Feature/parent node derivation (#35)

lib/money-tree/node.rb

@@ -86,6 +86,17 @@ def derive_private_key(i = 0)
       return child_private_key, child_chain_code
     end
 
+    def derive_parent_node(parent_key)
+      message = parent_key.public_key.to_bytes << i_as_bytes(index)
+      hash = hmac_sha512 hex_to_bytes(parent_key.chain_code_hex), message
+      priv = (private_key.to_i - left_from_hash(hash)) % MoneyTree::Key::ORDER
+      MoneyTree::Node.new(depth: parent_key.depth,
+                          index: parent_key.index,
+                          private_key: MoneyTree::PrivateKey.new(key: priv),
+                          public_key: parent_key.public_key,
+                          chain_code: parent_key.chain_code)
+    end
+
     def derive_public_key(i = 0)
       raise PrivatePublicMismatch if i >= PRIVATE_RANGE_LIMIT
       message = public_derivation_message(i)

spec/money-tree/node_spec.rb

@@ -1311,5 +1311,34 @@
         end
       end
     end
+
+    describe "deriving a parent node" do
+      before do
+        @master = MoneyTree::Master.new seed_hex: "000102030405060708090a0b0c0d0e0f"
+        @node = @master.node_for_path("m/101p")
+        @subnode = @node.node_for_path("1")
+      end
+      context "m/101'/1 -> m/101'" do
+        it "correctly derives from a subnode with priv key to a node knowing it's public key" do
+          node_priv_hex = @node.private_key.to_hex
+          @node.strip_private_info!
+          expect(@subnode.derive_parent_node(@node).private_key.to_hex).to eq(node_priv_hex)
+        end
+      end
+      context "m/101' -> master" do
+        it "unable to derive from a hardened node with priv key to a master node knowing it's public key" do
+          master_priv_hex = @master.private_key.to_hex
+          @master.strip_private_info!
+          expect(@node.derive_parent_node(@master).private_key.to_hex).to_not eq(master_priv_hex)
+        end
+      end
+      context "m/101 -> master" do
+        it "correctly derives from a non-hardened node with priv key to a master node knowing it's public key" do
+          @node = @master.node_for_path("m/101")
+          master_priv_hex = @master.private_key.to_hex
+          expect(@node.derive_parent_node(@master).private_key.to_hex).to eq(master_priv_hex)
+        end
+      end
+    end
   end
 end