Add client method for initializing from root metadata

This will allow users of go-tuf to initialize a client
without requiring a remote connection. This is useful
for when a root has been updated and the local root
verification keys can no longer verify the the latest
remote root, except by verifying the chain with
client.Update().

Ref theupdateframework#208

Signed-off-by: Hayden Blauzvern <[email protected]>

client/client.go

@@ -147,6 +147,20 @@ func (c *Client) Init(rootKeys []*data.PublicKey, threshold int) error {
 	return c.local.SetMeta("root.json", rootJSON)
 }
 
+// InitLocal initializes a local repository from root metadata.
+//
+// The root's keys are extracted from the root and saved in local storage.
+// Root expiration is not checked.
+// It is expected that rootJSON was securely distributed with the software
+// being updated.
+func (c *Client) InitLocal(rootJSON []byte) error {
+	err := c.loadAndVerifyRootMeta(rootJSON, true /*ignoreExpiredCheck*/)
+	if err != nil {
+		return err
+	}
+	return c.local.SetMeta("root.json", rootJSON)
+}
+
 // Update downloads and verifies remote metadata and returns updated targets.
 // It always performs root update (5.2 and 5.3) section of the v1.0.19 spec.
 //
@@ -430,6 +444,12 @@ func (c *Client) loadAndVerifyLocalRootMeta(ignoreExpiredCheck bool) error {
 	if !ok {
 		return ErrNoRootKeys
 	}
+	return c.loadAndVerifyRootMeta(rootJSON, ignoreExpiredCheck)
+}
+
+// loadAndVerifyRootMeta decodes and verifies root metadata and loads the top-level keys.
+// This method first clears the DB for top-level keys and then loads the new keys.
+func (c *Client) loadAndVerifyRootMeta(rootJSON []byte, ignoreExpiredCheck bool) error {
 	// unmarshal root.json without verifying as we need the root
 	// keys first
 	s := &data.Signed{}

client/client_test.go

@@ -280,6 +280,50 @@ func (s *ClientSuite) TestInit(c *C) {
 	c.Assert(err, IsNil)
 }
 
+func (s *ClientSuite) TestInitLocalAllowsExpired(c *C) {
+	s.genKeyExpired(c, "targets")
+	c.Assert(s.repo.Snapshot(), IsNil)
+	c.Assert(s.repo.Timestamp(), IsNil)
+	c.Assert(s.repo.Commit(), IsNil)
+	s.syncRemote(c)
+	client := NewClient(MemoryLocalStore(), s.remote)
+	bytes, err := io.ReadAll(s.remote.meta["root.json"])
+	c.Assert(err, IsNil)
+	s.withMetaExpired(func() {
+		c.Assert(client.InitLocal(bytes), IsNil)
+	})
+}
+
+func (s *ClientSuite) TestInitLocal(c *C) {
+	client := NewClient(MemoryLocalStore(), s.remote)
+	bytes, err := io.ReadAll(s.remote.meta["root.json"])
+	c.Assert(err, IsNil)
+	dataSigned := &data.Signed{}
+	c.Assert(json.Unmarshal(bytes, dataSigned), IsNil)
+	root := &data.Root{}
+	c.Assert(json.Unmarshal(dataSigned.Signed, root), IsNil)
+
+	// check Update() returns ErrNoRootKeys when uninitialized
+	_, err = client.Update()
+	c.Assert(err, Equals, ErrNoRootKeys)
+
+	// check InitLocal() returns ErrInvalid when the root's signature is
+	// invalid
+	// modify root and marshal without regenerating signatures
+	root.Version = root.Version + 1
+	rootBytes, err := json.Marshal(root)
+	c.Assert(err, IsNil)
+	dataSigned.Signed = rootBytes
+	dataBytes, err := json.Marshal(dataSigned)
+	c.Assert(err, IsNil)
+	c.Assert(client.InitLocal(dataBytes), Equals, verify.ErrInvalid)
+
+	// check Update() does not return ErrNoRootKeys after initialization
+	c.Assert(client.InitLocal(bytes), IsNil)
+	_, err = client.Update()
+	c.Assert(err, IsNil)
+}
+
 func (s *ClientSuite) TestFirstUpdate(c *C) {
 	files, err := s.newClient(c).Update()
 	c.Assert(err, IsNil)