Griphook climbs out; the wizards follow. On his direction, Harry places the palm of his hand on the door and watches the goblin do the same. A sharp needle pricks his finger all of a sudden; Harry hisses in surprise. Touch and blood lock? Probably senses magic signatures through the skin, as well. Touch can probably be fooled by something like a Polyjuice Potion, maybe even a glamour. Blood... That’s a bit more complicated. It would take some serious ritual magic to mimic blood well enough to fool a goblin security system. The magic signature... As far as Harry knows, that can’t be impersonated.