This invention describes a pickproof lock consisting of a rotating cylinder provided with two sets of opposing tumblers. The tumblers permit a very thin key to be inserted between them. The key is made of a ribbon material of the order of one-hundredth of an inch thick and the tumblers are displaced in both directions from the center so that manipulation of a lockpicking tool becomes very difficult, since the tool must be as thin as the key and each set of tumblers must be in the correct position for the cylinder in turn.