diff --git a/watchtower/wtclient/interface.go b/watchtower/wtclient/interface.go
index df8f376e..5aef8619 100644
--- a/watchtower/wtclient/interface.go
+++ b/watchtower/wtclient/interface.go
@@ -23,6 +23,14 @@ type DB interface {
 	// LoadTower retrieves a tower by its tower ID.
 	LoadTower(uint64) (*wtdb.Tower, error)
 
+	// NextSessionKeyIndex reserves a new session key derivation index for a
+	// particular tower id. The index is reserved for that tower until
+	// CreateClientSession is invoked for that tower and index, at which
+	// point a new index for that tower can be reserved. Multiple calls to
+	// this method before CreateClientSession is invoked should return the
+	// same index.
+	NextSessionKeyIndex(uint64) (uint32, error)
+
 	// CreateClientSession saves a newly negotiated client session to the
 	// client's database. This enables the session to be used across
 	// restarts.
diff --git a/watchtower/wtmock/client_db.go b/watchtower/wtmock/client_db.go
index 5853c363..6ce73dbc 100644
--- a/watchtower/wtmock/client_db.go
+++ b/watchtower/wtmock/client_db.go
@@ -22,6 +22,9 @@ type ClientDB struct {
 	activeSessions map[wtdb.SessionID]*wtdb.ClientSession
 	towerIndex     map[towerPK]uint64
 	towers         map[uint64]*wtdb.Tower
+
+	nextIndex uint32
+	indexes   map[uint64]uint32
 }
 
 // NewClientDB initializes a new mock ClientDB.
@@ -31,6 +34,7 @@ func NewClientDB() *ClientDB {
 		activeSessions: make(map[wtdb.SessionID]*wtdb.ClientSession),
 		towerIndex:     make(map[towerPK]uint64),
 		towers:         make(map[uint64]*wtdb.Tower),
+		indexes:        make(map[uint64]uint32),
 	}
 }
 
@@ -118,6 +122,27 @@ func (m *ClientDB) CreateClientSession(session *wtdb.ClientSession) error {
 	return nil
 }
 
+// NextSessionKeyIndex reserves a new session key derivation index for a
+// particular tower id. The index is reserved for that tower until
+// CreateClientSession is invoked for that tower and index, at which point a new
+// index for that tower can be reserved. Multiple calls to this method before
+// CreateClientSession is invoked should return the same index.
+func (m *ClientDB) NextSessionKeyIndex(towerID uint64) (uint32, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if index, ok := m.indexes[towerID]; ok {
+		return index, nil
+	}
+
+	index := m.nextIndex
+	m.indexes[towerID] = index
+
+	m.nextIndex++
+
+	return index, nil
+}
+
 // CommitUpdate persists the CommittedUpdate provided in the slot for (session,
 // seqNum). This allows the client to retransmit this update on startup.
 func (m *ClientDB) CommitUpdate(id *wtdb.SessionID, seqNum uint16,