watchtower/wtdb: only accept properly sized blobs

Modifies the bbolt and mock tower databases to only accept blobs that
are the expected size of the session's blob type. This prevents resource
exhaustion attacks where a client may provide disproportionately large
encrypted blob, even though all supported blob types are of fixed-size.

watchtower/wtdb/tower_db.go

@@ -46,6 +46,10 @@ var (
 	// ErrNoSessionHintIndex signals that an active session does not have an
 	// initialized index for tracking its own state updates.
 	ErrNoSessionHintIndex = errors.New("session hint index missing")
+
+	// ErrInvalidBlobSize indicates that the encrypted blob provided by the
+	// client is not valid according to the blob type of the session.
+	ErrInvalidBlobSize = errors.New("invalid blob size")
 )
 
 // TowerDB is single database providing a persistent storage engine for the
@@ -233,6 +237,13 @@ func (t *TowerDB) InsertStateUpdate(update *SessionStateUpdate) (uint16, error)
 			return err
 		}
 
+		// Assert that the blob is the correct size for the session's
+		// blob type.
+		expBlobSize := blob.Size(session.Policy.BlobType)
+		if len(update.EncryptedBlob) != expBlobSize {
+			return ErrInvalidBlobSize
+		}
+
 		// Validate the update against the current state of the session.
 		err = session.AcceptUpdateSequence(
 			update.SeqNum, update.LastApplied,