ESP32 Forum

For a production environment where secure boot and flash encryption are required, the recommendation seems to be to flash a plaintext firmware either via preprogrammed flash chips or over uart and let the chip generate its own unique key and encrypt the flash in place on first boot. Obviously one wants to minimize programming and test time in production, so is this the most optimized way? What is the duration of this operation for a typical firmware image?

Encrypting in-place can take some time (up to a minute for large partitions.)

The other option is to generate a high entropy random key off-device and program it into the efuse memory, but again the recommendation is to flash a plaintext firmware first.

So the question becomes is it possible to flash a pre-encrypted firmware over uart or to have the stub loader encrypt on write or to write pre-encrypted directly to the flash chip using external spi interface and then manually set the FLASH_CRYPT_CNT to 1 so the flash is not reencrypted?

The goal being to save any precious seconds the device has to spend on the programming/testing jigs.

WiFive wrote: The other option is to generate a high entropy random key off-device and program it into the efuse memory, but again the recommendation is to flash a plaintext firmware first.

This is primarily a security recommendation, because in other configuration it's tempting to share the same key across all devices. This means that disclosure of one key (via, for example, a factory leak or side channel power analysis) would potentially compromise all devices. Whereas with the on-device key generation, there is no predistributed key for a factory to leak and side channel analysis is only per-device.

If you're not worried about these kind of compromises then you can do whatever you think is wise.

WiFive wrote: So the question becomes is it possible to flash a pre-encrypted firmware over uart or to have the stub loader encrypt on write or to write pre-encrypted directly to the flash chip using external spi interface and then manually set the FLASH_CRYPT_CNT to 1 so the flash is not reencrypted?

You can pre-encrypt data using the 'espsecure.py encrypt_flash_data` command (mentioned in the flash encryption documents) and then flash it. This is also a relative slow process if using per-device keys (the encryption algorithm is implemented unoptimised in Python so far) but it happens off-device so you have some flexibility about when/how you do it.

However the only supported way to write the pre-generated key to efuse and also burn FLASH_CRYPT_CNT is via the serial loader, or following the "encrypt all partitions on first boot" flow.

You could write a custom bootloader that does this on first boot, however. The existing "encrypt on first boot" logic will skip any app partition which doesn't hold an unencrypted app, so you should be able still make an initial unencrypted boot and have it only encrypt the bootloader and partiton table (which would need to be plaintext, but are much smaller so quicker to encrypt) and set FLASH_CRYPT_CNT before rebooting for a fully encrypted boot.

The additional first boot logic you'd need would involve reading the flash key from a preprogrammed flash sector or partition, writing it to efuse, then erasing that sector of flash (maybe write/erase several times if worried about forensic analysis - I'm not an expert on how secure NOR flash erasing is), and then following the default first boot encryption process for the rest. We don't support that workflow, but I can't think of a reason it wouldn't work.

WiFive wrote: The goal being to save any precious seconds the device has to spend on the programming/testing jigs.

Understood. There may also be some optimisations we can do for on-device encryption as well, although the process will always take some amount of time.

Angus

Thanks, the custom bootloader and key to be programmed and deleted on flash sounds like a good option in combination with accelerated/parallel off-chip encryption routines. Will dig into the source code.

Otherwise is there a way to use the on-chip encryption within the stub loader to receive plaintext over serial and encrypt it before writing to flash? Or is there a conflict with DISABLE_DL_ENCRYPT? And if so is there an option to reimplement the encryption algorithm in the stub with hardware AES support? Or would the performance be too reduced?

What ℅ of the encrypt in place operation would you say is due to erase times and do you mean to optimize it by increasing read/erase block size or is this already optimized?

WiFive wrote:Thanks, the custom bootloader and key to be programmed and deleted on flash sounds like a good option in combination with accelerated/parallel off-chip encryption routines. Will dig into the source code.

Great. Let us know if there's any changes you need in esp-idf in order for this to be easy to modify.

WiFive wrote:Otherwise is there a way to use the on-chip encryption within the stub loader to receive plaintext over serial and encrypt it before writing to flash? Or is there a conflict with DISABLE_DL_ENCRYPT? And if so is there an option to reimplement the encryption algorithm in the stub with hardware AES support? Or would the performance be too reduced?

What ℅ of the encrypt in place operation would you say is due to erase times and do you mean to optimize it by increasing read/erase block size or is this already optimized?

I don't have numbers on this, but most of the slow part is encrypting the data. If DISABLE_DL_ENCRYPT efuse is not burned then it is theoretically possible to put plaintext into esptool.py over serial and have it written to the flash encrypted, but this would take close to the same time that encrypting-in-place takes.