ESP HTTPS OTA
=============

:link_to_translation:`zh_CN:[中文]`

Overview
--------

``esp_https_ota`` provides simplified APIs to perform firmware upgrades over HTTPS. It is an abstraction layer over the existing OTA APIs.

    .. code-block:: c

        esp_err_t do_firmware_upgrade()
        {
            esp_http_client_config_t config = {
                .url = CONFIG_FIRMWARE_UPGRADE_URL,
                .cert_pem = (char *)server_cert_pem_start,
            };
            esp_https_ota_config_t ota_config = {
                .http_config = &config,
            };
            esp_err_t ret = esp_https_ota(&ota_config);
            if (ret == ESP_OK) {
                esp_restart();
            } else {
                return ESP_FAIL;
            }
            return ESP_OK;
        }


Server Verification
-------------------

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.

.. note::

    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.

Partial Image Download over HTTPS
---------------------------------

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.

OTA Resumption
--------------

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.

Signature Verification
----------------------

For additional security, signature of OTA firmware images can be verified. For more information, please refer to :ref:`secure-ota-updates`.

.. _ota_updates_pre-encrypted-firmware:

OTA Upgrades with Pre-Encrypted Firmware
----------------------------------------

Pre-encrypted firmware is a completely independent scheme from :doc:`../../security/flash-encryption`. Primary reasons for this are as follows:

 * 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.

Design
^^^^^^

* This scheme requires a unique RSA-3072 public-private key pair to be generated first. The public key stays on the OTA update server for encryption purpose and the private key is part of the device (e.g., embedded in firmware) for decryption purpose.
* Pre-encrypted firmware is encrypted using AES-GCM key which is then appended to the image as header (along with config parameters).
* Further the AES-GCM key gets encrypted using RSA public key and the resultant image gets hosted on the OTA update server.
* On the device side, first the AES-GCM key is retrieved by decrypting the image header using RSA private key available to the device.
* Finally, the contents of the image are decrypted using AES-GCM key (and config parameters) and written to the flash storage.

This whole workflow is managed by an external component `esp_encrypted_image <https://github.com/espressif/idf-extra-components/blob/master/esp_encrypted_img>`_ and it gets plugged into the OTA update framework through decryption callback (:cpp:member:`esp_https_ota_config_t::decrypt_cb`) mechanism.

.. note::

    The supported scheme is based on RSA-3072 and the private key on device side must be protected using platform security features.

OTA System Events
-----------------

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.

:cpp:enum:`esp_https_ota_event_t` has all possible events that can occur when performing OTA upgrade using ESP HTTPS OTA.

Event Handler Example
^^^^^^^^^^^^^^^^^^^^^

    .. code-block:: c

        /* Event handler for catching system events */
        static void event_handler(void* arg, esp_event_base_t event_base,
                                int32_t event_id, void* event_data)
        {
            if (event_base == ESP_HTTPS_OTA_EVENT) {
                switch (event_id) {
                    case ESP_HTTPS_OTA_START:
                        ESP_LOGI(TAG, "OTA started");
                        break;
                    case ESP_HTTPS_OTA_CONNECTED:
                        ESP_LOGI(TAG, "Connected to server");
                        break;
                    case ESP_HTTPS_OTA_GET_IMG_DESC:
                        ESP_LOGI(TAG, "Reading Image Description");
                        break;
                    case ESP_HTTPS_OTA_VERIFY_CHIP_ID:
                        ESP_LOGI(TAG, "Verifying chip id of new image: %d", *(esp_chip_id_t *)event_data);
                        break;
                    case ESP_HTTPS_OTA_VERIFY_CHIP_REVISION:
                        ESP_LOGI(TAG, "Verifying chip revision of new image: %d", *(uint16_t *)event_data);
                        break;
                    case ESP_HTTPS_OTA_DECRYPT_CB:
                        ESP_LOGI(TAG, "Callback to decrypt function");
                        break;
                    case ESP_HTTPS_OTA_WRITE_FLASH:
                        ESP_LOGD(TAG, "Writing to flash: %d written", *(int *)event_data);
                        break;
                    case ESP_HTTPS_OTA_UPDATE_BOOT_PARTITION:
                        ESP_LOGI(TAG, "Boot partition updated. Next Partition: %d", *(esp_partition_subtype_t *)event_data);
                        break;
                    case ESP_HTTPS_OTA_FINISH:
                        ESP_LOGI(TAG, "OTA finish");
                        break;
                    case ESP_HTTPS_OTA_ABORT:
                        ESP_LOGI(TAG, "OTA abort");
                        break;
                }
            }
        }

Expected data type for different ESP HTTPS OTA events in the system event loop:

    - ESP_HTTPS_OTA_START                     : ``NULL``
    - ESP_HTTPS_OTA_CONNECTED                 : ``NULL``
    - ESP_HTTPS_OTA_GET_IMG_DESC              : ``NULL``
    - ESP_HTTPS_OTA_VERIFY_CHIP_ID            : ``esp_chip_id_t``
    - ESP_HTTPS_OTA_VERIFY_CHIP_REVISION      : ``uint16_t``
    - ESP_HTTPS_OTA_DECRYPT_CB                : ``NULL``
    - ESP_HTTPS_OTA_WRITE_FLASH               : ``int``
    - ESP_HTTPS_OTA_UPDATE_BOOT_PARTITION     : ``esp_partition_subtype_t``
    - ESP_HTTPS_OTA_FINISH                    : ``NULL``
    - ESP_HTTPS_OTA_ABORT                     : ``NULL``

Application Examples
--------------------

- :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`.

API Reference
-------------

.. include-build-file:: inc/esp_https_ota.inc
