watchtower: extend and rename SecretKeyRing

watchtower/wtmock/keyring.go

@@ -42,3 +42,57 @@ func (m *SecretKeyRing) DerivePrivKey(
 
 	return privKey, nil
 }
+
+// DeriveKey attempts to derive an arbitrary key specified by the
+// passed KeyLocator. This may be used in several recovery scenarios,
+// or when manually rotating something like our current default node
+// key.
+//
+// NOTE: This is part of the wtclient.ECDHKeyRing interface.
+func (m *SecretKeyRing) DeriveKey(
+	keyLoc keychain.KeyLocator) (keychain.KeyDescriptor, error) {
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if key, ok := m.keys[keyLoc]; ok {
+		return keychain.KeyDescriptor{
+			KeyLocator: keyLoc,
+			PubKey:     key.PubKey(),
+		}, nil
+	}
+
+	privKey, err := btcec.NewPrivateKey(btcec.S256())
+	if err != nil {
+		return keychain.KeyDescriptor{}, err
+	}
+
+	m.keys[keyLoc] = privKey
+
+	return keychain.KeyDescriptor{
+		KeyLocator: keyLoc,
+		PubKey:     privKey.PubKey(),
+	}, nil
+}
+
+// ECDH performs a scalar multiplication (ECDH-like operation) between the
+// target key descriptor and remote public key. The output returned will be the
+// sha256 of the resulting shared point serialized in compressed format. If k is
+// our private key, and P is the public key, we perform the following operation:
+//
+//  sx := k*P
+//  s := sha256(sx.SerializeCompressed())
+//
+// NOTE: This is part of the wtclient.ECDHKeyRing interface.
+func (m *SecretKeyRing) ECDH(keyDesc keychain.KeyDescriptor,
+	pub *btcec.PublicKey) ([32]byte, error) {
+
+	_, err := m.DeriveKey(keyDesc.KeyLocator)
+	if err != nil {
+		return [32]byte{}, err
+	}
+
+	privKey := m.keys[keyDesc.KeyLocator]
+	ecdh := &keychain.PrivKeyECDH{PrivKey: privKey}
+	return ecdh.ECDH(pub)
+}