Flash encryption process and implementation of esp32

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Preface

        After a project is completed , To prevent secondary burning or firmware piracy , Usually program encryption , It's like STM32 Of flash Write protection 、 only ID Program encryption is similar to .
        Lexin provides us flash Encryption scheme ,flash The encryption function is used to encrypt and ESP32 Off chip for use flash The content in . Enable flash After the encryption function , Firmware will be burned in clear text , Then encrypt the data at the first startup . therefore , Physical read flash Will not be able to recover most flash Content .

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One 、 Be careful

        Please familiarize yourself with this article and try to do the encryption experiment , Because encryption is one-time , It may cause the module to become... Due to encryption “ Soft brick ”, After encryption , Will not be available flash_download_tool Tool for firmware burning , You'll get a hint like this ：
 

Two 、flash The encryption process

hypothesis eFuse The value is in the default state , And the boot loader of the firmware is compiled to support flash encryption , be flash The specific process of encryption is as follows ：

• When the first power on reset ,flash All data in is unencrypted （ Plaintext ）.ROM Boot loader loads firmware boot loader .

• The boot loader of the firmware will read FLASH_CRYPT_CNT eFuse value （0b0000000）. Because the value is 0（ Even digit ）, The boot loader for the firmware will be configured and enabled flash Encryption block , At the same time FLASH_CRYPT_CONFIG eFuse The value of is programmed to 0xF.

• The boot loader of the firmware uses RNG（ Random number generation ） Module generation AES-256 A key , And then write it in flash_encryption eFuse in . because flash_encryption eFuse Write and read protection bits have been set , You will not be able to access the key through the software .Flash The encryption operation is completely completed in hardware , The key cannot be accessed through software .

• Flash The encryption block will encrypt flash The content of （ Boot loader for firmware 、 Applications 、 And marked with “ encryption ” Mark the partition ）. In place encryption may take some time （ For large partitions, it takes up to one minute ）.

• The firmware boot loader will be in FLASH_CRYPT_CNT (0b0000001) Set the first available bit in to encrypt the encrypted flash Mark the content . Set an odd number of bits .

• about Development mode , The firmware boot loader only sets DISABLE_DL_DECRYPT and DISABLE_DL_CACHE Of eFuse position , In order to UART Boot loader reburning encrypted binaries . Besides , FLASH_CRYPT_CNT Of eFuse Bits are not write protected .

• about Release pattern , Firmware boot loader settings DISABLE_DL_ENCRYPT、DISABLE_DL_DECRYPT and DISABLE_DL_CACHE Of eFuse Position as 1, To prevent UART Boot loader decrypts flash Content . It's also write protected FLASH_CRYPT_CNT eFuse position . To modify this behavior , see also Enable UART Boot loader encryption / Decrypt .

• Restart the device to start the encrypted image . The firmware boot loader calls flash Decrypt the block flash Content , Load the decrypted content into IRAM in .
          In short ： After the encrypted program starts , First, it will be encrypted by the program boot . Encryption takes time , After encryption is successful , Will be able to FLASH_CRYPT_CNT From 0x0 Turn into 1 or 0xf, Finally start executing the user program .

3、 ... and 、 Encryption mode

       Flash There are two modes of encryption , One is the development model 、 The other is the production mode .

3.1 Development mode encryption

        Since it's encryption , That must require a key , Lexin also provides us with two methods of encryption or generation of keys ：

3.1.1 Use ESP32 The generated key is encrypted

        Using this encryption method , The key is unique , And invisible . The key will not be saved in the system file , It will only be randomly generated and burned to efuse Partition .
Be careful ：
        Before doing the encryption experiment , And ensure that the module does not do any encryption , The query instruction of encryption status is ,（PORT Is the serial port number ）：

espefuse.py -p PORT summary

        This article will be hello_world Routine as an example to encrypt , stay hello_world Run in directory ：

idf.py menuconfig

Do the following ：

• Enable at startup flash encryption
• Select publishing mode （ Note that once the publishing mode is selected ,DISABLE_DL_ENCRYPT and DISABLE_DL_DECRYPT eFuse The bit will be programmed at ROM Disable in download mode flash Encryption hardware ）
• choice UART ROM Download mode （ It is recommended to permanently disable ） （ Note that this option is only available in CONFIG_ESP32_REV_MIN Level set to 3 when (ESP32 V3) You can use .） The default option is to keep enabled UART ROM Download mode , However, it is recommended to disable this mode permanently , To reduce the options available to attackers .
• Select the boot loader log with the appropriate level of detail
• Save the configuration and exit
  
  And then run directly ：

idf.py flash monitor

Screenshot of successful encryption :

Then we can take a look at ：FLASH_CRYPT_CNT

You can see ,FLASH_CRYPT_CNT The value of is set to 1; It means that it has been successfully encrypted .

3.1.2 Use the self generated key for encryption

        Using the self generated key is actually the step of burning the key , Key generation instruction ：

espsecure.py generate_flash_encryption_key my_flash_encryption_key.bin

I suggest saving the key in a separate file , Prevent false deletion . After generating the key , Use the following command to burn the key ：

espefuse.py --port PORT burn_key flash_encryption my_flash_encryption_key.bin

next step , It can be based on 3.1.1 Configuration burning in

3.2 Production mode encryption

        Use the development mode , Can still be used idf.py Script burning , It's just a limit on the number of times , But you can turn off encryption ：

espefuse.py burn_efuse FLASH_CRYPT_CNT

However, the encrypted burning of production mode has no chance of repeated burning , It is usually used in mass production , Because it's disposable , After burning, you can only pass OTA To upgrade the program . Its configuration is as follows ：

Use it directly ：

idf.py flash monitor

So , The blogger's module becomes “ Soft brick ”：

For more information, please skip ：

https://docs.espressif.com/projects/esp-idf/zh_CN/latest/esp32/security/flash-encryption.html#flash-encryption-status
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