Keys: Allow easy sharing of access without commands

This code adds the key concept to minetest_game, and integrates it
with lockable nodes. Currently supported lockable items are the Steel
Door, the Steel Trapdoor, and the Locked Chest.

The goal of this modification is to introduce a fine-grained multi-
player permission system that is intuitive and usable without any
console or chat commands, and doesn't require extra privileges to
be granted or setup. Keys can also physically be conveyed to other
players, adding to gameplay and adding some personality that is
preferable to console commands or editing formspecs.

A skeleton key can be crafted with 1 gold ingot. Skeleton keys can
then be matched to a lockable node by right-clicking the skeleton
key on a lockable node, which changes the skeleton key to a "key".

Gold was chosen as it's currently a not-so very useful item, and
therefore it's likely that players have some, but aren't really
using it for any purpose.

This key can subsequently used by any player to open or access that
lockable node, including retrieving items from Locked Chests, or
putting items in them.

They key is programmed to fit only the particular locked node it is
programmed to. This is achieved by storing a secret value in both
key and locked node. If this secret value doesn't match, the key
will not open the locked node. This allows many keys to be created
for one chest or door, but a key will only fit one node ever. The
secrets are stored in node, and item meta for the key.

If a locked node is removed, all keys that opened it are no longer
valid. Even if a new door/chest is placed in exactly the same spot,
the old keys will no longer fit that node.

Keys can be smelted back in gold ingots if they are no longer useful.

The method of storing a secret in nodemeta and itemstackmeta is secure
as there is no way for the client to create new items on the server
with a particular secret metadata value. Even if you could possible
create such an itemstack on the client, the server does not ever read
itemstackmeta from a client package.

The patch adds an API that allows other nodes and nodes added by
mods to use the same keys as well. The method how to implement this
is described in game_api.txt. The mod should add 2 callbacks to it's
node definition. Example code is given.

Textures are from PixelBOX, thanks to Gambit.

mods/default/README.txt

@@ -177,6 +177,8 @@ Gambit (CC BY-SA 3.0):
   default_snow.png
   default_snow_side.png
   default_snowball.png
+  default_key.png
+  default_key_skeleton.png
 
 asl97 (CC BY-SA 3.0):
   default_ice.png

mods/default/crafting.lua

@@ -352,6 +352,13 @@ minetest.register_craft({
 	}
 })
 
+minetest.register_craft({
+	output = 'default:skeleton_key',
+	recipe = {
+		{'default:gold_ingot'},
+	}
+})
+
 minetest.register_craft({
 	output = 'default:chest',
 	recipe = {
@@ -781,6 +788,20 @@ minetest.register_craft({
 	recipe = "default:clay_lump",
 })
 
+minetest.register_craft({
+	type = 'cooking',
+	output = 'default:gold_ingot',
+	recipe = 'default:skeleton_key',
+	cooktime = 5,
+})
+
+minetest.register_craft({
+	type = 'cooking',
+	output = 'default:gold_ingot',
+	recipe = 'default:key',
+	cooktime = 5,
+})
+
 --
 -- Fuels
 --

mods/default/nodes.lua

@@ -1619,16 +1619,30 @@ local function get_locked_chest_formspec(pos)
 end
 
 local function has_locked_chest_privilege(meta, player)
-	local name = ""
 	if player then
 		if minetest.check_player_privs(player, "protection_bypass") then
 			return true
 		end
-		name = player:get_player_name()
-	end
-	if name ~= meta:get_string("owner") then
+	else
 		return false
 	end
+
+	-- is player wielding the right key?
+	local item = player:get_wielded_item()
+	if item:get_name() == "default:key" then
+		local key_meta = minetest.parse_json(item.get_metadata())
+		local secret = meta:get_string("key_lock_secret")
+		if secret ~= key_meta.secret then
+			return false
+		end
+
+		return true
+	end
+
+	if player:get_player_name() ~= meta:get_string("owner") then
+		return false
+	end
+
 	return true
 end
 
@@ -1748,6 +1762,41 @@ minetest.register_node("default:chest_locked", {
 		return itemstack
 	end,
 	on_blast = function() end,
+	on_key_use = function(pos, player)
+		local secret = minetest.get_meta(pos):get_string("key_lock_secret")
+		local itemstack = player:get_wielded_item()
+		local key_meta = minetest.parse_json(itemstack:get_metadata())
+
+		if secret ~= key_meta.secret then
+			return
+		end
+
+		minetest.show_formspec(
+			player:get_player_name(),
+			"default:chest_locked",
+			get_locked_chest_formspec(pos)
+		)
+	end,
+	on_skeleton_key_use = function(pos, player, newsecret)
+		local meta = minetest.get_meta(pos)
+		local owner = meta:get_string("owner")
+		local name = player:get_player_name()
+
+		-- verify placer is owner of lockable chest
+		if owner ~= name then
+			minetest.record_protection_violation(pos, name)
+			minetest.chat_send_player(name, "You do not own this chest.")
+			return nil
+		end
+
+		local secret = meta:get_string("key_lock_secret")
+		if secret == "" then
+			secret = newsecret
+			meta:set_string("key_lock_secret", secret)
+		end
+
+		return secret, "a locked chest", owner
+	end,
 })
 
 

mods/default/tools.lua

@@ -378,3 +378,75 @@ minetest.register_tool("default:sword_diamond", {
 	},
 	sound = {breaks = "default_tool_breaks"},
 })
+
+minetest.register_tool("default:skeleton_key", {
+	description = "Skeleton Key",
+	inventory_image = "default_key_skeleton.png",
+	groups = {key = 1},
+	on_place = function(itemstack, placer, pointed_thing)
+		if pointed_thing.type ~= "node" then
+			return itemstack
+		end
+
+		local pos = pointed_thing.under
+		local node = minetest.get_node(pos)
+
+		if not node then
+			return itemstack
+		end
+
+		local on_skeleton_key_use = minetest.registered_nodes[node.name].on_skeleton_key_use
+		if on_skeleton_key_use then
+			-- make a new key secret in case the node callback needs it
+			local random = math.random
+			local newsecret = string.format(
+				"%04x%04x%04x%04x",
+				random(2^16) - 1, random(2^16) - 1,
+				random(2^16) - 1, random(2^16) - 1)
+
+			local secret, _, _ = on_skeleton_key_use(pos, placer, newsecret)
+
+			if secret then
+				-- finish and return the new key
+				itemstack:take_item()
+				itemstack:add_item("default:key")
+				itemstack:set_metadata(minetest.write_json({
+					secret = secret
+				}))
+				return itemstack
+			end
+		end
+		return nil
+	end
+})
+
+minetest.register_tool("default:key", {
+	description = "Key",
+	inventory_image = "default_key.png",
+	groups = {key = 1, not_in_creative_inventory = 1},
+	stack_max = 1,
+	on_place = function(itemstack, placer, pointed_thing)
+		if pointed_thing.type ~= "node" then
+			return itemstack
+		end
+
+		local pos = pointed_thing.under
+		local node = minetest.get_node(pos)
+
+		if not node or node.name == "ignore" then
+			return itemstack
+		end
+
+		local ndef = minetest.registered_nodes[node.name]
+		if not ndef then
+			return itemstack
+		end
+
+		local on_key_use = ndef.on_key_use
+		if on_key_use then
+			on_key_use(pos, placer)
+		end
+
+		return nil
+	end
+})