tka: validate key after UpdateKey before applying state

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

tka/state.go

@@ -29,8 +29,6 @@ type State struct {
 
 	// DisablementSecrets are KDF-derived values which can be used
 	// to turn off the TKA in the event of a consensus-breaking bug.
-	// An AUM of type DisableNL should contain a secret when results
-	// in one of these values when run through the disablement KDF.
 	//
 	// TODO(tom): This is an alpha feature, remove this mechanism once
 	//            we have confidence in our implementation.
@@ -169,6 +167,9 @@ func (s State) applyVerifiedAUM(update AUM) (State, error) {
 		if update.Meta != nil {
 			k.Meta = update.Meta
 		}
+		if err := k.StaticValidate(); err != nil {
+			return State{}, fmt.Errorf("updated key fails validation: %v", err)
+		}
 		out := s.cloneForUpdate(&update)
 		for i := range out.Keys {
 			if bytes.Equal(out.Keys[i].ID(), update.KeyID) {

tka/state_test.go

@@ -181,6 +181,7 @@ func TestApplyUpdatesChain(t *testing.T) {
 }
 
 func TestApplyUpdateErrors(t *testing.T) {
+	tooLargeVotes := uint(99999)
 	tcs := []struct {
 		Name    string
 		Updates []AUM
@@ -205,6 +206,12 @@ func TestApplyUpdateErrors(t *testing.T) {
 			State{},
 			ErrNoSuchKey,
 		},
+		{
+			"UpdateKey now fails validation",
+			[]AUM{{MessageKind: AUMUpdateKey, KeyID: []byte{1}, Votes: &tooLargeVotes}},
+			State{Keys: []Key{{Kind: Key25519, Public: []byte{1}}}},
+			errors.New("updated key fails validation: excessive key weight: 99999 > 4096"),
+		},
 		{
 			"Bad lastAUMHash",
 			[]AUM{