Editing Permissive action link (section)

{{Short description|Access control device for nuclear weapons}}
{{Use mdy dates|date=December 2014}}
[[File:Pal controller.jpg|thumb|right|250px|UC1583 PAL controller (early 1990s), based on a commercial [[Compaq LTE]] laptop]]

A '''permissive action link''' ('''PAL''') is an [[access control]] security device for [[nuclear weapon]]s. Its purpose is to prevent unauthorized [[arming plug|arming]] or [[Nuclear chain reaction|detonation]] of a nuclear weapon.<ref>{{cite book |chapter-url=http://www.cl.cam.ac.uk/~rja14/Papers/SE-11.pdf |title=Security Engineering: A Guide to Building Dependable Distributed Systems |chapter=Nuclear Command and Control |publisher=Ross Anderson, University of Cambridge Computing Laboratory |access-date=April 29, 2010 |archive-date=February 19, 2011 |archive-url=https://web.archive.org/web/20110219030234/http://www.cl.cam.ac.uk/~rja14/Papers/SE-11.pdf |url-status=live }}</ref>
The [[United States Department of Defense]] definition is:

{{quote|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 [[Nuclear weapon design|safety features]]. More recent innovations have included [[Encryption|encrypting]] the firing parameters it is programmed with, which must be decrypted to properly detonate the [[warhead]], and [[anti-handling device|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.