docs: def _get_rsa_key(self):
          ''' get steam RSA key, build and return cipher '''
          url = 'https://steamcommunity.com/mobilelogin/getrsakey/'
          values = {
                  'username': self._username,
                  'donotcache' : self._get_donotcachetime(),
          }
          req = self.post(url, data=values)
          data = req.json()
          if not data['success']:
              raise SteamWebError('Failed to get RSA key', data)
          # Construct RSA and cipher
          mod = int(str(data['publickey_mod']), 16)
          exp = int(str(data['publickey_exp']), 16)
          rsa = RSA.construct((mod, exp))
          self.rsa_cipher = PKCS1_v1_5.new(rsa)
          self.rsa_timestamp = data['timestamp']