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Espressif ESP32 Flash Encryption Test

Overview

Flash encryption is intended for encrypting the contents of the ESP32’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. This is a hardware feature that can be enabled in MCUboot build process and is an additional security measure beyond MCUboot existent features.

The following flash encryption modes are available:

  • Development Mode

    Recommended for use ONLY DURING DEVELOPMENT, as it does not prevent modification and readout of encrypted flash contents.

  • Release Mode

    Recommended for manufacturing and production to prevent physical readout of encrypted flash contents. When release mode is selected..

With flash encryption enabled, the following types of data are encrypted by default:

  • Bootloader area

  • Application area

  • Storage area

For more details, check ESP32 Flash Encryption and MCUBoot Readme.

Software Requirements

The following Python modules are required when building flash encryption sample:

  • cryptography

  • imgtool>=1.9.0

Setup

This sample code isn’t enough to enable flash encryption as it only consists on displaying encrypted/decrypted data. It requires MCUBoot bootloader previously configured to enable the feature. Follow the instructions provided at MCUBoot Readme prior to running this sample.

Warning

When enabling the Flash Encryption, user can encrypt the content either using a device generated key (remains unknown and unreadable) or a host generated key (owner is responsible for keeping the key private and safe as .bin file). After the flash encryption gets enabled through eFuse burning on the device, all read and write operations are decrypted/encrypted in runtime.

Supported SoCs

The following SoCs are supported by this sample code so far:

  • ESP32

Building and Running

Make sure you have your board connected over USB port.

west build -b esp32 samples/boards/esp32/flash_encryption
west flash

Sample Output

To check the output of this sample, run west espressif monitor or any other serial console program (e.g., minicom, putty, screen, etc). This example uses west espressif monitor, which automatically detects the serial port at /dev/ttyUSB0:

$ west espressif monitor

The sample code writes a known buffer into the storage area defined in DTS file. Then, it dumps 32-bytes of the same memory area in plaintext mode. The content is encrypted, meaning that a reading attack to the off-chip memory is safe. Last step is to perform the memory reading using proper spi_flash call, which decrypts the content as expected.

*** Booting Zephyr OS build v2.7.99-2729-geb08584393d6  ***
[00:00:00.453,000] <inf> flash_encryption: Found flash controller FLASH_CTRL.

[00:00:00.453,000] <inf> flash_encryption: BUFFER CONTENT
                                        00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e 0f |........ ........
                                        10 11 12 13 14 15 16 17  18 19 1a 1b 1c 1d 1e 1f |........ ........
[00:00:00.482,000] <inf> flash_encryption: FLASH RAW DATA (Encrypted)
                                        9a 06 93 76 12 cb 0f 7e  ec c5 12 6f 64 db d1 ff |...v...~ ...od...
                                        08 e6 be 0c cd 06 6d ad  7d 55 3d 57 bf b7 be 0a |......m. }U=W....
[00:00:00.482,000] <inf> flash_encryption: FLASH DECRYPTED DATA
                                        00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e 0f |........ ........
                                        10 11 12 13 14 15 16 17  18 19 1a 1b 1c 1d 1e 1f |........ ........