# File lib_core/rucy/passwd.rb, line 37
    def add_user(user, password)
      if (! user || user.strip.empty?) then
        raise 'not allowed empty user.'
      end
      if (! password || password.empty?) then
        raise 'not allowd empty password.'
      end
      @pw_db[user] = @pw_enc.crypt(password)
      nil
    end