Card key

Snake can acquire Personal Area Network Cards on his mission throughout Shadow Moses Island. He needs them to open certain doors, which require different card numbers. Snake can collect from Cards one through seven, and each card is backwards compatible. This means that if Snake has Key Cards one, two, and three, his Level three Card will open Level one, two, and three doors. The Card works with Snake's own natural electrical field, transmitting data using the salts in his body as the transmission medium. The Big Shell also utilized PAN cards. During the Big Shell Incident, Otacon, while undercover as a chief architect of the Big Shell's security systems, was supposed to get PAN cards for the Bravo Team, but apparently failed to do so.

The Warehouse at Ponizovoje utilized a card key that had holes in it, called Punch cards. San Hieronymo's missile silo complex, during the early 1970s, utilized a similar key system, referred to as Key E, which were strips of paper with dots to get through magnetic locks.

Trivia
Hideo Kojima, the father of Metal Gear, originally wanted to force the player to use trial-and-error to figure out which Cards worked for which doors. He was inspired by the classic George Romero horror film Dawn of the Dead. In one of the scenes, one of the characters has to go through several keys in order to try and open a door with zombies approaching. Just as the zombies are about to attack, he finally picks out the right key and moves to safety. Kojima wanted to give players this sense of dread and urgency in his game. While he was able to implement this in Metal Gear and Metal Gear 2, he decided not to implement it in further games as it would become too "troublesome".