code: @VisibleForTesting
      String obfuscateConfidentialData(final CharSequence confidentialSequence, @Nullable final CharSequence unmasked) {
          final int maskedLength = unmasked == null ? confidentialSequence.length() : confidentialSequence.length() - unmasked.length();
          return new String(new char[maskedLength]).replace('\0', PAD_CHAR);
      }
docs: Get a mask string for masking the given `confidentialSequence`.
  
  @param confidentialSequence the string to be obfuscated
  @param unmasked             the section of `confidentialSequence` to be left unmasked
  @return a mask string