.cdi.org/blair/permissive-action-links.cfm</a>.