minetest.override_item("default:chest_locked", {

on_rightclick = function(pos, node, clicker, itemstack, pointed_thing)

local name = clicker:get_player_name()

if minetest.is_protected(pos,name) then
minetest.chat_send_all(name,"You cannot use this shared chest, because you are not part of the protection.")
return itemstack
end

end

})