docs: Add description of manual encryption steps

Closes https://github.com/espressif/esp-idf/issues/5037

docs/en/security/flash-encryption.rst

@@ -373,7 +373,9 @@ Enabling flash encryption will increase the size of bootloader, which might requ
 
   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.
 
-At this stage, if you need to update and re-flash binaries, see :ref:`encrypt-partitions`.
+If using Development Mode, then the easiest way to update and re-flash binaries is :ref:`encrypt-partitions`.
+
+If using Release Mode, then it is possible to pre-encrypt the binaries on the host and then flash them as ciphertext. See :ref:`manual-encryption`.
 
 
 .. _encrypt-partitions:
@@ -440,9 +442,9 @@ For subsequent plaintext field updates, use :ref:`OTA scheme <updating-encrypted
 
 .. note::
 
-    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 using ``{IDF_TARGET_ENCRYPT_COMMAND}`` and then flashing the ciphertext.
+    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`.
 
-. _flash-encrypt-best-practices:
+.. _flash-encrypt-best-practices:
 
 Best Practices
 ^^^^^^^^^^^^^^
@@ -661,7 +663,14 @@ For general information about ESP-IDF OTA updates, please refer to :doc:`OTA <..
 
 .. _updating-encrypted-flash-serial:
 
+Updating Encrypted Flash via Serial
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Flashing an encrypted device via serial bootloader requires that the serial bootloader download interface has not been permanently disabled via eFuse.
+
+In Development Mode, the recommended method is :ref:`encrypt-partitions`.
 
+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`.
 
 Disabling Flash Encryption
 --------------------------
@@ -857,6 +866,48 @@ By default, when Flash Encryption is enabled (in either Development or Release m
 
 See :ref:`jtag-debugging-security-features` for more information about using JTAG Debugging with Flash Encryption.
 
+
+.. _manual-encryption:
+
+Manually Encrypting Files
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+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 key file should be a single raw binary file (example: ``key.bin``).
+
+.. only:: SOC_FLASH_ENCRYPTION_XTS_AES_256
+
+    .. note::
+
+       If using AES-XTS-256 then the key file is generated in two parts for programming via ``espefuse.py`` (``XTS_AES_256_KEY_1`` and ``XTS_AES_256_KEY_2``). ``espsecure.py`` currently only supports a single key file for encrypt/decrypt, so the individual files used for ``XTS_AES_256_KEY_1`` and ``XTS_AES_256_KEY_2`` should be manually concatenated to create a single file 64 bytes long.
+
+For example, these are the steps to encrypt the file ``build/my-app.bin`` to flash at offset 0x10000. Run espsecure.py as follows:
+
+.. only:: esp32
+
+    .. code-block:: bash
+
+       espsecure.py encrypt_flash_data --keyfile /path/to/key.bin --address 0x10000 --output my-app-ciphertext.bin build/my-app.bin
+
+.. only:: not esp32
+
+    .. code-block:: bash
+
+       espsecure.py encrypt_flash_data --aes-xts --keyfile /path/to/key.bin --address 0x10000 --output my-app-ciphertext.bin build/my-app.bin
+
+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.
+
+   .. only:: esp32
+
+       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.
+
 Technical Details
 -----------------