code: def generate(bits, progress_func=None):
          
          key = rsa.generate_private_key(
              public_exponent=65537, key_size=bits, backend=default_backend()
          )
          return RSAKey(key=key)
docs: Generate a new private RSA key.  This factory function can be used to
          generate a new host key or authentication key.
  
          :param int bits: number of bits the generated key should be.
          :param progress_func: Unused
          :return: new `.RSAKey` private key