Permissive action link

A permissive action link (PAL) is an access control security device for nuclear weapons. Its purpose is to prevent unauthorized arming or detonation of a nuclear weapon.^[1] The United States Department of Defense definition is:

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.

The earliest PALs were little more than locks introduced into the control and firing systems of a nuclear weapon, designed to prevent a person from detonating it or removing its safety features. More recent innovations have included encrypting the firing parameters it is programmed with, which must be decrypted to properly detonate the warhead, and anti-tamper systems which intentionally mis-detonate the weapon if its other security features are defeated, destroying it without giving rise to a nuclear explosion.