Permissive Action Link (PAL) is a security device for nuclear weapons. Its purpose is to prevent unauthorized arming or detonation of the nuclear weapon.
An ArmsTech-designed PAL system was used as a safety device to regulate the release and launch of nuclear warheads at the nuclear disposal site on Shadow Moses Island. Two detonation codes had to be entered to enable control of the nukes, which were known only to ArmsTech president Kenneth Baker and DARPA Chief Donald Anderson.
In case of emergency, PAL was also prepared with a special card key that enabled emergency release and detonation of the nukes without inputting the codes. The system required three types of keys, but there was only one actual key. The key was made of a shape memory alloy that would change shape depending on the ambient temperature, essentially functioning as three keys in one. Each key configuration was inserted into one of three computer terminals, each displaying a symbol corresponding to the key's shape. The keys were input in the order of room temperature key, low temperature key, and high temperature key. After analyzing and registering each key, the tower device behind the terminal would descend into the control panel and lock inside. Being an emergency system, the card key could only be used on these three occasions. When the PAL was engaged, it would either arm or disarm the nuke, depending on its current state.
After both Donald Anderson and Kenneth Baker died during the Shadow Moses Incident, the rogue FOXHOUND unit had only obtained one of the PAL codes. Unable to discover the second code, FOXHOUND's commander Liquid Snake needed another way to activate the nuke. Leading Solid Snake to believe that the nuke had already been armed, he arranged for Snake to obtain the PAL card key and use it. The key was given to Snake by Meryl Silverburgh, who had earlier received it from Baker. Unaware of Liquid's plan, Snake and Meryl made their way to the underground maintenance base where Metal Gear REX was housed. Otacon eventually managed to hack into Baker's files and locate the information relating to the PAL card key and tell Snake about the information, including its shape memory alloy nature. Snake dropped the key into a drainage ditch after being shot at by Revolver Ocelot, but later retrieved it. Changing the shape of the key card as necessary, through cooling and heating, Snake engaged the PAL, unwittingly arming the nuclear missile and activating Metal Gear.
Behind the scenes
In Metal Gear Solid, the PAL key is obtained after Snake meets with Meryl in the nuclear warhead storage building.
After Snake drops the card key in the underground maintenance base, the method of retrieving it differs depending on what actions the player has taken during the game. Snake has to either: wade through the water in the drainage ditch to find it, taking damage to health due to contamination by radioactive waste; or locate and kill a rat that has swallowed the key. In the novelization, Snake dives into the water itself to search for the key among the debris and litter, and locates it before he becomes poisoned by the radioactivity.
While engaging the PAL system in the original game, the player must backtrack to the warehouse in order to cool the key, then return to the blast furnace later on to heat it. The symbol and color of the key in the item menu changes as its shape is altered. In the 2004 remake The Twin Snakes, a liquid nitrogen and steam pipe are located near to the control room, providing the player with a much quicker method of changing the key's shape. The Twin Snakes also displays the key's current configuration in the item window, whereas the player had to manually check the key's description within the item menu in the original game.