It is recommended to have an uninterrupted power supply while enabling security features on ESP32 SoCs. Power failures during secure manufacturing process could cause issues that are hard to debug and, in some cases, may cause permanent boot-up failures.
This guide highlights an approach where security features are enabled with the assistance of an external host machine. Security workflows are broken down into various stages and key material is generated on the host machine; thus, allowing greater recovery chances in case of power or other failures. It also offers better timings for secure manufacturing, e.g., case of encryption of firmware on host machine vs on the device.
Goals
-----
#. Simplify the traditional workflow with stepwise instructions.
#. Design a more flexible workflow as compared to the traditional firmware based workflow.
#. Improve reliability by dividing the workflow in small operations.
To enable the Secure Boot (SB) V2, it is necessary to keep the SB V2 key readable. To protect the key's readability, the write-protection for RD_DIS (ESP_EFUSE_WR_DIS_RD_DIS) is applied. However, this action poses a challenge when attempting to enable Flash Encryption, as the Flash Encryption (FE) key needs to remain unreadable. This conflict arises because the RD_DIS is already write-protected, making it impossible to read protect the FE key.
In this case all the eFuses related to flash encryption are written with help of the espefuse tool. More details about flash encryption can process can be found in the :doc:`Flash Encryption Guide </security/flash-encryption>`
1. Ensure that you have an {IDF_TARGET_NAME} device with default flash encryption eFuse settings as shown in :ref:`flash-encryption-efuse`.
where ``BLOCK`` is a free keyblock between ``BLOCK_KEY0`` and ``BLOCK_KEY5``. And ``KEYPURPOSE`` is either ``XTS_AES_256_KEY_1``, ``XTS_AES_256_KEY_2``, ``XTS_AES_128_KEY``. See `{IDF_TARGET_NAME} Technical Reference Manual <{IDF_TARGET_TRM_EN_URL}>`_ for a description of the key purposes.
For AES-256 (512-bit key) - ``XTS_AES_256_KEY_1`` and ``XTS_AES_256_KEY_2``. ``espefuse.py`` supports burning both these two key purposes together with a 512 bit key to two separate key blocks via the virtual key purpose ``XTS_AES_256_KEY``. When this is used ``espefuse.py`` will burn the first 256 bit of the key to the specified ``BLOCK`` and burn the corresponding block key purpose to ``XTS_AES_256_KEY_1``. The last 256 bit of the key will be burned to the first free key block after ``BLOCK`` and the corresponding block key purpose to ``XTS_AES_256_KEY_2``
If you wish to specify exactly which two blocks are used then it is possible to divide key into two 256 bit keys, and manually burn each half with ``XTS_AES_256_KEY_1`` and ``XTS_AES_256_KEY_2`` as key purposes:
For AES-128 key derived from 128 bits (SHA256(128 bits)) - ``XTS_AES_128_KEY_DERIVED_FROM_128_EFUSE_BITS``. The FE key will be written in the lower part of eFuse BLOCK_KEY0. The upper 128 bits are not used and will remain available for reading by software. Using the special mode of the espefuse tool, shown in the ``For burning both keys together`` section below, the user can write their data to it using any espefuse commands.
If you only want to enable flash encryption in **Development** mode and want to keep the ability to disable it in future, Update the {IDF_TARGET_CRYPT_CNT} value in the below command from {IDF_TARGET_CRYPT_CNT_MAX_VAL} to 0x1. (not recommended for production)
At this point the flash encryption on the device has been enabled. You may test the flash encryption process as given in step 5. Please note that the security related eFuses have not been burned at this point. It is recommended that they should be burned in production use-cases as explained in step 6.
-:ref:`Enable flash encryption on boot <CONFIG_SECURE_FLASH_ENC_ENABLED>`
:esp32:- :ref:`Select Release mode <CONFIG_SECURE_FLASH_ENCRYPTION_MODE>` (Note that once Release mode is selected, the ``DISABLE_DL_ENCRYPT`` and ``DISABLE_DL_DECRYPT`` eFuse bits will be burned to disable flash encryption hardware in ROM Download Mode.)
:esp32:- :ref:`Select UART ROM download mode (Permanently disabled (recommended)) <CONFIG_SECURE_UART_ROM_DL_MODE>` (Note that this option is only available when :ref:`CONFIG_ESP32_REV_MIN` is set to 3 (ESP32 V3).) The default choice is to keep UART ROM download mode enabled, however it is recommended to permanently disable this mode to reduce the options available to an attacker.
:not esp32:- :ref:`Select Release mode <CONFIG_SECURE_FLASH_ENCRYPTION_MODE>` (Note that once Release mode is selected, the ``EFUSE_DIS_DOWNLOAD_MANUAL_ENCRYPT`` eFuse bit will be burned to disable flash encryption hardware in ROM Download Mode.)
:not esp32:- :ref:`Select UART ROM download mode (Permanently switch to Secure mode (recommended)) <CONFIG_SECURE_UART_ROM_DL_MODE>`. This is the default option, and is recommended. It is also possible to change this configuration setting to permanently disable UART ROM download mode, if this mode is not needed.
-:ref:`Select the appropriate bootloader log verbosity <CONFIG_BOOTLOADER_LOG_LEVEL>`
The above files can then be flashed to their respective offset using ``esptool.py``. To see all of the command line options recommended for ``esptool.py``, see the output printed when ``idf.py build`` succeeds. In the above command the offsets are used for a sample firmware, the actual offset for your firmware can be obtained by checking the partition table entry or by running `idf.py partition-table`. When the application contains following partition: ``otadata``, ``nvs_encryption_keys`` they need to be encrypted as well. Please refer to :ref:`encrypted-partitions` for more details about encrypted partitions.
If the flashed ciphertext file is not recognized by the {IDF_TARGET_NAME} when it boots, check that the keys match and that the command line arguments match exactly, including the correct offset. It is important to provide the correct offset as the ciphertext changes when the offset changes.
If your ESP32 uses non-default :ref:`FLASH_CRYPT_CONFIG value in eFuse <setting-flash-crypt-config>` then you will need to pass the ``--flash_crypt_conf`` argument to ``espsecure.py`` to set the matching value. This will not happen if the device configured flash encryption by itself, but may happen if burning eFuses manually to enable flash encryption.
The command ``espsecure.py decrypt_flash_data`` can be used with the same options (and different input/output files), to decrypt ciphertext flash contents or a previously encrypted file.
Please update the EFUSE_NAME with the eFuse that you need to burn. Multiple eFuses can be burned at the same time by appending them to the above command (e.g., EFUSE_NAME VAL EFUSE_NAME2 VAL2). More documentation about `espefuse.py` can be found `here <https://docs.espressif.com/projects/esptool/en/latest/esp32/espefuse/index.html>`_
Once the flash encryption has been enabled for the device, the key **must be deleted immediately**. This ensures that the host cannot produce encrypted binaries for the same device going forward. This step is important to reduce the vulnerability of the flash encryption key.
In this workflow we shall use ``espsecure`` tool to generate signing keys and use the ``espefuse`` tool to burn the relevant eFuses. The details about the Secure Boot V2 process can be found at :doc:`Secure Boot V2 Guide </security/secure-boot-v2>`
A total of 3 keys can be used for Secure Boot V2 at once. These should be computed independently and stored separately. The same command with different keyfile names can be used to generated multiple Secure Boot V2 signing keys. It is recommended to use multiple keys in order to reduce dependency on a single key.
In case of multiple digests, the other digests can be burned sequentially by changing the key purpose to ``SECURE_BOOT_DIGEST1`` and ``SECURE_BOOT_DIGEST2`` respectively.
At this stage the secure boot V2 has been enabled for the {IDF_TARGET_NAME}. The ROM bootloader shall now verify the authenticity of the :ref:`second-stage-bootloader` on every bootup. You may test the secure boot process by executing Step 5 & 6. Please note that security related eFuses have not been burned at this point. For production use cases, all security related eFuses must be burned as given in step 7.
By default the ROM bootloader would only verify the :ref:`second-stage-bootloader` (firmware bootloader). The firmware bootloader would verify the app partition only when the :ref:`CONFIG_SECURE_BOOT` option is enabled (and :ref:`CONFIG_SECURE_BOOT_VERSION` is set to ``SECURE_BOOT_V2_ENABLED``) while building the bootloader.
For ESP32, Secure Boot V2 is available only ESP32 ECO3 onwards. To view the "Secure Boot V2" option the chip revision should be changed to revision v3.0 (ECO3). To change the chip revision, set "Minimum Supported ESP32 Revision" to "Rev 3.0 (ECO3)" in "Component Config" -> "Hardware Settings" -> "Chip Revision".
b) Disable the option :ref:`CONFIG_SECURE_BOOT_BUILD_SIGNED_BINARIES` for the project in the :ref:`project-configuration-menu`. This shall make sure that all the generated binaries are secure padded and unsigned. This step is done to avoid generating signed binaries as we are going to manually sign the binaries using ``espsecure`` tool.
The Secure Boot V2 workflow only verifies the ``bootloader`` and ``application`` binaries, hence only those binaries need to be signed. The other binaries (e.g., ``partition-table.bin``) can be flashed as they are generated in the build stage.
The above files along with other binaries (e.g., partition table) can then be flashed to their respective offset using ``esptool.py``. To see all of the command line options recommended for ``esptool.py``, see the output printed when ``idf.py build`` succeeds. The flash offset for your firmware can be obtained by checking the partition table entry or by running ``idf.py partition-table``.
:SOC_EFUSE_REVOKE_BOOT_KEY_DIGESTS:- ``SECURE_BOOT_AGGRESSIVE_REVOKE``: Aggressive revocation of key digests, see :ref:`secure-boot-v2-aggressive-key-revocation` for more details.
Please update the EFUSE_NAME with the eFuse that you need to burn. Multiple eFuses can be burned at the same time by appending them to the above command (e.g., EFUSE_NAME VAL EFUSE_NAME2 VAL2). More documentation about `espefuse.py` can be found `here <https://docs.espressif.com/projects/esptool/en/latest/esp32/espefuse/index.html>`_
The secure boot digest burned in the eFuse must be kept readable otherwise secure boot operation would result in a failure. To prevent the accidental enabling of read protection for this key block we need to burn following eFuse:
After this eFuse has been burned, read protection cannot be enabled for any key. E.g., if Flash Encryption which requires read protection for its key is not enabled at this point then it cannot be enabled afterwards. Please ensure that no efuse keys are going to need read protection after this.
..only:: SOC_EFUSE_REVOKE_BOOT_KEY_DIGESTS
ii) Revoke key digests:
The unused digest slots need to be revoked when we are burning the secure boot key. The respective slots can be revoked by running
..code:: bash
espefuse.py --port PORT --chip {IDF_TARGET_PATH_NAME} burn_efuse EFUSE_REVOKE_BIT
The ``EFUSE_REVOKE_BIT`` in the above command can be ``SECURE_BOOT_KEY_REVOKE0`` or ``SECURE_BOOT_KEY_REVOKE1`` or ``SECURE_BOOT_KEY_REVOKE2``. Please note that only the unused key digests must be revoked. Once reovked, the respective digest cannot be used again.
..only:: esp32
C) Disable UART ROM DL mode:
..only:: not esp32
C) Enable Security Download mode:
..warning::
Please burn the following bit at the very end. After this bit is burned, the espefuse tool can no longer be used to burn additional eFuses.
..list::
:esp32:- ``UART_DOWNLOAD_DIS`` : Disable the UART ROM Download mode.
:not esp32:- ``ENABLE_SECURITY_DOWNLOAD``: Enable Secure ROM download mode
..only:: esp32
The eFuse can be burned by running:
..code:: bash
espefuse.py --port PORT burn_efuse UART_DOWNLOAD_DIS
..only:: not esp32
The eFuse can be burned by running:
..code:: bash
espefuse.py --port PORT burn_efuse ENABLE_SECURITY_DOWNLOAD
Secure Boot V2 Guidelines
~~~~~~~~~~~~~~~~~~~~~~~~~
* It is recommended to store the Secure boot key at a highly secure place. A physical or a cloud HSM may be used for secure storage of the Secure Boot Private key. Please take a look at :ref:`remote-sign-v2-image` for more details.
..only:: SOC_EFUSE_REVOKE_BOOT_KEY_DIGESTS
* It is recommended to use all the available digest slots to reduce dependency on a single private key.
