fun toggleMask(
    cursor: Buffer.UnsafeCursor,
    key: ByteArray,
  ) {
    var keyIndex = 0
    val keyLength = key.size
    do {
      val buffer = cursor.data
      var i = cursor.start
      val end = cursor.end
      if (buffer != null) {
        while (i < end) {
          keyIndex %= keyLength // Reassign to prevent overflow breaking counter.