tka: Use strict decoding settings, implement Unserialize()

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

tka/sig_test.go

@@ -7,6 +7,8 @@ package tka
 import (
 	"crypto/ed25519"
 	"testing"
+
+	"github.com/google/go-cmp/cmp"
 )
 
 func TestSigDirect(t *testing.T) {
@@ -32,3 +34,24 @@ func TestSigDirect(t *testing.T) {
 		t.Fatalf("verifySignature() failed: %v", err)
 	}
 }
+
+func TestSigSerializeUnserialize(t *testing.T) {
+	nodeKeyPub := []byte{1, 2, 3, 4}
+	pub, priv := testingKey25519(t, 1)
+	key := Key{Kind: Key25519, Public: pub, Votes: 2}
+	sig := NodeKeySignature{
+		SigKind: SigDirect,
+		KeyID:   key.ID(),
+		Pubkey:  nodeKeyPub,
+	}
+	sigHash := sig.sigHash()
+	sig.Signature = ed25519.Sign(priv, sigHash[:])
+
+	var decoded NodeKeySignature
+	if err := decoded.Unserialize(sig.Serialize()); err != nil {
+		t.Fatalf("Unserialize() failed: %v", err)
+	}
+	if diff := cmp.Diff(sig, decoded); diff != "" {
+		t.Errorf("unmarshalled version differs (-want, +got):\n%s", diff)
+	}
+}