This is a quick start guide to {IDF_TARGET_NAME}'s flash encryption feature. Using an application code example, it demonstrates how to test and verify flash encryption operations during development and production.
Flash encryption is intended for encrypting the contents of the {IDF_TARGET_NAME}'s off-chip flash memory. Once this feature is enabled, firmware is flashed as plaintext, and then the data is encrypted in place on the first boot. As a result, physical readout of flash will not be sufficient to recover most flash contents.
Enabling flash encryption limits the options for further updates of {IDF_TARGET_NAME}. Before using this feature, read the document and make sure to understand the implications.
The flash encryption operation is controlled by various eFuses available on {IDF_TARGET_NAME}. The list of eFuses and their descriptions is given in the table below. The names in eFuse column are also used by espefuse.py tool. For usage in the eFuse API, modify the name by adding ``ESP_EFUSE_``, for example: esp_efuse_read_field_bit(ESP_EFUSE_DISABLE_DL_ENCRYPT).
.. Comment: As text in cells of list-table header rows does not wrap, it is necessary to make 0 header rows and apply bold typeface to the first row. Otherwise, the table goes beyond the html page limits on the right.
- Controls actual number of block1 bits used to derive final 256-bit AES key. Possible values: ``0`` for 256 bits, ``1`` for 192 bits, ``2`` for 128 bits. Final AES key is derived based on the ``FLASH_CRYPT_CONFIG`` value.
- Enables/disables encryption at boot time. If even number of bits set (0, 2, 4, 6) - encrypt flash at boot time. If odd number of bits set (1, 3, 5, 7) - do not encrypt flash at boot time.
- Controls the purpose of eFuse block ``BLOCK_KEYN``, where N is between 0 and 5. Possible values: ``2`` for ``XTS_AES_256_KEY_1`` , ``3`` for ``XTS_AES_256_KEY_2``, and ``4`` for ``XTS_AES_128_KEY``. Final AES key is derived based on the value of one or two of these purpose eFuses. For a detailed description of the possible combinations, see *{IDF_TARGET_NAME} Technical Reference Manual* > *External Memory Encryption and Decryption (XTS_AES)* [`PDF <{IDF_TARGET_TRM_EN_URL}#extmemencr>`__].
- Controls the purpose of eFuse block ``BLOCK_KEYN``, where N is between 0 and 5. For flash encryption the only valid value is ``4`` for ``XTS_AES_128_KEY``.
..only:: SOC_FLASH_ENCRYPTION_XTS_AES_128 and SOC_EFUSE_CONSISTS_OF_ONE_KEY_BLOCK
..list-table:: eFuses Used in Flash Encryption
:widths:25 40 10
:header-rows:0
* - **eFuse**
-**Description**
-**Bit Depth**
* - ``XTS_KEY_LENGTH_256``
- Controls actual number of eFuse bits used to derive final 256-bit AES key. Possible values: ``0`` use all 256 bits of the eFuse block for the key, ``1`` use the lower 128 bits of the eFuse block for the key (the higher 128 bits are reserved for Secure Boot key). For 128 bits option, the final AES key is derived as SHA256(EFUSE_KEY0_FE_128BIT).
- 1
* - ``BLOCK_KEY0``
- AES key storage.
- 256 or 128 key block
* - ``DIS_DOWNLOAD_MANUAL_ENCRYPT``
- If set, disables flash encryption when in download bootmodes.
- 1
* - ``{IDF_TARGET_CRYPT_CNT}``
- Enables encryption and decryption, when an SPI boot mode is set. Feature is enabled if 1 or 3 bits are set in the eFuse, disabled otherwise.
Read and write access to eFuse bits is controlled by appropriate fields in the registers ``WR_DIS`` and ``RD_DIS``. For more information on {IDF_TARGET_NAME} eFuses, see :doc:`eFuse manager <../api-reference/system/efuse>`. To change protection bits of eFuse field using espefuse.py, use these two commands: read_protect_efuse and write_protect_efuse. Example ``espefuse.py write_protect_efuse DISABLE_DL_ENCRYPT``.
{IDF_TARGET_NAME} has only one eFuse key block for both keys: Secure Boot and Flash Encryption. The eFuse key block can only be burned once. Therefore these keys should be burned together at the same time. Please note that "Secure Boot" and "Flash Encryption" can not be enabled separately as subsequent writes to eFuse key block shall return an error.
Assuming that the eFuse values are in their default states and the firmware bootloader is compiled to support flash encryption, the flash encryption process executes as shown below:
2. Firmware bootloader reads the ``{IDF_TARGET_CRYPT_CNT}`` eFuse value (``0b0000000``). Since the value is ``0`` (even number of bits set), it configures and enables the flash encryption block. It also sets the ``FLASH_CRYPT_CONFIG`` eFuse to 0xF. For more information on the flash encryption block, see *{IDF_TARGET_NAME} Technical Reference Manual* > *eFuse Controller (eFuse)* > *Flash Encryption Block* [`PDF <{IDF_TARGET_TRM_EN_URL}#efuse>`__].
3. Firmware bootloader uses RNG (random) module to generate an AES-256 bit key and then writes it into the ``flash_encryption`` eFuse. The key cannot be accessed via software as the write and read protection bits for the ``flash_encryption`` eFuse are set. The flash encryption operations happen entirely by hardware, and the key cannot be accessed via software.
4. Flash encryption block encrypts the flash contents - the firmware bootloader, applications and partitions marked as ``encrypted``. Encrypting in-place can take time, up to a minute for large partitions.
5. Firmware bootloader sets the first available bit in ``{IDF_TARGET_CRYPT_CNT}`` (0b0000001) to mark the flash contents as encrypted. Odd number of bits is set.
6. For :ref:`flash-enc-development-mode`, the firmware bootloader sets only the eFuse bits ``DISABLE_DL_DECRYPT`` and ``DISABLE_DL_CACHE`` to allow the UART bootloader to re-flash encrypted binaries. Also, the ``{IDF_TARGET_CRYPT_CNT}`` eFuse bits are NOT write-protected.
7. For :ref:`flash-enc-release-mode`, the firmware bootloader sets the eFuse bits ``DISABLE_DL_ENCRYPT``, ``DISABLE_DL_DECRYPT``, and ``DISABLE_DL_CACHE`` to 1 to prevent the UART bootloader from decrypting the flash contents. It also write-protects the ``{IDF_TARGET_CRYPT_CNT}`` eFuse bits. To modify this behavior, see :ref:`uart-bootloader-encryption`.
8. The device is then rebooted to start executing the encrypted image. The firmware bootloader calls the flash decryption block to decrypt the flash contents and then loads the decrypted contents into IRAM.
2. Firmware bootloader reads the ``{IDF_TARGET_CRYPT_CNT}`` eFuse value (``0b000``). Since the value is ``0`` (even number of bits set), it configures and enables the flash encryption block. For more information on the flash encryption block, see *{IDF_TARGET_NAME} Technical Reference Manual* > *eFuse Controller (eFuse)* > *Auto Encryption Block* [`PDF <{IDF_TARGET_TRM_EN_URL}#efuse>`__].
3. Firmware bootloader uses RNG (random) module to generate an 256 bit or 512 bit key, depending on the value of :ref:`Size of generated AES-XTS key <CONFIG_SECURE_FLASH_ENCRYPTION_KEYSIZE>`, and then writes it into respectively one or two `BLOCK_KEYN` eFuses. The software also updates the ``KEY_PURPOSE_N`` for the blocks where the keys were stored. The key cannot be accessed via software as the write and read protection bits for one or two `BLOCK_KEYN` eFuses are set. ``KEY_PURPOSE_N`` field is write-protected as well. The flash encryption operations happen entirely by hardware, and the key cannot be accessed via software.
4. Flash encryption block encrypts the flash contents - the firmware bootloader, applications and partitions marked as ``encrypted``. Encrypting in-place can take time, up to a minute for large partitions.
5. Firmware bootloader sets the first available bit in ``{IDF_TARGET_CRYPT_CNT}`` (0b001) to mark the flash contents as encrypted. Odd number of bits is set.
6. For :ref:`flash-enc-development-mode`, the firmware bootloader allows the UART bootloader to re-flash encrypted binaries. Also, the ``{IDF_TARGET_CRYPT_CNT}`` eFuse bits are NOT write-protected. In addition, the firmware bootloader by default sets the eFuse bits ``DIS_BOOT_REMAP``, ``DIS_DOWNLOAD_ICACHE``, ``DIS_DOWNLOAD_DCACHE``, ``HARD_DIS_JTAG`` and ``DIS_LEGACY_SPI_BOOT``.
7. For :ref:`flash-enc-release-mode`, the firmware bootloader sets all the eFuse bits set under development mode as well as ``DIS_DOWNLOAD_MANUAL_ENCRYPT``. It also write-protects the ``{IDF_TARGET_CRYPT_CNT}`` eFuse bits. To modify this behavior, see :ref:`uart-bootloader-encryption`.
8. The device is then rebooted to start executing the encrypted image. The firmware bootloader calls the flash decryption block to decrypt the flash contents and then loads the decrypted contents into IRAM.
2. Firmware bootloader reads the ``{IDF_TARGET_CRYPT_CNT}`` eFuse value (``0b000``). Since the value is ``0`` (even number of bits set), it configures and enables the flash encryption block. For more information on the flash encryption block, see `{IDF_TARGET_NAME} Technical Reference Manual <{IDF_TARGET_TRM_EN_URL}>`_.
3. Firmware bootloader uses RNG (random) module to generate an 256 bit key and then writes it into `BLOCK_KEYN` eFuse. The software also updates the ``KEY_PURPOSE_N`` for the block where the key were stored. The key cannot be accessed via software as the write and read protection bits for `BLOCK_KEYN` eFuse are set. ``KEY_PURPOSE_N`` field is write-protected as well. The flash encryption operations happen entirely by hardware, and the key cannot be accessed via software.
4. Flash encryption block encrypts the flash contents - the firmware bootloader, applications and partitions marked as ``encrypted``. Encrypting in-place can take time, up to a minute for large partitions.
5. Firmware bootloader sets the first available bit in ``{IDF_TARGET_CRYPT_CNT}`` (0b001) to mark the flash contents as encrypted. Odd number of bits is set.
6. For :ref:`flash-enc-development-mode`, the firmware bootloader allows the UART bootloader to re-flash encrypted binaries. Also, the ``{IDF_TARGET_CRYPT_CNT}`` eFuse bits are NOT write-protected. In addition, the firmware bootloader by default sets the eFuse bits ``DIS_DOWNLOAD_ICACHE``, ``DIS_PAD_JTAG``, ``DIS_USB_JTAG`` and ``DIS_LEGACY_SPI_BOOT``.
7. For :ref:`flash-enc-release-mode`, the firmware bootloader sets all the eFuse bits set under development mode as well as ``DIS_DOWNLOAD_MANUAL_ENCRYPT``. It also write-protects the ``{IDF_TARGET_CRYPT_CNT}`` eFuse bits. To modify this behavior, see :ref:`uart-bootloader-encryption`.
8. The device is then rebooted to start executing the encrypted image. The firmware bootloader calls the flash decryption block to decrypt the flash contents and then loads the decrypted contents into IRAM.
..only:: SOC_FLASH_ENCRYPTION_XTS_AES_128 and SOC_EFUSE_CONSISTS_OF_ONE_KEY_BLOCK
1. On the first power-on reset, all data in flash is un-encrypted (plaintext). The ROM bootloader loads the firmware bootloader.
2. Firmware bootloader reads the ``{IDF_TARGET_CRYPT_CNT}`` eFuse value (``0b000``). Since the value is ``0`` (even number of bits set), it configures and enables the flash encryption block. For more information on the flash encryption block, see `{IDF_TARGET_NAME} Technical Reference Manual <{IDF_TARGET_TRM_EN_URL}>`_.
3. Firmware bootloader uses RNG (random) module to generate an 256 or 128 bit key (depends on :ref:`Size of generated AES-XTS key <CONFIG_SECURE_FLASH_ENCRYPTION_KEYSIZE>`) and then writes it into `BLOCK_KEY0` eFuse. The software also updates the ``XTS_KEY_LENGTH_256`` according to the chosen option. The key cannot be accessed via software as the write and read protection bits for `BLOCK_KEY0` eFuse are set. The flash encryption operations happen entirely by hardware, and the key cannot be accessed via software. If 128-bit flash encryption key is used, then only the lower 128 bits of the eFuse key block are read-protected, the remaining 128 bits are readable, which is required for secure boot. The entire eFuse block is write-protected. If the FE key is 256 bits long, then ``XTS_KEY_LENGTH_256`` is 1, otherwise it is 0. To prevent this eFuse from being accidentally changed in the future (from 0 to 1), we set a write-protect bit for the RELEASE mode.
4. Flash encryption block encrypts the flash contents - the firmware bootloader, applications and partitions marked as ``encrypted``. Encrypting in-place can take time, up to a minute for large partitions.
5. Firmware bootloader sets the first available bit in ``{IDF_TARGET_CRYPT_CNT}`` (0b001) to mark the flash contents as encrypted. Odd number of bits is set.
6. For :ref:`flash-enc-development-mode`, the firmware bootloader allows the UART bootloader to re-flash encrypted binaries. Also, the ``{IDF_TARGET_CRYPT_CNT}`` eFuse bits are NOT write-protected. In addition, the firmware bootloader by default sets the eFuse bits ``DIS_DOWNLOAD_ICACHE``, ``DIS_PAD_JTAG``, and ``DIS_DIRECT_BOOT``.
7. For :ref:`flash-enc-release-mode`, the firmware bootloader sets all the eFuse bits set under development mode as well as ``DIS_DOWNLOAD_MANUAL_ENCRYPT``. It also write-protects the ``{IDF_TARGET_CRYPT_CNT}`` eFuse bits. To modify this behavior, see :ref:`uart-bootloader-encryption`.
8. The device is then rebooted to start executing the encrypted image. The firmware bootloader calls the flash decryption block to decrypt the flash contents and then loads the decrypted contents into IRAM.
During the development stage, there is a frequent need to program different plaintext flash images and test the flash encryption process. This requires that Firmware Download mode is able to load new plaintext images as many times as it might be needed. However, during manufacturing or production stages, Firmware Download mode should not be allowed to access flash contents for security reasons.
Hence, two different flash encryption configurations were created: for development and for production. For details on these configurations, see Section `Flash Encryption Configuration`_.
-:ref:`flash-enc-development-mode` - recommended for use ONLY DURING DEVELOPMENT, as it does not prevent modification and readout of encrypted flash contents.
:esp32:- :ref:`Select UART ROM download mode <CONFIG_SECURE_UART_ROM_DL_MODE>` (**enabled** by default. Note that for the esp32 target, the choice is only available when :ref:`CONFIG_ESP32_REV_MIN` level is set to 3 (ESP32 V3)).
:not esp32:- :ref:`Select UART ROM download mode <CONFIG_SECURE_UART_ROM_DL_MODE>` (**enabled** by default.)
This command does not include any user files which should be written to the partitions on the flash memory. Please write them manually before running this command otherwise the files should be encrypted separately before writing.
This command will write to flash memory unencrypted images: the firmware bootloader, the partition table and applications. Once the flashing is complete, {IDF_TARGET_NAME} will reset. On the next boot, the firmware bootloader encrypts: the firmware bootloader, application partitions and partitions marked as ``encrypted`` then resets. Encrypting in-place can take time, up to a minute for large partitions. After that, the application is decrypted at runtime and executed.
It is possible to pre-generate a flash encryption key on the host computer and burn it into the eFuse. This allows you to pre-encrypt data on the host and flash already encrypted data without needing a plaintext flash update. This feature can be used in both :ref:`flash-enc-development-mode` and :ref:`flash-enc-release-mode`. Without a pre-generated key, data is flashed in plaintext and then {IDF_TARGET_NAME} encrypts the data in-place.
Note that {IDF_TARGET_NAME} only has one eFuse key block for both Secure Boot and Flash Encryption keys. Therefore, writing the host-generated Flash Encryption key must be done with Secure Boot key (if used), otherwise Secure Boot cannot be used.
where ``BLOCK`` is a free keyblock between ``BLOCK_KEY0`` and ``BLOCK_KEY5``. And ``KEYPURPOSE`` is either ``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:
..only:: SOC_FLASH_ENCRYPTION_XTS_AES_128 and SOC_EFUSE_CONSISTS_OF_ONE_KEY_BLOCK
For AES-128 (256-bit key) - ``XTS_AES_128_KEY`` (the ``XTS_KEY_LENGTH_256`` eFuse will be burn to 1):
..code-block:: bash
espefuse.py --port PORT burn_key BLOCK_KEY0 flash_encryption_key256.bin XTS_AES_128_KEY
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.
..code-block:: bash
espefuse.py --port PORT burn_key BLOCK_KEY0 flash_encryption_key128.bin XTS_AES_128_KEY_DERIVED_FROM_128_EFUSE_BITS
For burning both keys together (Secure Boot and Flash Encryption):
..code-block:: bash
espefuse.py --port PORT --chip esp32c2 burn_key_digest secure_boot_signing_key.pem \
If the key is not burned and the device is started after enabling flash encryption, the {IDF_TARGET_NAME} will generate a random key that software cannot access or modify.
This command does not include any user files which should be written to the partitions on the flash memory. Please write them manually before running this command otherwise the files should be encrypted separately before writing.
This command will write to flash memory unencrypted images: the firmware bootloader, the partition table and applications. Once the flashing is complete, {IDF_TARGET_NAME} will reset. On the next boot, the firmware bootloader encrypts: the firmware bootloader, application partitions and partitions marked as ``encrypted`` then resets. Encrypting in-place can take time, up to a minute for large partitions. After that, the application is decrypted at runtime and executed.
If you update your application code (done in plaintext) and want to re-flash it, you will need to encrypt it before flashing. To encrypt the application and flash it in one step, run:
In Release mode, UART bootloader cannot perform flash encryption operations. New plaintext images can ONLY be downloaded using the over-the-air (OTA) scheme which will encrypt the plaintext image before writing to flash.
: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's 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.
This command does not include any user files which should be written to the partitions on the flash memory. Please write them manually before running this command otherwise the files should be encrypted separately before writing.
This command will write to flash memory unencrypted images: the firmware bootloader, the partition table and applications. Once the flashing is complete, {IDF_TARGET_NAME} will reset. On the next boot, the firmware bootloader encrypts: the firmware bootloader, application partitions and partitions marked as ``encrypted`` then resets. Encrypting in-place can take time, up to a minute for large partitions. After that, the application is decrypted at runtime and executed.
If you have pre-generated the flash encryption key and stored a copy, and the UART download mode is not permanently disabled via :ref:`CONFIG_SECURE_UART_ROM_DL_MODE` {IDF_TARGET_ESP32_V3_ONLY}, then it is possible to update the flash locally by pre-encrypting the files and then flashing the ciphertext. See :ref:`manual-encryption`.
- Do not reuse the same flash encryption key between multiple devices. This means that an attacker who copies encrypted data from one device cannot transfer it to a second device.
:esp32:- When using ESP32 V3, if the UART ROM Download Mode is not needed for a production device then it should be disabled to provide an extra level of protection. Do this by calling :cpp:func:`esp_efuse_disable_rom_download_mode` during application startup. Alternatively, configure the project :ref:`CONFIG_ESP32_REV_MIN` level to 3 (targeting ESP32 V3 only) and select the :ref:`CONFIG_SECURE_UART_ROM_DL_MODE` to "Permanently disable ROM Download Mode (recommended)". The ability to disable ROM Download Mode is not available on earlier ESP32 versions.
:not esp32:- The UART ROM Download Mode should be disabled entirely if it is not needed, or permanently set to "Secure Download Mode" otherwise. Secure Download Mode permanently limits the available commands to basic flash read and write only. The default behaviour is to set Secure Download Mode on first boot in Release mode. To disable Download Mode entirely select select the :ref:`CONFIG_SECURE_UART_ROM_DL_MODE` to "Permanently disable ROM Download Mode (recommended)" or call :cpp:func:`esp_efuse_disable_rom_download_mode` at runtime.
- Enable :doc:`Secure Boot <secure-boot-v2>` as an extra layer of protection, and to prevent an attacker from selectively corrupting any part of the flash before boot.
Once flash encryption is enabled, the ``{IDF_TARGET_CRYPT_CNT}`` eFuse value will have an odd number of bits set. It means that all the partitions marked with the encryption flag are expected to contain encrypted ciphertext. Below are the three typical failure cases if the {IDF_TARGET_NAME} is erroneously loaded with plaintext data:
1. If the bootloader partition is re-flashed with a **plaintext firmware bootloader image**, the ROM bootloader will fail to load the firmware bootloader resulting in the following failure:
2. If the firmware bootloader is encrypted, but the partition table is re-flashed with a **plaintext partition table image**, the bootloader will fail to read the partition table resulting in the following failure:
3. If the bootloader and partition table are encrypted, but the application is re-flashed with a **plaintext application image**, the bootloader will fail to load the application resulting in the following failure:
- flash the application example :example:`security/flash_encryption` onto your device. This application prints the ``{IDF_TARGET_CRYPT_CNT}`` eFuse value and if flash encryption is enabled or disabled.
-:doc:`Find the serial port name <../get-started/establish-serial-connection>` under which your {IDF_TARGET_NAME} device is connected, replace ``PORT`` with your port name in the following command, and run it:
{IDF_TARGET_NAME} application code can check if flash encryption is currently enabled by calling :cpp:func:`esp_flash_encryption_enabled`. Also, a device can identify the flash encryption mode by calling :cpp:func:`esp_get_flash_encryption_mode`.
Whenever the ``{IDF_TARGET_CRYPT_CNT}`` eFuse is set to a value with an odd number of bits, all flash content accessed via the MMU's flash cache is transparently decrypted. It includes:
The MMU flash cache unconditionally decrypts all existing data. Data which is stored unencrypted in flash memory will also be "transparently decrypted" via the flash cache and will appear to software as random garbage.
To read data without using a flash cache MMU mapping, you can use the partition read function :cpp:func:`esp_partition_read`. This function will only decrypt data when it is read from an encrypted partition. Data read from unencrypted partitions will not be decrypted. In this way, software can access encrypted and non-encrypted flash in the same way.
Data stored using the Non-Volatile Storage (NVS) API is always stored and read decrypted from the perspective of flash encryption. It is up to the library to provide encryption feature if required. Refer to :ref:`NVS Encryption <nvs_encryption>` for more details.
It is recommended to use the partition write function :cpp:func:`esp_partition_write`. This function will only encrypt data when it is written to an encrypted partition. Data written to unencrypted partitions will not be encrypted. In this way, software can access encrypted and non-encrypted flash in the same way.
Before building the application image for OTA updating of an already encrypted device, enable the option :ref:`Enable flash encryption on boot <CONFIG_SECURE_FLASH_ENC_ENABLED>` in project configuration menu.
For general information about ESP-IDF OTA updates, please refer to :doc:`OTA <../api-reference/system/ota>`
In Release Mode, if a copy of the same key stored in eFuse is available on the host then it's possible to pre-encrypt files on the host and then flash them. See :ref:`manual-encryption`.
If flash encryption was enabled accidentally, flashing of plaintext data will soft-brick the {IDF_TARGET_NAME}. The device will reboot continuously, printing the error ``flash read err, 1000`` or ``invalid header: 0xXXXXXX``.
For flash encryption in Development mode, encryption can be disabled by burning the ``{IDF_TARGET_CRYPT_CNT}`` eFuse. It can only be done three times per chip by taking the following steps:
For flash encryption in Development mode, encryption can be disabled by burning the ``{IDF_TARGET_CRYPT_CNT}`` eFuse. It can only be done one time per chip by taking the following steps:
#. In :ref:`project-configuration-menu`, disable :ref:`Enable flash encryption on boot <CONFIG_SECURE_FLASH_ENC_ENABLED>`, then save and exit.
#. Open project configuration menu again and **double-check** that you have disabled this option! If this option is left enabled, the bootloader will immediately re-enable encryption when it boots.
#. With flash encryption disabled, build and flash the new bootloader and application by running ``idf.py flash``.
:esp32:- Flash memory contents is encrypted using AES-256. The flash encryption key is stored in the ``flash_encryption`` eFuse internal to the chip and, by default, is protected from software access.
:esp32:- The flash encryption algorithm is AES-256, where the key is "tweaked" with the offset address of each 32 byte block of flash. This means that every 32-byte block (two consecutive 16 byte AES blocks) is encrypted with a unique key derived from the flash encryption key.
:esp32s2 or esp32s3:- Flash memory contents is encrypted using XTS-AES-128 or XTS-AES-256. The flash encryption key is 256 bits and 512 bits respectively and stored in one or two ``BLOCK_KEYN`` eFuses internal to the chip and, by default, is protected from software access.
:esp32c3:- Flash memory contents is encrypted using XTS-AES-128. The flash encryption key is 256 bits and stored in one ``BLOCK_KEYN`` eFuse internal to the chip and, by default, is protected from software access.
:esp32c2:- Flash memory contents is encrypted using XTS-AES-128. The flash encryption key is 256 or 128 bits and stored in ``BLOCK_KEY0`` eFuse internal to the chip and, by default, is protected from software access.
- Flash access is transparent via the flash cache mapping feature of {IDF_TARGET_NAME} - any flash regions which are mapped to the address space will be transparently decrypted when read.
Some data partitions might need to remain unencrypted for ease of access or might require the use of flash-friendly update algorithms which are ineffective if the data is encrypted. NVS partitions for non-volatile storage cannot be encrypted since the NVS library is not directly compatible with flash encryption. For details, refer to :ref:`NVS Encryption <nvs_encryption>`.
- If flash encryption might be used in future, the programmer must keep it in mind and take certain precautions when writing code that :ref:`uses encrypted flash <reading-writing-content>`.
- If secure boot is enabled, re-flashing the bootloader of an encrypted device requires a "Re-flashable" secure boot digest (see :ref:`flash-encryption-and-secure-boot`).
Do not interrupt power to the {IDF_TARGET_NAME} while the first boot encryption pass is running. If power is interrupted, the flash contents will be corrupted and will require flashing with unencrypted data again. In this case, re-flashing will not count towards the flashing limit.
Flash encryption protects firmware against unauthorised readout and modification. It is important to understand the limitations of the flash encryption feature:
- Flash encryption is only as strong as the key. For this reason, we recommend keys are generated on the device during first boot (default behaviour). If generating keys off-device, ensure proper procedure is followed and don't share the same key between all production devices.
- Not all data is stored encrypted. If storing data on flash, check if the method you are using (library, API, etc.) supports flash encryption.
- Flash encryption does not prevent an attacker from understanding the high-level layout of the flash. This is because the same AES key is used for every pair of adjacent 16 byte AES blocks. When these adjacent 16 byte blocks contain identical content (such as empty or padding areas), these blocks will encrypt to produce matching pairs of encrypted blocks. This may allow an attacker to make high-level comparisons between encrypted devices (i.e. to tell if two devices are probably running the same firmware version).
:esp32:- For the same reason, an attacker can always tell when a pair of adjacent 16 byte blocks (32 byte aligned) contain two identical 16 byte sequences. Keep this in mind if storing sensitive data on the flash, design your flash storage so this doesn't happen (using a counter byte or some other non-identical value every 16 bytes is sufficient). :ref:`NVS Encryption <nvs_encryption>` deals with this and is suitable for many uses.
- Flash encryption alone may not prevent an attacker from modifying the firmware of the device. To prevent unauthorised firmware from running on the device, use flash encryption in combination with :doc:`Secure Boot <secure-boot-v2>`.
It is recommended to use flash encryption in combination with Secure Boot. However, if Secure Boot is enabled, additional restrictions apply to device re-flashing:
-:ref:`Plaintext serial flash updates <updating-encrypted-flash-serial>` are only possible if the :ref:`Re-flashable <CONFIG_SECURE_BOOTLOADER_MODE>` Secure Boot mode is selected and a Secure Boot key was pre-generated and burned to the {IDF_TARGET_NAME} (refer to :ref:`Secure Boot <secure-boot-reflashable>`). In such configuration, ``idf.py bootloader`` will produce a pre-digested bootloader and secure boot digest file for flashing at offset 0x0. When following the plaintext serial re-flashing steps it is necessary to re-flash this file before flashing other plaintext data.
-:ref:`Re-flashing via Pregenerated Flash Encryption Key <pregenerated-flash-encryption-key>` is still possible, provided the bootloader is not re-flashed. Re-flashing the bootloader requires the same :ref:`Re-flashable <CONFIG_SECURE_BOOTLOADER_MODE>` option to be enabled in the Secure Boot config.
Some partitions are encrypted by default. Other partitions can be marked in the partition table description as requiring encryption by adding the flag ``encrypted`` to the partitions' flag field. As a result, data in these marked partitions will be treated as encrypted in the same manner as an app partition.
- If flash encryption is not enabled, the flag "encrypted" has no effect.
- You can also consider protecting ``phy_init`` data from physical access, readout, or modification, by marking the optional ``phy`` partition with the flag ``encrypted``.
- The ``nvs`` partition cannot be encrypted, because the NVS library is not directly compatible with flash encryption.
-``DISABLE_DL_DECRYPT`` which disables transparent flash decryption when running in UART bootloader mode, even if the eFuse ``{IDF_TARGET_CRYPT_CNT}`` is set to enable it in normal operation.
:esp32s2 or esp32s3:- ``DIS_DOWNLOAD_ICACHE`` and ``DIS_DOWNLOAD_DCACHE`` which disables the entire MMU flash cache when running in UART bootloader mode.
However, before the first boot you can choose to keep any of these features enabled by burning only selected eFuses and write-protect the rest of eFuses with unset value 0. For example:
Write protection of all the three eFuses is controlled by one bit. It means that write-protecting one eFuse bit will inevitably write-protect all unset eFuse bits.
The eFuse ``FLASH_CRYPT_CONFIG`` determines the number of bits in the flash encryption key which are "tweaked" with the block offset. For details, see :ref:`flash-encryption-algorithm`.
It is possible to burn this eFuse manually and write protect it before the first boot in order to select different tweak values. However, this is not recommended.
It is strongly recommended to never write-protect ``FLASH_CRYPT_CONFIG`` when it is unset. Otherwise, its value will remain zero permanently, and no bits in the flash encryption key will be tweaked. As a result, the flash encryption algorithm will be equivalent to AES ECB mode.
By default, when Flash Encryption is enabled (in either Development or Release mode) then JTAG debugging is disabled via eFuse. The bootloader does this on first boot, at the same time it enables flash encryption.
See :ref:`jtag-debugging-security-features` for more information about using JTAG Debugging with Flash Encryption.
Manually encrypting or decrypting files requires the flash encryption key to be pre-burned in eFuse (see :ref:`pregenerated-flash-encryption-key`) and a copy to be kept on the host. If the flash encryption is configured in Development Mode then it's not necessary to keep a copy of the key or follow these steps, the simpler :ref:`encrypt-partitions` steps can be used.
The file ``my-app-ciphertext.bin`` can then be flashed to offset 0x10000 using ``esptool.py``. To see all of the command line options recommended for ``esptool.py``, see the output printed when ``idf.py build`` succeeds.
..note::
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.
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.
When Flash Encryption is enabled any data read from and written to external SPI RAM through the cache will also be encrypted/decrypted. This happens the same way and with the same key as for Flash Encryption. If Flash Encryption is enabled then encryption for external SPI RAM is also always enabled, it is not possible to separately control this functionality.
- AES-256 key size is 256 bits (32 bytes) read from the ``flash_encryption`` eFuse. The hardware AES engine uses the key in reversed byte order as compared to the storage order in ``flash_encryption``.
- If the ``CODING_SCHEME`` eFuse is set to ``0`` (default, "None" Coding Scheme) then the eFuse key block is 256 bits and the key is stored as-is (in reversed byte order).
- If the ``CODING_SCHEME`` eFuse is set to ``1`` (3/4 Encoding) then the eFuse key block is 192 bits (in reversed byte order), so overall entropy is reduced. The hardware flash encryption still operates on a 256-bit key, after being read (and un-reversed), the key is extended as ``key = key[0:255] + key[64:127]``.
- AES algorithm is used inverted in flash encryption, so the flash encryption "encrypt" operation is AES decrypt and the "decrypt" operation is AES encrypt. This is for performance reasons and does not alter the effeciency of the algorithm.
- Each 32-byte block (two adjacent 16-byte AES blocks) is encrypted with a unique key. The key is derived from the main flash encryption key in ``flash_encryption``, XORed with the offset of this block in the flash (a "key tweak").
- The specific tweak depends on the ``FLASH_CRYPT_CONFIG`` eFuse setting. This is a 4-bit eFuse where each bit enables XORing of a particular range of the key bits:
It is recommended that ``FLASH_CRYPT_CONFIG`` is always left at the default value ``0xF``, so that all key bits are XORed with the block offset. For details, see :ref:`setting-flash-crypt-config`.
- The high 19 bits of the block offset (bit 5 to bit 23) are XORed with the main flash encryption key. This range is chosen for two reasons: the maximum flash size is 16MB (24 bits), and each block is 32 bytes so the least significant 5 bits are always zero.
- There is a particular mapping from each of the 19 block offset bits to the 256 bits of the flash encryption key to determine which bit is XORed with which. See the variable ``_FLASH_ENCRYPTION_TWEAK_PATTERN`` in the ``espsecure.py`` source code for complete mapping.
- To see the full flash encryption algorithm implemented in Python, refer to the ``_flash_encryption_operation()`` function in the ``espsecure.py`` source code.
- XTS-AES is a block cipher mode specifically designed for disc encryption and addresses the weaknesses other potential modes (e.g. AES-CTR) have for this use case. A detailed description of the XTS-AES algorithm can be found in `IEEE Std 1619-2007 <https://ieeexplore.ieee.org/document/4493450>`_.
- To see the full flash encryption algorithm implemented in Python, refer to the `_flash_encryption_operation()` function in the ``espsecure.py`` source code.
- {IDF_TARGET_NAME} use the XTS-AES block chiper mode with 256 bit size for flash encryption.
- XTS-AES is a block chiper mode specifically designed for disc encryption and addresses the weaknesses other potential modes (e.g. AES-CTR) have for this use case. A detailed description of the XTS-AES algorithm can be found in `IEEE Std 1619-2007 <https://ieeexplore.ieee.org/document/4493450>`_.
- To see the full flash encryption algorithm implemented in Python, refer to the `_flash_encryption_operation()` function in the ``espsecure.py`` source code.
..only:: SOC_FLASH_ENCRYPTION_XTS_AES_128 and SOC_EFUSE_CONSISTS_OF_ONE_KEY_BLOCK
.._flash-encryption-algorithm:
Flash Encryption Algorithm
^^^^^^^^^^^^^^^^^^^^^^^^^^
- {IDF_TARGET_NAME} use the XTS-AES block chiper mode with 256 bit size for flash encryption. In case the 128-bit key is stored in the eFuse key block, the final 256-bit AES key is obtained as SHA256(EFUSE_KEY0_FE_128BIT).
- XTS-AES is a block chiper mode specifically designed for disc encryption and addresses the weaknesses other potential modes (e.g. AES-CTR) have for this use case. A detailed description of the XTS-AES algorithm can be found in `IEEE Std 1619-2007 <https://ieeexplore.ieee.org/document/4493450>`_.
- The flash encryption key is stored in ``BLOCK_KEY0`` eFuse and, by default, is protected from further writes or software readout.
- To see the full flash encryption algorithm implemented in Python, refer to the `_flash_encryption_operation()` function in the ``espsecure.py`` source code.
