<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

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']