code: def step1(self, username, password):
          
          self._check_initialized()
          context = AtvSRPContext(
              str(username), str(password),
              prime=constants.PRIME_2048,
              generator=constants.PRIME_2048_GEN)
          self.session = SRPClientSession(
              context, binascii.hexlify(self._auth_private).decode())
docs: First authentication step.