watchtower/wtclient/interface: add SecretKeyRing iface

watchtower/wtclient/interface.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/lightningnetwork/lnd/brontide"
+	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
 	"github.com/lightningnetwork/lnd/watchtower/wtserver"
@@ -77,3 +78,11 @@ func AuthDial(localPriv *btcec.PrivateKey, netAddr *lnwire.NetAddress,
 
 	return brontide.Dial(localPriv, netAddr, dialer)
 }
+
+// SecretKeyRing abstracts the ability to derive HD private keys given a
+// description of the derivation path.
+type SecretKeyRing interface {
+	// DerivePrivKey derives the private key from the root seed using a
+	// key descriptor specifying the key's derivation path.
+	DerivePrivKey(loc keychain.KeyDescriptor) (*btcec.PrivateKey, error)
+}

watchtower/wtmock/keyring.go

@@ -0,0 +1,44 @@
+package wtmock
+
+import (
+	"sync"
+
+	"github.com/btcsuite/btcd/btcec"
+	"github.com/lightningnetwork/lnd/keychain"
+)
+
+// SecretKeyRing is a mock, in-memory implementation for deriving private keys.
+type SecretKeyRing struct {
+	mu   sync.Mutex
+	keys map[keychain.KeyLocator]*btcec.PrivateKey
+}
+
+// NewSecretKeyRing creates a new mock SecretKeyRing.
+func NewSecretKeyRing() *SecretKeyRing {
+	return &SecretKeyRing{
+		keys: make(map[keychain.KeyLocator]*btcec.PrivateKey),
+	}
+}
+
+// DerivePrivKey derives the private key for a given key descriptor. If
+// this method is called twice with the same argument, it will return the same
+// private key.
+func (m *SecretKeyRing) DerivePrivKey(
+	desc keychain.KeyDescriptor) (*btcec.PrivateKey, error) {
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if key, ok := m.keys[desc.KeyLocator]; ok {
+		return key, nil
+	}
+
+	privKey, err := btcec.NewPrivateKey(btcec.S256())
+	if err != nil {
+		return nil, err
+	}
+
+	m.keys[desc.KeyLocator] = privKey
+
+	return privKey, nil
+}