Spoofer

The spoofer (referred to as the "lockpick" in the game files) is used by the UNSC forces to force open doors and override certain pieces of equipment by sending in an electrical current that stabilizes the door that may be locked, damaged, or otherwise unable to function.

Description
They are standard equipment to Marine fireteams when deployed into ground engagements. They appear to be very advanced, as they are capable of even overriding the Forerunner locks on doors in Installation 04.

In 2552, Private Kappus used one to open the door to the Flood containment facility on Installation 04. He was ordered by Captain Jacob Keyes to open the door that happened to be the only thing holding back the Flood. Unwillingly and unknowingly, Kappus unleashed the Flood on Installation 04, dooming him and the rest of his squad, with only some few survivors, including Sgt. Johnson.

Trivia

 * In the action horror film Aliens and the video game Aliens vs. Predator 2, one of the marines uses a device similar or identical to the spoofer. In fact, considering how most of the game is based of the aspects of the movie, it could have been inspired by the device in the movie.