Questions about SDFU implentation and security

[i_4556]

Hello and thanks for reading,

We are developing for the NRF5340 and one of the features we are very excited about is the OTA firmware updates via the SDFU.

I have followed the instructions here :  Add DFU support to your application and things seem to be working in that the correct services appear when I then look using the NRF app.

I have not yet been able to fully test DFU, because every time I try the app crashes to the homescreen. This is unfortunate; it looks to me like a problem with the mobile app, hopefully it gets fixed soon.

In the meantime, I have two questions about SDFU.

Firstly, does the bootloader perform a full chip erase when new firmware is loaded? Like if I use the DFU to upload a new firmware that is substantially smaller in size, is there a bunch of old code in flash waiting to be stolen?

Secondly, is there any way to tell the NRF to only accept firmware images that are signed or otherwise authenticated? I assume there must be, could you provide me a link to a guide or example or something of how to implement that?

Thank you very much!

[Amanda Hsieh]

Hi,

I have not yet been able to fully test DFU, because every time I try the app crashes to the homescreen. This is unfortunate; it looks to me like a problem with the mobile app, hopefully it gets fixed soon.

Which phone are you using?

Are you using the nRF Connect Device Manager app?

Which NCS version and sample are you using?

In NCS v2.5.1, you could enable the DFU function with "CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU=y" in the peripheral_lbs sample.

Firstly, does the bootloader perform a full chip erase when new firmware is loaded?

No, it would swap the image with the current and new image. See my colleague's explanation in https://github.com/hellesvik-nordic/samples_for_nrf_connect_sdk/tree/main/bootloader_samples/smp#firmware-update 

Secondly, is there any way to tell the NRF to only accept firmware images that are signed or otherwise authenticated?

See my colleague's explanation in https://github.com/hellesvik-nordic/samples_for_nrf_connect_sdk/tree/main/bootloader_samples/keys_and_signatures

Regards,
Amanda H.

[i_4556]

Thank you for your reply. I am using an Iphone 12 running IOS 16.6. I am using the NRF connect app on the phone. I am using SDK 2.5.0, and per the example simply added

CONFIG_BOOTLOADER_MCUBOOT=y

CONFIG_NCS_SAMPLE_MCUMGR_BT_OTA_DFU=y

to my proj.config file for my existing codebase.

If the firmware just swaps the images, what is to stop a malicious actor uploading firmware that then dumps the old code out the serial port or otherwise bypasses readback protections? I assume the only solution is, as described in your second link, I need to add keys or something to block those uploads in the first place?

Regarding the second link, is there anything special I need to know because I am using the nrf5340 with its seperate BLE processor, or will I still be able to use the code broadly as shown?

Thank you!

[Amanda Hsieh]

Hi,

Here is my example peripheral_hids_mouse.7z. After uploading with the update_image/nRF53_HID_app_update.bin, it would show "build time: Jan 29 2024 09:51:56"

Beware that this code/configuration is not fully tested or qualified and should be considered provided “as-is”. Please test it with your application and let me know if you find any issues.

i_4556 said:

EDIT: Tested the base case with no signature on my other coworker's iphone 13 and that works fine. Not sure if its all Iphone12 or just mine it doesnt work on...

Could you try to use the nRF Connect Device Manager App https://apps.apple.com/us/app/nrf-connect-device-manager/id1519423539? Please make sure it's the latest version v1.5.3. 

The minimum required iOS version is 9.0, originally released in Fall of 2015.

i_4556 said:

EDIT2: Tested with my old android phone running Android 7.0 and once again app crashes when trying to perform DFU. It doesnt crash on my Android 9.0 but also doesnt work, just flashes through the screens like above.

Please try to use the nRF Connect Device Manager app https://play.google.com/store/apps/details?id=no.nordicsemi.android.nrfconnectdevicemanager&hl=en_US 

The minimum required Android version is 5.0 (Android Lollipop) due to a requirement for high MTU.

-Amanda H.

[i_4556]

Hello,

Thank you for your continued assistance. Per your suggestion, I tried starting with the base case (no security) with the NRF Connect Device Manager app, and that has worked with my iPhone, and every other phone I have tried.

I am having a problem with the encryption however, in that it wont build. It fails during the signing/encrypting phase, with the following error:

PermissionError: [Errno 13] Permission denied: 'C:/Users/Isaac/Downloads/peripheral_hids_mouse/peripheral_hids_mouse/child_image/mcuboot/'

The code embed tool has suddenly stopped working, but here is the relevant terminal message

[3/12] Generating ../../zephyr/app_update.bin
FAILED: zephyr/app_update.bin
cmd.exe /C "cd /D C:\Users\Isaac\Downloads\peripheral_hids_mouse\peripheral_hids_mouse\build\modules\mcuboot && C:\ncs\toolchains\c57af46cb7\opt\zephyr-sdk\arm-zephyr-eabi\bin\arm-zephyr-eabi-objcopy.exe --input-target=ihex --output-target=binary --gap-fill=0xff C:/Users/Isaac/Downloads/peripheral_hids_mouse/peripheral_hids_mouse/build/zephyr/mcuboot_primary_app.hex C:/Users/Isaac/Downloads/peripheral_hids_mouse/peripheral_hids_mouse/build/zephyr/app_to_sign.bin && C:\ncs\toolchains\c57af46cb7\opt\bin\python.exe C:/ncs/v2.5.0/bootloader/mcuboot/scripts/imgtool.py sign --key C:/Users/Isaac/Downloads/peripheral_hids_mouse/peripheral_hids_mouse/child_image/mcuboot/enc-ec256-priv.pem --header-size 0x200 --align 4 --version 0.0.0+0 --pad-header --encrypt C:/Users/Isaac/Downloads/peripheral_hids_mouse/peripheral_hids_mouse/child_image/mcuboot/ --slot-size 0x74000 C:/Users/Isaac/Downloads/peripheral_hids_mouse/peripheral_hids_mouse/build/zephyr/app_to_sign.bin C:/Users/Isaac/Downloads/peripheral_hids_mouse/peripheral_hids_mouse/build/zephyr/app_update.bin"
Traceback (most recent call last):

  File "C:/ncs/v2.5.0/bootloader/mcuboot/scripts/imgtool.py", line 22, in <module>

    main.imgtool()

  File "C:\ncs\toolchains\c57af46cb7\opt\bin\Lib\site-packages\click\core.py", line 1128, in __call__

    return self.main(*args, **kwargs)

  File "C:\ncs\toolchains\c57af46cb7\opt\bin\Lib\site-packages\click\core.py", line 1053, in main

    rv = self.invoke(ctx)

  File "C:\ncs\toolchains\c57af46cb7\opt\bin\Lib\site-packages\click\core.py", line 1659, in invoke

    return _process_result(sub_ctx.command.invoke(sub_ctx))

  File "C:\ncs\toolchains\c57af46cb7\opt\bin\Lib\site-packages\click\core.py", line 1395, in invoke

    return ctx.invoke(self.callback, **ctx.params)

  File "C:\ncs\toolchains\c57af46cb7\opt\bin\Lib\site-packages\click\core.py", line 754, in invoke

    return __callback(*args, **kwargs)

  File "C:\ncs\v2.5.0\bootloader\mcuboot\scripts\imgtool\main.py", line 425, in sign

    enckey = load_key(encrypt) if encrypt else None

  File "C:\ncs\v2.5.0\bootloader\mcuboot\scripts\imgtool\main.py", line 91, in load_key

    key = keys.load(keyfile)

  File "C:\ncs\v2.5.0\bootloader\mcuboot\scripts\imgtool\keys\__init__.py", line 49, in load

    with open(path, 'rb') as f:

PermissionError: [Errno 13] Permission denied: 'C:/Users/Isaac/Downloads/peripheral_hids_mouse/peripheral_hids_mouse/child_image/mcuboot/'

The error seems related to a path issue, where instead of pointing to the key file it only points to the directory with the key file. I have the same problem with my code, where I have basically copied your work related to SDFU.

I am using 2.5.0 if that matters in some way.

Thanks again for all your help!

EDIT: I tried invoking the tool in standalone mode, using the exact same command as the visual studio build, and the problem becomes evident:

C:/ncs/v2.5.0/bootloader/mcuboot/scripts/imgtool.py sign --key C:/Users/Isaac/Documents/peripheral_hids_mouse/child_image/mcuboot/enc-ec256-priv.pem --header-size 0x200 --align 4 --version 0.0.0+0 --pad-header --encrypt C:/Users/Isaac/Documents/peripheral_hids_mouse/child_image/mcuboot/ --slot-size 0x74000 C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_to_sign.bin C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_update.bin"

Notice that after the --encrypt it points to a directory and not a file.

I dont know how to fix this in the Visual Studio configs, but I tried fixing it in the terminal, and that led to another interesting error:

TypeError: ECPrivateKey.sign() got an unexpected keyword argument 'signature_algorithm'

A quick google search shows this is a known issue from basically last week that has already been patched, so im going to apply the patch and report back. github.com/.../10237

[Amanda Hsieh]

Hi, 

Try to shorter the path, for example:  C:/Users/Isaac/Downloads/peripheral_hids_mouse/peripheral_hids_mouse/

Windows has character limitations with the executed path, so too many characters in the path would cause unexpected errors. 

-Amanda H.

[i_4556]

Hello,

Thank you for your reply. I dont think the issue is path length.

I tried invoking the imgtool.py tool in the windows terminal, using the exact same command as the visual studio build, and the problem becomes evident:

C:/ncs/v2.5.0/bootloader/mcuboot/scripts/imgtool.py sign --key C:/Users/Isaac/Documents/peripheral_hids_mouse/child_image/mcuboot/enc-ec256-priv.pem --header-size 0x200 --align 4 --version 0.0.0+0 --pad-header --encrypt C:/Users/Isaac/Documents/peripheral_hids_mouse/child_image/mcuboot/ --slot-size 0x74000 C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_to_sign.bin C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_update.bin"

Notice that after the --encrypt it points to a directory and not a file.

I dont know how to fix this in the Visual Studio configs, but I tried fixing it in the terminal by simply adding the location of the key file, making the command

scripts/imgtool.py sign --key C:/Users/Isaac/Documents/peripheral_hids_mouse/enc-ec256-priv.pem --header-size 0x200 --align 4 --version 0.0.0+0 --pad-header --encrypt C:/Users/Isaac/Documents/peripheral_hids_mouse/enc-ec256-priv.pem --slot-size 0x74000 C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_to_sign.bin C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_update.bin"

And now it seems to be working. How do I make that change in the config files for the project?

[Amanda Hsieh]

Hi, 

I am trying to reproduce it with the Visual Studio. Could you provide the details on how you encrypt?

-Amanda H.

[i_4556]

Hello, and thanks for your constant support.

When I get it to work I manually invoke imgtool.py from windows command prompt with the command

C:\ncs\v2.5.0\bootloader\mcuboot>python scripts/imgtool.py sign --key C:/Users/Isaac/Documents/peripheral_hids_mouse/enc-ec256-priv.pem --header-size 0x200 --align 4 --version 0.0.0+0 --pad-header --encrypt C:/Users/Isaac/Documents/peripheral_hids_mouse/enc-ec256-priv.pem --slot-size 0x74000 C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_to_sign.bin C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_update.bin"

[Amanda Hsieh]

Hi,

Not sure if this is what you ask for VS Code or not. 

You could add the following configs to child_image/mcuboot/prj.conf to encrypt the image

CONFIG_BOOT_ENCRYPT_IMAGE=y
CONFIG_BOOT_ENCRYPTION_KEY_FILE="enc-ec256-priv.pem"

-Amanda H.

[i_4556]

Hello,

Thank you for all your support. Per your suggestion, I added the two Configs. This has resulted in some strange behaviour from the build in visual studio. Please take a look at the error message below, I have bolded the relevant portion, and then underlined the paths to the key files:

FAILED: zephyr/app_update.bin
cmd.exe /C "cd /D C:\Users\Isaac\Documents\peripheral_hids_mouse\build\modules\mcuboot && C:\ncs\toolchains\c57af46cb7\opt\zephyr-sdk\arm-zephyr-eabi\bin\arm-zephyr-eabi-objcopy.exe --input-target=ihex --output-target=binary --gap-fill=0xff C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/mcuboot_primary_app.hex C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_to_sign.bin && C:\ncs\toolchains\c57af46cb7\opt\bin\python.exe C:/ncs/v2.5.0/bootloader/mcuboot/scripts/imgtool.py sign --key C:/ncs/v2.5.0/bootloader/mcuboot/mcubootenc-ec256-priv.pem --header-size 0x200 --align 4 --version 0.0.0+0 --pad-header --encrypt mcubootenc-ec256-priv.pem --slot-size 0x74000 C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_to_sign.bin C:/Users/Isaac/Documents/peripheral_hids_mouse/build/zephyr/app_update.bin"
Traceback (most recent call last):

As you can see, the path to the key file is wrong, and has somehow set itself to the location of the default key files that come with NRF SDK.

Where did it get this path from, and how do I correct it?

Earlier in the build it correctly located the key files:

MCUBoot bootloader key file: C:/Users/Isaac/Documents/peripheral_hids_mouse/child_image/mcuboot/enc-ec256-priv.pem
MCUBoot bootloader encryption key file: C:/Users/Isaac/Documents/peripheral_hids_mouse/child_image/mcuboot/enc-ec256-priv.pem

EDIT: It seems to have worked, let me do a little more testing but somehow doing a second clean build seems to have worked.

[i_4556]

Everything seems to now be working; I can successfully do an update with the correct key, and it rejects attempts to update with an incorrect key.

Thank you so much for all your help.

I have one last little question: right now in the mcuboot proj.config file I have

CONFIG_BOOT_SIGNATURE_KEY_FILE="enc-ec256-priv.pem"

Could I instead define it in cmakelists.txt, that way I can make it easier to swap between the dev key and the release key? Something like:

set(CONFIG_BOOT_SIGNATURE_KEY_FILE \"${CMAKE_CURRENT_SOURCE_DIR}/child_image/mcuboot/enc-ec256-priv.pem\")

[i_4556]

Interesting but mixed results.

I started by making the changes in the sandbox, basically the nrf mouse example we have been playing around with. I change the CMakeLists to include

if (EXISTS "K/path_to_key/key_priv.pem")

set(mcuboot_CONFIG_BOOT_SIGNATURE_KEY_FILE \"K/path_to_key/key_priv.pem\")

set(mcuboot_CONFIG_BOOT_ENCRYPTION_KEY_FILE \"K/path_to_key/key_priv.pem\")

else()

add_compile_definitions(USING_DEV_KEY)

set(mcuboot_CONFIG_BOOT_SIGNATURE_KEY_FILE \"${CMAKE_CURRENT_SOURCE_DIR}/keys/dev_ec256_priv.pem\")

set(mcuboot_CONFIG_BOOT_ENCRYPTION_KEY_FILE \"${CMAKE_CURRENT_SOURCE_DIR}/keys/dev_ec256_priv.pem\")

endif()

And in the nrf mouse example that worked fine.

I then tried moving everything to my main project with all the real code, and its not working.

Here is my entire CmakeList from the main project:

cmake_minimum_required(VERSION 3.20.0)
find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
project(none)

#check for key file
if (EXISTS "K/release_key/key.pem")
set(mcuboot_CONFIG_BOOT_SIGNATURE_KEY_FILE \"K/release_key/key.pem\")
set(mcuboot_CONFIG_BOOT_ENCRYPTION_KEY_FILE \"K/release_key/key.pem\")
else()
add_compile_definitions(USING_DEV_KEY)
set(mcuboot_CONFIG_BOOT_SIGNATURE_KEY_FILE \"${CMAKE_CURRENT_SOURCE_DIR}/keys/dev_ec256_priv.pem\")
set(mcuboot_CONFIG_BOOT_ENCRYPTION_KEY_FILE \"${CMAKE_CURRENT_SOURCE_DIR}/keys/dev_ec256_priv.pem\")
endif()

ADD_CUSTOM_COMMAND(TARGET app
   PRE_BUILD
   COMMAND python ${CMAKE_CURRENT_SOURCE_DIR}/version_control_hook.py
)

FILE(GLOB app_sources src/*.c*)

target_sources(app PRIVATE ${app_sources})

Instead, it reverts to the keys in the NRF folder.

Do you have any suggestions for what im doing wrong?