Remember all those cold war movies where nuclear missile crews are frantically dialing in the secret codes sent by the White House to launch nuclear-tipped intercontinental ballistic missiles? Well, for two decades, all the Minuteman nuclear missiles in the US used the same eight-digit numeric passcode to enable their warheads: 00000000.

That fact, originally revealed in a column in 2004 by then-president of the Center for Defense Information Dr. Bruce G. Blair, a former US Air Force officer who manned Minuteman silos, was also mentioned in a paper by Steven M. Bellovin, a computer science professor at Columbia University who teaches security architecture. Both of these sources were cited this week in an article on the site Today I Found Out written by Karl Smallwood, as well as in an article in the UK's Daily Mail.

The codes, known as Permissive Action Links (PALs), were supposed to prevent the use of nuclear weapons—and the nuclear weapons under joint control with NATO countries in particular—without the authorization of the president of the United States. The need for such controls became clear during the 1963-1964 Cyprus crisis, when NATO members Turkey and Greece were reportedly seeking control of NATO nuclear weapons—to use on each other.

At least that's the way it was supposed to work, following an executive order from President John F. Kennedy. But at the time of the Cuban Missile Crisis, more than half of the missiles in Europe, including those in Turkey, lacked PAL controls. And while Secretary of Defense Robert McNamara directly oversaw the installation of PALs on the US-based ICBM arsenal, US Strategic Command generals almost immediately had the PAL codes all reset to 00000000 to ensure that the missiles were ready for use regardless of whether the president was available to give authorization.

Dr. Blair stated that that the not-so-secret code was hardly a secret from the crews manning the silos. "Our launch checklist in fact instructed us, the firing crew, to double-check the locking panel in our underground launch bunker to ensure that no digits other than zero had been inadvertently dialed into the panel," Blair noted.

Update: This article was updated with information from Steven Bellovin.