def step3(self):
ios_device_x = hkdf_expand(<EOL>'<STR_LIT>',<EOL>'<STR_LIT>',<EOL>binascii.unhexlify(self._client_session_key))<EOL>self._session_key = hkdf_expand(<EOL>'<STR_LIT>',<EOL>'<STR_LIT>',<EOL>binascii.unhexlify(self._client_session_key))<EOL>device_info = ios_device_x + self.pairing_id + self._auth_public<EOL>device_signature = self._signing_key.sign(device_info)<EOL>tlv = tlv8.write_tlv({tlv8.TLV_IDENTIFIER: self.pairing_id,<EOL>tlv8.TLV_PUBLIC_KEY: self._auth_public,<EOL>tlv8.TLV_SIGNATURE: device_signature})<EOL>chacha = chacha20.Chacha20Cipher(self._session_key, self._session_key)<EOL>encrypted_data = chacha.encrypt(tlv, nounce='<STR_LIT>'.encode())<EOL>log_binary(_LOGGER, '<STR_LIT>', Encrypted=encrypted_data)<EOL>return encrypted_data<EOL>
Third pairing step.