Permissive Action Link

A Permissive Action Link (PAL) is a security device for nuclear weapons. Its purpose is to prevent unauthorized arming or detonation of the nuclear weapon. 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, that would inhibit either the detonation, or the removal of safety features of the weapon. More recent innovations have included encrypted firing parameters, which must be decrypted to properly detonate the warhead, plus anti-tamper systems which intentionally mis-detonate the weapon, destroying it without giving rise to a nuclear explosion.

Permissive Action Links were developed in the United States over a gradual process from the first use of atomic weapons to the early 1960s. Importantly, in 1953 the United States Atomic Energy Commission and the Department of Defense signed the Missiles and Rockets Agreement, which paved the way for the development and implementation of PALs. Certain national laboratories, under the auspices of the AEC, would develop and produce nuclear weapons, while the responsibility for the use and deployment remained with the military. The laboratories were also free to conduct their own research in the field of arms control and security. The thinking behind this was that if the government would ever be interested in such a security device, the research and development of prototypes would already be well advanced. At the beginning of the 1960s, the desire for the usage of such a system grew for both political and technological reasons.

...
Wikipedia