getProvidedPassphrases() {
    var passphrases = new goog.structs.Map();
    this.forEachChild(function(chip) {
      if (chip.isPassphrase()) {
        passphrases.set(chip.getValue(), true);
      }
    });

    return passphrases.getKeys();
  }
Returns a list with the user-provided passphrases (for symmetric
encryption).
@return {!Array.<string>} A list with the provided passphrases.