Please refer to :ref:`ESP-TLS: TLS Server Verification <esp_tls_server_verification>` for more information on server verification. The root certificate in PEM format needs to be provided to the :cpp:member:`esp_http_client_config_t::cert_pem` member.
The server-endpoint **root** certificate should be used for verification instead of any intermediate ones from the certificate chain. The reason is that the root certificate has the maximum validity and usually remains the same for a long period of time. Users can also use the :cpp:member:`esp_http_client_config_t::crt_bundle_attach` member for verification by the ``ESP x509 Certificate Bundle`` feature, which covers most of the trusted root certificates.
To use the partial image download feature, enable ``partial_http_download`` configuration in ``esp_https_ota_config_t``. When this configuration is enabled, firmware image will be downloaded in multiple HTTP requests of specified sizes. Maximum content length of each request can be specified by setting ``max_http_request_size`` to the required value.
This option is useful while fetching image from a service like AWS S3, where mbedTLS Rx buffer size (:ref:`CONFIG_MBEDTLS_SSL_IN_CONTENT_LEN`) can be set to a lower value which is not possible without enabling this configuration.
Default value of mbedTLS Rx buffer size is set to 16 KB. By using ``partial_http_download`` with ``max_http_request_size`` of 4 KB, size of mbedTLS Rx buffer can be reduced to 4 KB. With this configuration, memory saving of around 12 KB is expected.
To use the OTA resumption feature, enable the ``ota_resumption`` configuration in the :cpp:struct:`esp_https_ota_config_t`. When OTA resumption is enabled, an OTA image download which has failed previously can be resumed from its intermediate state instead of restarting the whole OTA process from the beginning. This is implemented using the HTTP partial range request feature.
To specify the point from where the image download should resume, you need to set the ``ota_image_bytes_written`` field in :cpp:struct:`esp_https_ota_config_t`. This value indicates the number of bytes already written to the OTA partition in the previous OTA attempt.
For reference, you can check the :example:`system/ota/advanced_https_ota`, which demonstrates OTA resumption. In this example, the intermediate OTA state is saved in NVS, allowing the OTA process to resume seamlessly from the last saved state and continue the download.
* Flash encryption scheme recommends using per-device unique encryption key that is internally generated. This makes pre-encryption of the firmware on OTA update server infeasible.
* Flash encryption scheme depends on the flash offset and generates different ciphertext for different flash offset. And hence it becomes difficult to manage different OTA update images based on the partition slots like ``ota_0``, ``ota_1`` etc.
* Even for devices where flash encryption is not enabled, it could be requirement that firmware image over OTA is still encrypted in nature.
Pre-encrypted firmware distribution ensures that the firmware image stays encrypted **in transit** from the server to the device (irrespective of the underlying transport security). First the pre-encrypted software layer will decrypt the firmware (received over network) on device and then re-encrypt the contents using platform flash encryption (if enabled) before writing to flash.
Pre-encrypted firmware is a **transport security scheme** that ensures firmware images remain encrypted **in transit** from the OTA server to the device (irrespective of the underlying transport security). This approach differs from :doc:`../../security/flash-encryption` in several key ways:
**Important Security Note**: Pre-encrypted firmware does not provide device-level security on its own. Once received, the firmware is decrypted on the device and stored according to the device's flash encryption configuration. For device-level security, flash encryption must be separately enabled.
This process is managed by the `esp_encrypted_img <https://github.com/espressif/idf-extra-components/tree/master/esp_encrypted_img>`_ component, which integrates with the OTA update framework via the decryption callback (:cpp:member:`esp_https_ota_config_t::decrypt_cb`).
For detailed information on the image format, key generation, and implementation details, refer to the `esp_encrypted_img component documentation <https://github.com/espressif/idf-extra-components/tree/master/esp_encrypted_img>`_.
ESP HTTPS OTA has various events for which a handler can be triggered by the :doc:`../system/esp_event` when the particular event occurs. The handler has to be registered using :cpp:func:`esp_event_handler_register`. This helps the event handling for ESP HTTPS OTA.
-:example:`system/ota/advanced_https_ota` demonstrates how to use the Advanced HTTPS OTA update functionality on {IDF_TARGET_NAME} using the `esp_https_ota` component's APIs. For the applicable SoCs, please refer to :example_file:`system/ota/advanced_https_ota/README.md`.
-:example:`system/ota/partitions_ota` demonstrates how to perform OTA updates for various partitions (app, bootloader, partition table, storage) using the `esp_https_ota` component's APIs.
-:example:`system/ota/simple_ota_example` demonstrates how to use the `esp_https_ota` component's APIs to support firmware upgrades through specific networking interfaces such as Ethernet or Wi-Fi Station on {IDF_TARGET_NAME}. For the applicable SoCs, please refer to :example_file:`system/ota/simple_ota_example/README.md`.
