tka: make rotation signatures use nested keyID

Duplicating this at each layer doesnt make any sense, and is another
invariant where things could go wrong.

Signed-off-by: Tom DNetto <tom@tailscale.com>

tka/sig.go

@@ -116,6 +116,27 @@ func (s NodeKeySignature) wrappingPublic() (pub ed25519.PublicKey, ok bool) {
 	}
 }
 
+// authorizingKeyID returns the KeyID of the key trusted by network-lock which authorizes
+// this signature.
+func (s NodeKeySignature) authorizingKeyID() (tkatype.KeyID, error) {
+	switch s.SigKind {
+	case SigDirect, SigCredential:
+		if len(s.KeyID) == 0 {
+			return tkatype.KeyID{}, errors.New("invalid signature: no keyID present")
+		}
+		return tkatype.KeyID(s.KeyID), nil
+
+	case SigRotation:
+		if s.Nested == nil {
+			return tkatype.KeyID{}, errors.New("invalid signature: rotation signature missing nested signature")
+		}
+		return s.Nested.authorizingKeyID()
+
+	default:
+		return tkatype.KeyID{}, fmt.Errorf("unhandled signature type: %v", s.SigKind)
+	}
+}
+
 // SigHash returns the cryptographic digest which a signature
 // is over.
 //

tka/sig_test.go

@@ -79,7 +79,6 @@ func TestSigNested(t *testing.T) {
 	// rotation key & embedding the original signature.
 	sig := NodeKeySignature{
 		SigKind: SigRotation,
-		KeyID:   k.ID(),
 		Pubkey:  nodeKeyPub,
 		Nested:  &nestedSig,
 	}
@@ -145,14 +144,13 @@ func TestSigNested_DeepNesting(t *testing.T) {
 
 	outer := nestedSig
 	var lastNodeKey key.NodePrivate
-	for i := 0; i < 100; i++ {
+	for i := 0; i < 15; i++ { // 15 = max nesting level for CBOR
 		lastNodeKey = key.NewNode()
 		nodeKeyPub, _ := lastNodeKey.Public().MarshalBinary()
 
 		tmp := outer
 		sig := NodeKeySignature{
 			SigKind: SigRotation,
-			KeyID:   k.ID(),
 			Pubkey:  nodeKeyPub,
 			Nested:  &tmp,
 		}
@@ -166,6 +164,16 @@ func TestSigNested_DeepNesting(t *testing.T) {
 		t.Fatalf("verifySignature(lastNodeKey) failed: %v", err)
 	}
 
+	// Test this works with our public API
+	a, _ := Open(newTestchain(t, "G1\nG1.template = genesis",
+		optTemplate("genesis", AUM{MessageKind: AUMCheckpoint, State: &State{
+			Keys:               []Key{k},
+			DisablementSecrets: [][]byte{DisablementKDF([]byte{1, 2, 3})},
+		}})).Chonk())
+	if err := a.NodeKeyAuthorized(lastNodeKey.Public(), outer.Serialize()); err != nil {
+		t.Errorf("NodeKeyAuthorized(lastNodeKey) failed: %v", err)
+	}
+
 	// Test verification fails if the inner signature is invalid
 	tmp := make([]byte, ed25519.SignatureSize)
 	copy(tmp, nestedSig.Signature)
@@ -206,7 +214,6 @@ func TestSigCredential(t *testing.T) {
 	// delegated key & embedding the original signature.
 	sig := NodeKeySignature{
 		SigKind: SigRotation,
-		KeyID:   k.ID(),
 		Pubkey:  nodeKeyPub,
 		Nested:  &nestedSig,
 	}

tka/tka.go

@@ -686,7 +686,12 @@ func (a *Authority) NodeKeyAuthorized(nodeKey key.NodePublic, nodeKeySignature t
 		return errors.New("credential signatures cannot authorize nodes on their own")
 	}
 
-	key, err := a.state.GetKey(decoded.KeyID)
+	kID, err := decoded.authorizingKeyID()
+	if err != nil {
+		return err
+	}
+
+	key, err := a.state.GetKey(kID)
 	if err != nil {
 		return fmt.Errorf("key: %v", err)
 	}