Permissive Action Link
A Permissive Action Link (PAL) is a U.S.-developed physical component of a nuclear weapon, specifically to prevent unauthorized activation by persons who have legitimate access to them, and by persons at intermediate levels in the chain of command over the weapon. They complement measures to prevent activation of the weapon by accidents of nature, and validation mechanisms within the command and control system.
It is unclear who both originated the idea and effectively shepherded through the government. The Joint Committee on Atomic Energy claimed it, there is evidence that the first idea came from Fred Ikle, at the RAND Corporation, in 1957-1958, although he did not advance the idea. Scientists at "Livermore, Los Alamos, and Sandia laboratories began investigating methods of controlling the use of nuclear weapons. Concepts were refined and a prototype built at Livermore was demonstrated in the fall of 1960 before a military audience in Washington and in December 1960 for incoming Secretary of Defense Robert S. McNamara. The military officers were unimpressed and considered the device redundant in light of what they considered adequate nonmechanical controls already in effect. McNamara's response is unknown, but apparently he did not consider it an urgent matter."  Congress can claim input, however, after an inspection trip in December 1960, showed European-based U.S. nuclear weapons in the possession of North Atlantic Treaty Organization (NATO) forces were under inadequate and possibly-illegal control (as defined by terms of the Atomic Energy Act of 1954). "The sight of fully-operational American weapons aboard NATO quick-reaction aircraft (QRA) manned by foreign pilots who were essentially free to "scramble" with or without proper authorization was enough to cause a renewed interest in a positive-control arming lock."
A system of systems
There is a continuum of safeguards, with U.S. weapons, from the legitimate authority for their use in the positive control National Command Authority, down to the mechanisms of the weapon. Command and control mechanisms are outside the scope of this article.
There is especially tight complementarity between PALs and Environmental Sensing Device (ESD). PALs prevent detonation without a positive human action. ESDs prevent detonation "until a series of events or forces peculiar to the warhead's delivery vehicle trajectory occur in a prescribed sequence , such as the sequential acceleration, deceleration, and thermal heating of a missile RV; the high acceleration and spin of an AFAP; or the changing barometric or hydrostatic pressures of air-dropped bombs or ship-launched ASW weapons." Even if the PAL were circumvented, a bomb, for example, with a barometric ESD would not detonate in its storage area. 
Less coupled to PALs, but important at the weapon level, are physical precautions such as the one-point safe criterion, insensitive high explosives, weak link-strong link and enhanced nuclear detonation safety, and fire-resistant pits.
There have been six general types of PAL. The multiple codes allow setting options such as the nuclear yield in weapons with the "Dial-a-yield" feature, but also things such as burst altitude. It is not always practical to give examples in the table, since many weapons had PAL upgrades during their service life.
|(none)||Mechanical combination lock||A single person may not have the complete combination||nuclear artillery shells|
|A||Four-digit, 10-position electromechanical coded switch||Most retired; enabled a single option||W28, W49, W50, and W52|
|B||Ground or airplane cabin-operable 4-digit coded switch||later version with limited try followed by lockout until reset||Gravity bombs|
|C||Single-code 6-digit switch||limited try followed by lockout|
|D||Multiple-code 6-digit switch||limited try followed by lockout||W80 for AGM-69 ALCM, gravity bombs, W79 and W82 artillery shells|
|F||Multiple-code 12-digit switch||limited try followed by lockout and disabling of weapon||B61 Mod 10 gravity bomb|
Since the PAL is a part of the weapon, each specific design will vary. It is generally agreed that their primary function is acting as switches in the high-voltage path interconnecting power supplies, high-voltage capacitors, timing components, and detonators.
- , The JCAE and the Development of the Permissive Action Link, The U.S. Nuclear Weapons Cost Study Project, Brookings Institution, August 1998
- Chuck Hansen (September 4, 1995), Part V: Arming & Fuzing: Techniques & Equipment, The Swords of Armageddon: U.S. Nuclear Weapons Development since 1945
- Steven Bellovin (21 October 2005), Permissive Action Links,Nuclear Weapons, and the History of Public Key Cryptography, Department of Computer Science, Columbia University