* @throws IllegalArgumentException if the user doesn't exist.
*/
public static void changeUserPassword(String newPassword, ParticipantId participantId,
AccountStore accountStore) throws PersistenceException, IllegalArgumentException {
PasswordDigest newPasswordDigest = new PasswordDigest(newPassword.toCharArray());
HumanAccountDataImpl account = new HumanAccountDataImpl(participantId, newPasswordDigest);
if (accountStore.getAccount(participantId) != null) {
accountStore.removeAccount(participantId);
accountStore.putAccount(account);
} else {
throw new IllegalArgumentException(String.format("User %s does not exist on this domain.",