Permissive Action Link

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A Permissive Action Link is a security device for nuclear weapons. The United States Department of Defense definition is:

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A device included in or attached to a nuclear weapon system to preclude arming and/or launching until the insertion of a prescribed discrete code or combination. It may include equipment and cabling external to the weapon or weapon system to activate components within the weapon or weapon system.

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PAL devices weren't primarily installed to prevent any terrorist acts as many people may think. The first reason were military commanders who could use the nuclear weapons without authorization.

The earliest PALs were little more than locks introduced into the control systems of a nuclear weapon, that would inhibit either the detonation, or the removal of safety features of the weapon. Modern PAL devices are more complex and their detailed principles of operation are classified. There are theories that insertion of the right code closes the arming and firing circuits, removes safety mechanism and decrypts the timing data needed to initiate proper detonation sequence.

These devices are buried deeply inside the weapon so they cannot be bypassed without complete weapon disassembly. They are still designed as electronic combination locks with limited retry. If too many wrong codes are inserted or any unauthorized tampering is detected the weapon can become permanently disabled. It is speculated that some weapons include small explosive charge which can disable the core of a nuclear weapon, requiring a specialized workshop to repair the weapon. Modern weapon designs probably use non-destructive method to disable a weapon.

These combination locks were installed on Minuteman ICBMs in the 1960s. However, the Strategic Air Command in Omaha worried that in case of need the codes would not be available, so they quietly decided to set them to 00000000; checking this combination was even present on the launch checklists. This was not changed until 1977.^[1] In 2007 the British Government revealed that its Nuclear Weapons were not equipped with Permissive Action Links. ^[2].

PAL devices were also installed on Sergeant, Pershing, and WAC Corporal missiles as well as the Nike-Hercules, Honest John rockets, the Davy Crockett system, and 155mm Howitzer, 8 inch howitzer rounds.

The PAL or PAL-like devices have been portrayed in various settings by the entertainment industry, such as in the computer game Metal Gear Solid, the motion picture WarGames, and the television series 24 season 4.