Key of Rassilon

The Key of Rassilon was a Time Lord artefact which permitted entrance to the Matrix. Time Lords were arrogant in the belief that it could not be stolen or copied; they were proven wrong in the second category when the Master used a copy of the Key to enter the Matrix. (TV: The Ultimate Foe)

The First Doctor once had a similar key to the Matrix. It was collected by Graak on a mission from the Master to free one of the Doctors' incarnations. (GAME: Destiny of the Doctors)