swap: reserve multi-address key families

Reserve separate key families for static receive and change addresses.
This keeps derived keys out of the legacy static-address and HTLC key
streams.

swap/keychain.go

@@ -5,7 +5,16 @@ var (
 	// spending of the htlc.
 	KeyFamily = int32(99)
 
-	// StaticAddressKeyFamily is the key family used to generate static
-	// address keys.
+	// StaticAddressKeyFamily is the legacy static-address key family. It is
+	// used for the V0 single static-address key and for static-address HTLC
+	// keys.
 	StaticAddressKeyFamily = int32(42060)
+
+	// StaticMultiAddressKeyFamily is the key family used to generate
+	// externally visible multi-address static-address receive keys.
+	StaticMultiAddressKeyFamily = int32(42061)
+
+	// StaticAddressChangeKeyFamily is the key family used to generate
+	// static-address change outputs.
+	StaticAddressChangeKeyFamily = int32(42062)
 )

swap/keychain_test.go

@@ -0,0 +1,24 @@
+package swap
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestStaticAddressKeyFamiliesAreDisjoint documents the key-family split used
+// by static-address HTLC, receive and change key derivation.
+func TestStaticAddressKeyFamiliesAreDisjoint(t *testing.T) {
+	families := map[int32]string{
+		KeyFamily:                    "swap htlc",
+		StaticAddressKeyFamily:       "legacy static address and htlc",
+		StaticMultiAddressKeyFamily:  "multi-address receive",
+		StaticAddressChangeKeyFamily: "static-address change",
+	}
+
+	require.Len(t, families, 4)
+	require.EqualValues(t, 99, KeyFamily)
+	require.EqualValues(t, 42060, StaticAddressKeyFamily)
+	require.EqualValues(t, 42061, StaticMultiAddressKeyFamily)
+	require.EqualValues(t, 42062, StaticAddressChangeKeyFamily)
+}