ESP HTTPS OTA

Overview

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

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 ESP-TLS: TLS Server Verification for more information on server verification. The root certificate in PEM format needs to be provided to the 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 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 (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.

Signature Verification

For additional security, signature of OTA firmware images can be verified. For more information， please refer to Secure OTA Updates Without Secure Boot.

OTA Upgrades with Pre-Encrypted Firmware

Pre-encrypted firmware is a completely independent scheme from 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 and it gets plugged into the OTA update framework through decryption callback (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 Event Loop Library when the particular event occurs. The handler has to be registered using esp_event_handler_register(). This helps the event handling for ESP HTTPS OTA.

esp_https_ota_event_t has all the events which can happen when performing OTA upgrade using ESP HTTPS OTA.

Event Handler Example

/* 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_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_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

system/ota/pre_encrypted_ota demonstrates how to perform OTA updates with pre-encrypted binary using the esp_encrypted_img component's APIs and tool, ensuring the confidentiality of the firmware on the network channel, but not its authenticity. To perform OTA upgrades with pre-encrypted firmware, please enable CONFIG_ESP_HTTPS_OTA_DECRYPT_CB in component menuconfig.
system/ota/advanced_https_ota demonstrates how to use the Advanced HTTPS OTA update functionality on ESP32 using the esp_https_ota component's APIs. For the applicable SoCs, please refer to system/ota/advanced_https_ota/README.md.
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 ESP32. For the applicable SoCs, please refer to system/ota/simple_ota_example/README.md.

API Reference

Header File

components/esp_https_ota/include/esp_https_ota.h
This header file can be included with:
#include "esp_https_ota.h"
This header file is a part of the API provided by the esp_https_ota component. To declare that your component depends on esp_https_ota, add the following to your CMakeLists.txt:
REQUIRES esp_https_ota
or
PRIV_REQUIRES esp_https_ota

Functions

esp_err_t esp_https_ota(const esp_https_ota_config_t *ota_config)

HTTPS OTA Firmware upgrade.

This function allocates HTTPS OTA Firmware upgrade context, establishes HTTPS connection, reads image data from HTTP stream and writes it to OTA partition and finishes HTTPS OTA Firmware upgrade operation. This API supports URL redirection, but if CA cert of URLs differ then it should be appended to cert_pem member of ota_config->http_config.

Note

This API handles the entire OTA operation, so if this API is being used then no other APIs from esp_https_ota component should be called. If more information and control is needed during the HTTPS OTA process, then one can use esp_https_ota_begin and subsequent APIs. If this API returns successfully, esp_restart() must be called to boot from the new firmware image.

Parameters

ota_config -- [in] pointer to esp_https_ota_config_t structure.

Returns

ESP_OK: OTA data updated, next reboot will use specified partition.
ESP_FAIL: For generic failure.
ESP_ERR_INVALID_ARG: Invalid argument
ESP_ERR_OTA_VALIDATE_FAILED: Invalid app image
ESP_ERR_NO_MEM: Cannot allocate memory for OTA operation.
ESP_ERR_FLASH_OP_TIMEOUT or ESP_ERR_FLASH_OP_FAIL: Flash write failed.
For other return codes, refer OTA documentation in esp-idf's app_update component.

esp_err_t esp_https_ota_begin(const esp_https_ota_config_t *ota_config, esp_https_ota_handle_t *handle)

Start HTTPS OTA Firmware upgrade.

This function initializes ESP HTTPS OTA context and establishes HTTPS connection. This function must be invoked first. If this function returns successfully, then esp_https_ota_perform should be called to continue with the OTA process and there should be a call to esp_https_ota_finish on completion of OTA operation or on failure in subsequent operations. This API supports URL redirection, but if CA cert of URLs differ then it should be appended to cert_pem member of http_config, which is a part of ota_config. In case of error, this API explicitly sets handle to NULL.

Note

This API is blocking, so setting is_async member of http_config structure will result in an error.

Parameters

ota_config -- [in] pointer to esp_https_ota_config_t structure
handle -- [out] pointer to an allocated data of type esp_https_ota_handle_t which will be initialised in this function

Returns

ESP_OK: HTTPS OTA Firmware upgrade context initialised and HTTPS connection established
ESP_FAIL: For generic failure.
ESP_ERR_INVALID_ARG: Invalid argument (missing/incorrect config, certificate, etc.)
For other return codes, refer documentation in app_update component and esp_http_client component in esp-idf.

esp_err_t esp_https_ota_perform(esp_https_ota_handle_t https_ota_handle)

Read image data from HTTP stream and write it to OTA partition.

This function reads image data from HTTP stream and writes it to OTA partition. This function must be called only if esp_https_ota_begin() returns successfully. This function must be called in a loop since it returns after every HTTP read operation thus giving you the flexibility to stop OTA operation midway.

Parameters

https_ota_handle -- [in] pointer to esp_https_ota_handle_t structure

Returns

ESP_ERR_HTTPS_OTA_IN_PROGRESS: OTA update is in progress, call this API again to continue.
ESP_OK: OTA update was successful
ESP_FAIL: OTA update failed
ESP_ERR_INVALID_ARG: Invalid argument
ESP_ERR_INVALID_VERSION: Invalid chip revision in image header
ESP_ERR_OTA_VALIDATE_FAILED: Invalid app image
ESP_ERR_NO_MEM: Cannot allocate memory for OTA operation.
ESP_ERR_FLASH_OP_TIMEOUT or ESP_ERR_FLASH_OP_FAIL: Flash write failed.
For other return codes, refer OTA documentation in esp-idf's app_update component.

bool esp_https_ota_is_complete_data_received(esp_https_ota_handle_t https_ota_handle)

Checks if complete data was received or not.

Note

This API can be called just before esp_https_ota_finish() to validate if the complete image was indeed received.

Parameters

https_ota_handle -- [in] pointer to esp_https_ota_handle_t structure

Returns

false
true

esp_err_t esp_https_ota_finish(esp_https_ota_handle_t https_ota_handle)

Clean-up HTTPS OTA Firmware upgrade and close HTTPS connection.

This function closes the HTTP connection and frees the ESP HTTPS OTA context. This function switches the boot partition to the OTA partition containing the new firmware image.

Note

If this API returns successfully, esp_restart() must be called to boot from the new firmware image esp_https_ota_finish should not be called after calling esp_https_ota_abort

Parameters

https_ota_handle -- [in] pointer to esp_https_ota_handle_t structure

Returns

ESP_OK: Clean-up successful
ESP_ERR_INVALID_STATE
ESP_ERR_INVALID_ARG: Invalid argument
ESP_ERR_OTA_VALIDATE_FAILED: Invalid app image

esp_err_t esp_https_ota_abort(esp_https_ota_handle_t https_ota_handle)

Clean-up HTTPS OTA Firmware upgrade and close HTTPS connection.

This function closes the HTTP connection and frees the ESP HTTPS OTA context.

Note

esp_https_ota_abort should not be called after calling esp_https_ota_finish

Parameters

https_ota_handle -- [in] pointer to esp_https_ota_handle_t structure

Returns

ESP_OK: Clean-up successful
ESP_ERR_INVALID_STATE: Invalid ESP HTTPS OTA state
ESP_FAIL: OTA not started
ESP_ERR_NOT_FOUND: OTA handle not found
ESP_ERR_INVALID_ARG: Invalid argument

esp_err_t esp_https_ota_get_img_desc(esp_https_ota_handle_t https_ota_handle, esp_app_desc_t *new_app_info)

Reads app description from image header. The app description provides information like the "Firmware version" of the image.

Note

This API can be called only after esp_https_ota_begin() and before esp_https_ota_perform(). Calling this API is not mandatory.

Parameters

https_ota_handle -- [in] pointer to esp_https_ota_handle_t structure
new_app_info -- [out] pointer to an allocated esp_app_desc_t structure

Returns

ESP_ERR_INVALID_ARG: Invalid arguments
ESP_ERR_INVALID_STATE: Invalid state to call this API. esp_https_ota_begin() not called yet.
ESP_FAIL: Failed to read image descriptor
ESP_OK: Successfully read image descriptor

int esp_https_ota_get_image_len_read(esp_https_ota_handle_t https_ota_handle)

This function returns OTA image data read so far.

Note

This API should be called only if esp_https_ota_perform() has been called at least once or if esp_https_ota_get_img_desc has been called before.

Parameters

https_ota_handle -- [in] pointer to esp_https_ota_handle_t structure

Returns

-1 On failure
total bytes read so far

int esp_https_ota_get_status_code(esp_https_ota_handle_t https_ota_handle)

This function returns the HTTP status code of the last HTTP response.

Note

This API should be called only after esp_https_ota_begin() has been called. This can be used to check the HTTP status code of the OTA download process.

Parameters

https_ota_handle -- [in] pointer to esp_https_ota_handle_t structure

Returns

-1 On failure
HTTP status code

int esp_https_ota_get_image_size(esp_https_ota_handle_t https_ota_handle)

This function returns OTA image total size.

Note

This API should be called after esp_https_ota_begin() has been already called. This can be used to create some sort of progress indication (in combination with esp_https_ota_get_image_len_read())

Parameters

https_ota_handle -- [in] pointer to esp_https_ota_handle_t structure

Returns

-1 On failure or chunked encoding
total bytes of image

Structures

struct decrypt_cb_arg_t

ESP HTTPS OTA decrypt callback args.

Public Members

const char *data_in: Pointer to data to be decrypted

size_t data_in_len: Input data length

char *data_out: Pointer to data decrypted using callback, this will be freed after data is written to flash

size_t data_out_len: Output data length

struct esp_https_ota_config_t

ESP HTTPS OTA configuration.

Public Members

const esp_http_client_config_t *http_config: ESP HTTP client configuration

http_client_init_cb_t http_client_init_cb: Callback after ESP HTTP client is initialised

bool bulk_flash_erase: Erase entire flash partition during initialization. By default flash partition is erased during write operation and in chunk of 4K sector size

bool partial_http_download: Enable Firmware image to be downloaded over multiple HTTP requests

int max_http_request_size: Maximum request size for partial HTTP download

uint32_t buffer_caps: The memory capability to use when allocating the buffer for OTA update. Default capability is MALLOC_CAP_DEFAULT

decrypt_cb_t decrypt_cb: Callback for external decryption layer

void *decrypt_user_ctx: User context for external decryption layer

uint16_t enc_img_header_size: Header size of pre-encrypted ota image header

Macros

ESP_ERR_HTTPS_OTA_BASE

ESP_ERR_HTTPS_OTA_IN_PROGRESS

Type Definitions

typedef void *esp_https_ota_handle_t

typedef esp_err_t (*http_client_init_cb_t)(esp_http_client_handle_t)

typedef esp_err_t (*decrypt_cb_t)(decrypt_cb_arg_t *args, void *user_ctx)

Enumerations

enum esp_https_ota_event_t

Events generated by OTA process.

Values:

enumerator ESP_HTTPS_OTA_START: OTA started

enumerator ESP_HTTPS_OTA_CONNECTED: Connected to server

enumerator ESP_HTTPS_OTA_GET_IMG_DESC: Read app description from image header

enumerator ESP_HTTPS_OTA_VERIFY_CHIP_ID: Verify chip id of new image

enumerator ESP_HTTPS_OTA_DECRYPT_CB: Callback to decrypt function

enumerator ESP_HTTPS_OTA_WRITE_FLASH: Flash write operation

enumerator ESP_HTTPS_OTA_UPDATE_BOOT_PARTITION: Boot partition update after successful ota update

enumerator ESP_HTTPS_OTA_FINISH: OTA finished

enumerator ESP_HTTPS_OTA_ABORT: OTA aborted

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