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A permissive action link (PAL) is an
access control In the fields of physical security and information security, access control (AC) is the selective restriction of access to a place or other resource, while access management describes the process. The act of ''accessing'' may mean consuming ...
security device for
nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bomb ...
s. Its purpose is to prevent unauthorized arming or detonation of a nuclear weapon. The
United States Department of Defense The United States Department of Defense (DoD, USDOD or DOD) is an executive branch department of the federal government charged with coordinating and supervising all agencies and functions of the government directly related to national sec ...
definition is: 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.


History


Background

Permissive action links were developed in the United States in a gradual process from the first use of atomic weapons to the early 1960s. In 1953 the
United States Atomic Energy Commission The United States Atomic Energy Commission (AEC) was an agency of the United States government established after World War II by U.S. Congress to foster and control the peacetime development of atomic science and technology. President ...
and the
Department of Defense Department of Defence or Department of Defense may refer to: Current departments of defence * Department of Defence (Australia) * Department of National Defence (Canada) * Department of Defence (Ireland) * Department of National Defense (Philipp ...
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. Newer nuclear weapons were simpler in their operation and were produced ''en masse'' and were less cumbersome to arm and use than previous designs. Accordingly, new controls were necessary to prevent their unauthorized use. As the
Cold War The Cold War is a term commonly used to refer to a period of geopolitical tension between the United States and the Soviet Union and their respective allies, the Western Bloc and the Eastern Bloc. The term '' cold war'' is used because t ...
came to a head in the 1960s, the government felt it best not to leave the use of nuclear weapons in the hands of possibly-renegade generals, including the commander of
Strategic Air Command Strategic Air Command (SAC) was both a United States Department of Defense Specified Command and a United States Air Force (USAF) Major Command responsible for command and control of the strategic bomber and intercontinental ballistic missile ...
(SAC). Without Permissive Action Links, nuclear weapons were effectively under the independent command of a number of generals. In order to protect its
NATO The North Atlantic Treaty Organization (NATO, ; french: Organisation du traité de l'Atlantique nord, ), also called the North Atlantic Alliance, is an intergovernmental military alliance between 30 member states – 28 European and two N ...
allies, the United States had stationed various nuclear weapons overseas; these weapons were thus at least under the partial control of the hosting allied state. This was especially concerning to the
United States Congress The United States Congress is the legislature of the federal government of the United States. It is Bicameralism, bicameral, composed of a lower body, the United States House of Representatives, House of Representatives, and an upper body, ...
, as this lack of control was in violation of federal law. Added to this was the fact that some of the allies were considered potentially unstable—particularly
West Germany West Germany is the colloquial term used to indicate the Federal Republic of Germany (FRG; german: Bundesrepublik Deutschland , BRD) between its formation on 23 May 1949 and the German reunification through the accession of East Germany on 3 ...
and Turkey.Peter Stein, Peter Feaver: ''Assuring Control of Nuclear Weapons: The Evolution of Permissive Action Links''. University Press of America, Lanham 1989, . There was considerable concern that in one of these countries the instructions of the civilian leadership of the host country could overrule the military. In addition, the U.S. realized that in the event of war, parts of West Germany would be overwhelmed early on, and nuclear weapons stationed there could fall into the hands of the Soviet Union. For a long time the U.S. military resisted the use of PALs. It feared the loss of independence and feared malfunction, which could put warheads out of action in a time of crisis. But the advantages of PALs outweighed the disadvantages: thanks to the PALs weapons were able to be distributed to a greater extent in Europe, so as to prevent a rapid and selective destruction or conquest by the Soviet bloc, while still retaining U.S. control over the farther-flung weapons.


Development and dissemination

The precursors of permissive action links were simple mechanical
combination lock A combination lock is a type of locking device in which a sequence of symbols, usually numbers, is used to open the lock. The sequence may be entered using a single rotating dial which interacts with several discs or ''cams'', by using a set o ...
s that were set into the control systems of nuclear weapons, such as the Minuteman
ICBM An intercontinental ballistic missile (ICBM) is a ballistic missile with a range greater than , primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads). Conventional, chemical, and biological weapons ...
. There they could perform different functions: some blocked the cavity through which the nuclear materials were shot to create a reaction; other locks blocked circuits; and some simply prevented access to the control panel. For testing, some of these mechanisms were installed during 1959 in weapons stationed in Europe. The work on PAL prototypes remained at low levels until 1960. Sandia National Laboratories successfully created a number of new combination locks that were adaptable to different types of weapons. In the spring of 1961, there was a series of hearings in Congress, where Sandia presented the prototype of a special electro-mechanical lock, which was then known still as a "proscribed action link". The military leadership, however, soon realized that this term had negative connotations for the use of weapons by the officer corps ("proscribed" meaning "prohibited"), and changed the meaning of PAL to "permissive action link" ("permissive" meaning "allowing" or "tolerating"). In June 1962, President John F. Kennedy signed the National Security Action Memorandum number 160. This presidential directive ordered the installation of PALs in all U.S. nuclear weapons in Europe. All other U.S. nuclear weapons were excluded at that time. The conversion was completed in September 1962 and cost $23 million ($ in dollars). According to nuclear safety expert
Bruce G. Blair Bruce Gentry Blair (November 16, 1947 – July 19, 2020) was an American nuclear security expert, research scholar, national security expert, the author of articles and books on nuclear topics, and a television show producer. Education and backgro ...
, the US Air Force's
Strategic Air Command Strategic Air Command (SAC) was both a United States Department of Defense Specified Command and a United States Air Force (USAF) Major Command responsible for command and control of the strategic bomber and intercontinental ballistic missile ...
worried that in times of need the codes for the Minuteman ICBM force would not be available, so it decided to set the codes to 00000000 in all missile launch control centers. Blair said the missile launch checklists included an item confirming this combination until 1977. A 2014 article in ''
Foreign Policy A state's foreign policy or external policy (as opposed to internal or domestic policy) is its objectives and activities in relation to its interactions with other states, unions, and other political entities, whether bilaterally or through ...
'' said that the US Air Force told the
United States House Committee on Armed Services The U.S. House Committee on Armed Services, commonly known as the House Armed Services Committee or HASC, is a standing committee of the United States House of Representatives. It is responsible for funding and oversight of the Department of De ...
that "A code consisting of eight zeroes has never been used to enable a MM ICBM, as claimed by Dr. Bruce Blair." The complete conversion to PAL systems was relatively slow. In 1974, U.S. Defense Secretary James Schlesinger found that a variety of tactical nuclear weapons were still not fitted with permissive action links, even though the technology had been available for some time.Thomas C. Reed: ''At the Abyss: An Insider’s History of the Cold War.'' Presidio Press, New York 2005, . It took another two years until all the tactical nuclear weapons were fully equipped with PALs. In 1981, almost 20 years after the invention of PALs, just over half of U.S. nuclear weapons were still equipped only with mechanical locks. It took until 1987 until these were completely replaced.


Modernization and the present

Over the years the permissive action links have been continuously maintained and upgraded. In 2002, PALs on older B61 nuclear bombs were replaced and upgraded with new systems to improve reliability and security, as a part of extending the weapons' service lives to at least 2025. Code management system The year 1995 saw the development of the code management system (CMS). The CMS has simplified the control and logistics for staff and improved the flexibility and speed in deploying and arming weapons. New codes can be used to recode, lock, and manage the weapons, while the secrecy and validity of the possible launch orders is still ensured. In total, CMS consists of fourteen custom products (nine software and five hardware products).Hans M. Kristensen: ''U.S. Nuclear Weapons in Europe''. Natural Resources Defense Council, New York 2005, S. 20–21.
PDF; 4,9 MB
, accessed February 4, 2009).
The software products were developed by Sandia National Laboratories while the hardware was created by the
National Nuclear Security Administration The National Nuclear Security Administration (NNSA) is a United States federal agency responsible for safeguarding national security through the military application of nuclear science. NNSA maintains and enhances the safety, security, and ef ...
. The CMS was fully operational for the first time in November 2001. A part of the system, a special cryptographic processor fitted into the weapons in 1997 had a potential Year 2000 problem. By the spring of 2004, all PAL systems were equipped with the CMS. It is thus currently the general foundation for future hardware and software improvements to PALs.


Features

Permissive action link are powered by low-maintenance radioisotope generators. Instead of conventional batteries, these generators produce electricity based on the heat evolved from the radioactive decay of
plutonium-238 Plutonium-238 (238Pu or Pu-238) is a fissile, radioactive isotope of plutonium that has a half-life of 87.7 years. Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suit ...
. Although the half-life of 238Pu is 87.7 years, these generators have shorter lifespans. This is due to the pressurization of the generator from helium produced by the
alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...
of the plutonium fuel. PALs are also linked directly or indirectly with a number of security measures, which together form a comprehensive security package. In general, elements of PAL systems are located deep within the nuclear device. This makes it almost impossible to bypass the system. PAL devices have been installed on all nuclear devices in the US arsenal. The US Navy was last to receive them with all weapons fitted with PALs by 1996 or 1997.


Two-man rule

Modern PALs use the two-man rule, which is designed to prevent accidental or malicious launch of
nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bomb ...
s by a single individual. For example, on a
ballistic missile submarine A ballistic missile submarine is a submarine capable of deploying submarine-launched ballistic missiles (SLBMs) with nuclear warheads. The United States Navy's hull classification symbols for ballistic missile submarines are SSB and SSBN � ...
(SSBN), both the
commanding officer The commanding officer (CO) or sometimes, if the incumbent is a general officer, commanding general (CG), is the officer in command of a military unit. The commanding officer has ultimate authority over the unit, and is usually given wide latitu ...
(CO) and
executive officer An executive officer is a person who is principally responsible for leading all or part of an organization, although the exact nature of the role varies depending on the organization. In many militaries and police forces, an executive officer, o ...
(XO) must agree that the order to launch is valid, and then mutually authorize the launch with their operations personnel. Instead of another party confirming a missile launch as in the case of land-based
ICBM An intercontinental ballistic missile (ICBM) is a ballistic missile with a range greater than , primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads). Conventional, chemical, and biological weapons ...
s, the set of
keys Key or The Key may refer to: Common meanings * Key (cryptography), a piece of information that controls the operation of a cryptography algorithm * Key (lock), device used to control access to places or facilities restricted by a lock * Key (m ...
is distributed among the key personnel on the submarine and kept in safes (each of these crew members has access only to his keys), some of which are locked by
combination lock A combination lock is a type of locking device in which a sequence of symbols, usually numbers, is used to open the lock. The sequence may be entered using a single rotating dial which interacts with several discs or ''cams'', by using a set o ...
s. Nobody onboard has the combination to open these safes; the unlock key comes as a part of the launch order from the higher authority. In the case of Minuteman missile launch crews, both operators must agree that the launch order is valid by comparing the order's authorization code against a code from a "sealed authenticator" (a special sealed envelope that holds a code). The sealed authenticators are stored in a safe that has two separate locks so that a single crew member cannot open the safe alone. Both crew members must simultaneously turn the four launch keys. An additional safeguard is provided by requiring the crew in another launch control center to do the same for the missiles to be launched.


Stronglinks and weaklinks

Another part of the PAL design is the inclusion of "stronglinks" and "weaklinks". These ensure resilience to accidental activation through damage. The stronglinks include an increased ruggedness of some components and the inclusion of insensitive munitions so that they will not be circumvented by fire, vibration, or magnetic fields, leaving the PAL vulnerable to bypass after such damage. Also, activation-critical electronics within the weapon, such as capacitors, are selected so that they will fail before the safety device in the event of damage, ensuring that the weapon fails safe.


Critical signal detection

Nuclear weapons will only respond to a specific arming signal. This is passed to the weapon by a unique signal generator located outside the weapon. This output is specific and well-defined, precluding approximation, emulation, noise, or interference from being accepted as a false positive.Donald R. Cotter: "Peacetime Operations: Safety and Security." In: Ashton B. Carter, John D. Steinbruner, Charles A. Zraket (Eds.): ''Managing Nuclear Operations''. Brookings Institution Press, Washington, D.C. 1987, .


Environmental sensing device

An environmental sensing device (ESD) determines through environmental sensors whether the weapon is operating in its combat environment. For example, on an ICBM, a nuclear warhead would first be exposed to a strong acceleration, then a period of free fall and then further acceleration as the warhead reenters the atmosphere. The ESD determines the external parameters such as acceleration curve, temperature and pressure, and only arms the weapon when these environments are sensed in the correct order. ESDs are not exclusive to weapons equipped with PALs and some weapons, such as the W25, also had ESDs despite not being fitted with PALs.


Limited retry and non-violent disablement

Modern PALs are believed to feature a limited number of code reentries before the weapon locks out, requiring that the weapon be returned to
Pantex Pantex is the primary United States nuclear weapons assembly and disassembly facility that aims to maintain the safety, security and reliability of the U.S. nuclear weapons stockpile. The facility is located in the Panhandle of Texas on a site ...
for rebuilding. This system may also include a non-violent disablement system, where some of the weapon's internal components are destroyed to hamper use. This system may be part of the ordinary limited retry lockout system, or may be a feature that can be enabled if the local situation calls for it. The non-violent disablement system may also be part of the weapon's anti-intrusion system, designed to activate if someone tries to enter one of the weapon's exclusion regions such as for the purpose of circumventing the weapon's PAL.


Versions

Over the years the design and feature set of PALs has increased, as has the length of the access code. US-manufactured PALs are divided into five categories; however, the earliest PALs were never assigned a category letter.


Usage by other states

The increase in the number of nuclear-armed states was a similar cause for concern for the United States government for reasons similar to the original impetus for PALs. Thus, since the 1960s, the US has offered its own PAL technologies to other nuclear powers. The US considered this a necessary step: if the technology were kept secret, it would only be half as effective as possible, since the other power in a conflict might not have such safety measures. In the early 1970s, France was an early recipient of United States assistance on this critical element of nuclear security. The Nuclear Non-Proliferation Treaty (NPT) went into effect in 1970 and precluded treaty members (including the US) from directly disseminating technology related to nuclear weapons development or enhancement. In order to get around this prohibition, the US developed a legal trick: "negative guidance". French nuclear scientists would regularly brief US scientists on French developments in the field of PALs, and the US scientists would tell their French counterparts when they were not on the right track. In 1971, the US also offered its technology to the Soviet Union, which developed a similar system. In the early 1990s, the
People's Republic of China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
requested information to develop its own PALs. The Clinton administration believed that to do so would give too much information to the Chinese about American weapon design, and therefore, refused the request. Following the
dissolution of the Soviet Union The dissolution of the Soviet Union, also negatively connoted as rus, Разва́л Сове́тского Сою́за, r=Razvál Sovétskogo Soyúza, ''Ruining of the Soviet Union''. was the process of internal disintegration within the Sov ...
, Ukraine had on its territory the world's third largest nuclear weapons stockpile. While Ukraine had physical control of the weapons, it did not have operational control of the weapons as they were dependent on Russian-controlled electronic permissive action links and the Russian command-and-control system. In 1994 Ukraine agreed to the destruction of the weapons, and to join the NPT. In 2007, the
UK government ga, Rialtas a Shoilse gd, Riaghaltas a Mhòrachd , image = HM Government logo.svg , image_size = 220px , image2 = Royal Coat of Arms of the United Kingdom (HM Government).svg , image_size2 = 180px , caption = Royal Arms , date_est ...
revealed that its nuclear weapons were not equipped with permissive action links. Instead, the UK's nuclear bombs to be dropped by aircraft were armed by inserting a key into a simple lock similar to those used to protect bicycles from theft. The UK withdrew all air-launched bombs in 1998. Detailed information about PAL systems design and their use is classified, although these mechanisms have been offered to Pakistan for protection of their nuclear weapons.New York Times
''U.S. Secretly Aids Pakistan in Guarding Nuclear Arms''
, Accessed on February 4, 2009.
In the end, the US decided that it could not do so for legal reasons; the Pakistanis were also concerned that such technology would be sabotaged by a "kill-switch" that the US could operate. However, many experts in the field of nuclear technology in the US government supported the publication of the PAL system because they considered Pakistan's arsenal as the world's most vulnerable to abuse by terrorist groups. In November 2007, ''
The New York Times ''The New York Times'' (''the Times'', ''NYT'', or the Gray Lady) is a daily newspaper based in New York City with a worldwide readership reported in 2020 to comprise a declining 840,000 paid print subscribers, and a growing 6 million paid ...
'' revealed that the US had invested $100 million since 2001 in a secret program to protect Pakistan's nuclear arsenal. Instead of transferring PAL technology, the US provided helicopters, night vision and nuclear detection devices, as well as training to Pakistani personnel in order to prevent the theft or misuse of Pakistan's nuclear material, warheads, and laboratories.


See also

*
Emergency Action Message In the United States military's strategic nuclear weapon nuclear command and control (NC2) system, an Emergency Action Message (EAM) is a preformatted message that directs nuclear-capable forces to execute specific Major Attack Options (MAOs) o ...
* Insensitive munitions * Nuclear football * Special Weapons Emergency Separation System


References


Further reading

*


External links

* *
The JCAE and the Development of the Permissive Action Link

Keeping Presidents in the Nuclear Dark

Chapter 11: Nuclear Command and Control
Ross Anderson, * {{cite news, url=http://news.bbc.co.uk/media/avdb/news/uk/video/133000/bb/133417_16x9_bb.asx , title=BBC News (video) , access-date=April 29, 2010
''The Dr. Strangelove scenario''
- a BBC '' Newsnight'' report featuring the lack of security features on the WE.177 (streaming video) United States nuclear command and control Nuclear weapon safety Nuclear weapons Nuclear command and control