watchtower/wtclient: use HD session key derivation

watchtower/wtclient/client.go

@@ -9,7 +9,6 @@ import (
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/lightningnetwork/lnd/input"
-	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb"
@@ -77,7 +76,7 @@ type Config struct {
 	// SecretKeyRing is used to derive the session keys used to communicate
 	// with the tower. The client only stores the KeyLocators internally so
 	// that we never store private keys on disk.
-	SecretKeyRing keychain.SecretKeyRing
+	SecretKeyRing SecretKeyRing
 
 	// Dial connects to an addr using the specified net and returns the
 	// connection object.
@@ -202,6 +201,7 @@ func New(config *Config) (*TowerClient, error) {
 	}
 	c.negotiator = newSessionNegotiator(&NegotiatorConfig{
 		DB:            cfg.DB,
+		SecretKeyRing: cfg.SecretKeyRing,
 		Policy:        cfg.Policy,
 		ChainHash:     cfg.ChainHash,
 		SendMessage:   c.sendMessage,
@@ -222,14 +222,25 @@ func New(config *Config) (*TowerClient, error) {
 	}
 
 	// Reload any towers from disk using the tower IDs contained in each
-	// candidate session.
+	// candidate session. We will also rederive any session keys needed to
+	// be able to communicate with the towers and authenticate session
+	// requests. This prevents us from having to store the private keys on
+	// disk.
 	for _, s := range c.candidateSessions {
 		tower, err := c.cfg.DB.LoadTower(s.TowerID)
 		if err != nil {
 			return nil, err
 		}
 
+		sessionPriv, err := DeriveSessionKey(
+			c.cfg.SecretKeyRing, s.KeyIndex,
+		)
+		if err != nil {
+			return nil, err
+		}
+
 		s.Tower = tower
+		s.SessionPrivKey = sessionPriv
 	}
 
 	// Finally, load the sweep pkscripts that have been generated for all

watchtower/wtclient/client_test.go

@@ -432,6 +432,7 @@ func newHarness(t *testing.T, cfg harnessCfg) *testHarness {
 		},
 		DB:            clientDB,
 		AuthDial:      mockNet.AuthDial,
+		SecretKeyRing: wtmock.NewSecretKeyRing(),
 		PrivateTower:  towerAddr,
 		Policy:        cfg.policy,
 		NewAddress: func() ([]byte, error) {

watchtower/wtclient/session_negotiator.go

@@ -42,6 +42,10 @@ type NegotiatorConfig struct {
 	// negotiated sessions.
 	DB DB
 
+	// SecretKeyRing allows the client to derive new session private keys
+	// when attempting to negotiate session with a tower.
+	SecretKeyRing SecretKeyRing
+
 	// Candidates is an abstract set of tower candidates that the negotiator
 	// will traverse serially when attempting to negotiate a new session.
 	Candidates TowerCandidateIterator
@@ -255,12 +259,23 @@ retryWithBackoff:
 			goto retryWithBackoff
 		}
 
+		towerPub := tower.IdentityKey.SerializeCompressed()
 		log.Debugf("Attempting session negotiation with tower=%x",
-			tower.IdentityKey.SerializeCompressed())
+			towerPub)
+
+		// Before proceeding, we will reserve a session key index to use
+		// with this specific tower. If one is already reserved, the
+		// existing index will be returned.
+		keyIndex, err := n.cfg.DB.NextSessionKeyIndex(tower.ID)
+		if err != nil {
+			log.Debugf("Unable to reserve session key index "+
+				"for tower=%x: %v", towerPub, err)
+			continue
+		}
 
 		// We'll now attempt the CreateSession dance with the tower to
 		// get a new session, trying all addresses if necessary.
-		err = n.createSession(tower)
+		err = n.createSession(tower, keyIndex)
 		if err != nil {
 			log.Debugf("Session negotiation with tower=%x "+
 				"failed, trying again -- reason: %v",
@@ -277,22 +292,21 @@ retryWithBackoff:
 // its stored addresses. This method returns after the first successful
 // negotiation, or after all addresses have failed with ErrFailedNegotiation. If
 // the tower has no addresses, ErrNoTowerAddrs is returned.
-func (n *sessionNegotiator) createSession(tower *wtdb.Tower) error {
+func (n *sessionNegotiator) createSession(tower *wtdb.Tower,
+	keyIndex uint32) error {
+
 	// If the tower has no addresses, there's nothing we can do.
 	if len(tower.Addresses) == 0 {
 		return ErrNoTowerAddrs
 	}
 
-	// TODO(conner): create with hdkey at random index
-	sessionPrivKey, err := btcec.NewPrivateKey(btcec.S256())
+	sessionPriv, err := DeriveSessionKey(n.cfg.SecretKeyRing, keyIndex)
 	if err != nil {
 		return err
 	}
 
-	// TODO(conner): write towerAddr+privkey
-
 	for _, lnAddr := range tower.LNAddrs() {
-		err = n.tryAddress(sessionPrivKey, tower, lnAddr)
+		err = n.tryAddress(sessionPriv, keyIndex, tower, lnAddr)
 		switch {
 		case err == ErrPermanentTowerFailure:
 			// TODO(conner): report to iterator? can then be reset
@@ -318,7 +332,7 @@ func (n *sessionNegotiator) createSession(tower *wtdb.Tower) error {
 // returns true if all steps succeed and the new session has been persisted, and
 // fails otherwise.
 func (n *sessionNegotiator) tryAddress(privKey *btcec.PrivateKey,
-	tower *wtdb.Tower, lnAddr *lnwire.NetAddress) error {
+	keyIndex uint32, tower *wtdb.Tower, lnAddr *lnwire.NetAddress) error {
 
 	// Connect to the tower address using our generated session key.
 	conn, err := n.cfg.Dial(privKey, lnAddr)
@@ -394,7 +408,8 @@ func (n *sessionNegotiator) tryAddress(privKey *btcec.PrivateKey,
 		clientSession := &wtdb.ClientSession{
 			TowerID:        tower.ID,
 			Tower:          tower,
-			SessionPrivKey: privKey, // remove after using HD keys
+			KeyIndex:       keyIndex,
+			SessionPrivKey: privKey,
 			ID:             sessionID,
 			Policy:         n.cfg.Policy,
 			SeqNum:         0,

watchtower/wtmock/client_db.go

@@ -109,7 +109,6 @@ func (m *ClientDB) CreateClientSession(session *wtdb.ClientSession) error {
 	m.activeSessions[session.ID] = &wtdb.ClientSession{
 		TowerID:          session.TowerID,
 		KeyIndex:         session.KeyIndex,
-		SessionPrivKey:   session.SessionPrivKey,
 		ID:               session.ID,
 		Policy:           session.Policy,
 		SeqNum:           session.SeqNum,