How to validate flash encryption in Development mode?
Posted: Sat Feb 11, 2023 11:47 pm
I am trying to learn more about flash encryption and have read the docs multiple times but am still confused as to why I can dump encrypted flash in development mode with esptool.py and see plain strings. I followed the instructions in the documentation by enabling flash encryption and allowing the first boot to happen. I saw the bootloader output saying it was encrypting the partitions and all booted fine. When I use idf.py flash I get the error as expected that the firmware isn't valid. If I use idf.py encrypted-flash everything works fine. However, when I use esptool.py to dump the contents of flash it is not encrypted at all as I can see all strings in plaintext.
In the ESP32 technical reference it says that the decrypt block is disabled in Download mode when DISABLE_DL_DECRYPT is set, which the first flash does after enabling flash encryption.
So I have a couple of questions:
1. What steps/settings can I follow with a brand new device to enable flash encryption and then see on a run of esptool.py that the contents are, indeed, encrypted?
2. What is idf.py encrypted-flash doing different than idf.py flash? I thought the flash was updated with plaintext and then encrypted by the device on boot.
Here is the dump of my efuses for validation. The count is higher because I disabled and reenabled encryption but the results are the same. I can still see everything in plaintext in the dumped binary.
Thanks so much for any assistance in helping me understand this better.
In the ESP32 technical reference it says that the decrypt block is disabled in Download mode when DISABLE_DL_DECRYPT is set, which the first flash does after enabling flash encryption.
So I have a couple of questions:
1. What steps/settings can I follow with a brand new device to enable flash encryption and then see on a run of esptool.py that the contents are, indeed, encrypted?
2. What is idf.py encrypted-flash doing different than idf.py flash? I thought the flash was updated with plaintext and then encrypted by the device on boot.
Here is the dump of my efuses for validation. The count is higher because I disabled and reenabled encryption but the results are the same. I can still see everything in plaintext in the dumped binary.
Code: Select all
EFUSE_NAME (Block) Description = [Meaningful Value] [Readable/Writeable] (Hex Value)
----------------------------------------------------------------------------------------
Calibration fuses:
BLK3_PART_RESERVE (BLOCK0): BLOCK3 partially served for ADC calibration data = False R/W (0b0)
ADC_VREF (BLOCK0): Voltage reference calibration = 1058 R/W (0b10110)
Config fuses:
XPD_SDIO_FORCE (BLOCK0): Ignore MTDI pin (GPIO12) for VDD_SDIO on reset = False R/W (0b0)
XPD_SDIO_REG (BLOCK0): If XPD_SDIO_FORCE, enable VDD_SDIO reg on reset = False R/W (0b0)
XPD_SDIO_TIEH (BLOCK0): If XPD_SDIO_FORCE & XPD_SDIO_REG = 1.8V R/W (0b0)
CLK8M_FREQ (BLOCK0): 8MHz clock freq override = 51 R/W (0x33)
SPI_PAD_CONFIG_CLK (BLOCK0): Override SD_CLK pad (GPIO6/SPICLK) = 0 R/W (0b00000)
SPI_PAD_CONFIG_Q (BLOCK0): Override SD_DATA_0 pad (GPIO7/SPIQ) = 0 R/W (0b00000)
SPI_PAD_CONFIG_D (BLOCK0): Override SD_DATA_1 pad (GPIO8/SPID) = 0 R/W (0b00000)
SPI_PAD_CONFIG_HD (BLOCK0): Override SD_DATA_2 pad (GPIO9/SPIHD) = 0 R/W (0b00000)
SPI_PAD_CONFIG_CS0 (BLOCK0): Override SD_CMD pad (GPIO11/SPICS0) = 0 R/W (0b00000)
DISABLE_SDIO_HOST (BLOCK0): Disable SDIO host = False R/W (0b0)
Efuse fuses:
WR_DIS (BLOCK0): Efuse write disable mask = 32896 R/W (0x8080)
RD_DIS (BLOCK0): Efuse read disable mask = 1 R/W (0x1)
CODING_SCHEME (BLOCK0): Efuse variable block length scheme
= NONE (BLK1-3 len=256 bits) R/W (0b00)
KEY_STATUS (BLOCK0): Usage of efuse block 3 (reserved) = False R/W (0b0)
Identity fuses:
MAC (BLOCK0): Factory MAC Address
= 24:0a:c4:c1:2e:88 (CRC 0x4b OK) R/W
MAC_CRC (BLOCK0): CRC8 for factory MAC address = 75 R/W (0x4b)
CHIP_VER_REV1 (BLOCK0): Silicon Revision 1 = True R/W (0b1)
CHIP_VER_REV2 (BLOCK0): Silicon Revision 2 = False R/W (0b0)
CHIP_VERSION (BLOCK0): Reserved for future chip versions = 2 R/W (0b10)
CHIP_PACKAGE (BLOCK0): Chip package identifier = 1 R/W (0b001)
CHIP_PACKAGE_4BIT (BLOCK0): Chip package identifier #4bit = False R/W (0b0)
MAC_VERSION (BLOCK3): Version of the MAC field = 0 R/W (0x00)
Security fuses:
FLASH_CRYPT_CNT (BLOCK0): Flash encryption mode counter = 7 R/W (0b0000111)
UART_DOWNLOAD_DIS (BLOCK0): Disable UART download mode (ESP32 rev3 only) = False R/W (0b0)
FLASH_CRYPT_CONFIG (BLOCK0): Flash encryption config (key tweak bits) = 15 R/W (0xf)
CONSOLE_DEBUG_DISABLE (BLOCK0): Disable ROM BASIC interpreter fallback = True R/- (0b1)
ABS_DONE_0 (BLOCK0): Secure boot V1 is enabled for bootloader image = False R/W (0b0)
ABS_DONE_1 (BLOCK0): Secure boot V2 is enabled for bootloader image = False R/W (0b0)
JTAG_DISABLE (BLOCK0): Disable JTAG = True R/W (0b1)
DISABLE_DL_ENCRYPT (BLOCK0): Disable flash encryption in UART bootloader = False R/- (0b0)
DISABLE_DL_DECRYPT (BLOCK0): Disable flash decryption in UART bootloader = True R/- (0b1)
DISABLE_DL_CACHE (BLOCK0): Disable flash cache in UART bootloader = True R/- (0b1)
BLOCK1 (BLOCK1): Flash encryption key
= ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? -/-
BLOCK2 (BLOCK2): Secure boot key
= 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 R/W
BLOCK3 (BLOCK3): Variable Block 3
= 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 R/W
Wow, don't I feel foolish. As part of enabling flash encryption I left bootloader logging on and, therefore, had to move the app partition after making more room for the bootloader. This is exactly what was going on. I did an idf.py erase_flash followed by an idf.py encrypted-flash. Everything ran fine and then I used esptool.py to dump the flash contents and confirmed it is fully encrypted.