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Aditya Patwardhan a57aa71190

feat(security): Add security_features_app example to demonstrate security features

2024-06-27 16:56:55 +05:30

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Supported Targets	ESP32-C3

All Secure

This example demonstrates how to enable all of the available security features on the esp32c3 target at once. This example shall use combination of host based python tools and firmware for enabling all the related security eFuses. For simplicity the security features such as Secure Boot V2, Flash Encryption, NVS Encryption shall be enabled through host based python tools (e.g., espefuse). Some additional security eFuses shall be enabled in the firmware.

The device that has followed all the steps mentioned below can be used for production use-cases.

Caution

The instructions in the example directly burn eFuses and once done, it cannot be reverted. Please go through the below steps carefully before executing the example. All the steps must be followed without any changes and in the exact sequence, otherwise the device may end up in an unrecoverable state.

Hardware Required

A development board with ESP32C3 SoC
A USB cable for power supply and programming

Note: The hardware is not required if you plan to run the example with help of QEMU.

Pre-requisites

1. Set ESPPORT

In the example, we need to use the Serial port in nearly all the commands. To make it easier, we shall set the ESPPORT environment variable at once and reuse it later. See the documentation about Connecting the ESP device to PC to find out the Serial port.

export ESPPORT=/* Serial port to which esp is connected */

Setup serial port for QEMU

If you are enabling eFuses on esp32c3 emulated using QEMU then we shall set the serial port as follows:

export ESPPORT=socket://localhost:5555

Please note that this value is set in correspondence with the command to start serial connection with QEMU, do not change the values.

Please make sure to perform this step every time when you open a new terminal to use esptool/espefuse commands.

2. Erase flash

We shall erase the flash on the device to ensure a clean state.

idf.py -p $ESPPORT erase_flash

3. Install esptool

We shall require esptool utility which can be installed as follows:

pip install esptool

4. Installing qemu (optional)

If you want to enable the security features on an esp32c3 which has been virtually emulated using qemu then you need to install the necessary packages.

The detailed instructions on how to use QEMU can be found in the QEMU documentation.

Enabling Security Features

We shall enable the necessary security features one by one as follows:

Enabling Secure Boot V2

For more details about Secure Boot V2 protocol checkout the Secure boot V2 documentation.

Please follow below steps to enable Secure Boot V2:

Generate Signing Key

espsecure.py generate_signing_key --version 2 --scheme rsa3072 secure_boot_signing_key.pem

Generate Public Key Digest

espsecure.py digest_sbv2_public_key --keyfile secure_boot_signing_key.pem --output digest.bin

Burn the key digest in eFuse
```
espefuse.py --port $ESPPORT --chip esp32c3 burn_key BLOCK_KEY0 digest.bin SECURE_BOOT_DIGEST0
```
where $ESPPORT is the serial port to which the esp32c3 chip is connected. We have used BLOCK_KEY0 here to store the Secure Boot V2 digest. Generally, the BLOCK can be a free key block from BLOCK_KE0 to BLOCK_KEY5.

Enable Secure Boot V2

espefuse.py --port $ESPPORT --chip esp32c3 burn_efuse SECURE_BOOT_EN

With above steps the Secure Boot V2 feature shall be enabled. The firmware build step is configured to generate signed binaries for bootloader and application by default (so there is no need to manually sign). The necessary security eFuses are yet to be burned. They shall be burned by the application when first launched.

Use multiple Secure Boot V2 signing keys

It is recommended to use multiple secure boot v2 signing keys.

When the application is built (later in the workflow) the bootloader and application shall only be signed with the first key. To sign it with multiple keys, please follow below additional steps:

Repeat Step 1 to Step 3 for secure_boot_signing_key_2.pem and secure_boot_signing_key_3.pem respectively.

Sign it with remaining two keys by executing following commands for secure_boot_signing_key_2.pem and secure_boot_signing_key_3.pem respectively:

espsecure.py sign_data --version 2 --keyfile 	/* Signing key placeholder */ --output bootloader-signed.bin build/bootloader/bootloader.bin

espsecure.py sign_data --version 2 --keyfile 	/* Signing key placeholder */ --output my-app-signed.bin build/security_features.bin

Enabling Flash Encryption

Details about the Flash Encryption protocol can be found at the Flash Encryption documentation The indicates the status of Flash Encryption feature for the chip. The example also demonstrates writing and reading encrypted partitions in flash.

Please follow below steps to enable Flash Encryption:

Generate Flash Encryption keys
```
espsecure.py generate_flash_encryption_key my_flash_encryption_key.bin
```
Note: It is recommended to ensure that the RNG used by host machine to generate the flash encryption key has good entropy
Burn Flash Encryption keys
```
espefuse.py --port $ESPPORT burn_key BLOCK_KEY1 my_flash_encryption_key.bin XTS_AES_128_KEY
```
We have used BLOCK_KEY1 here to store the Flash Encryption key. Generally, the BLOCK can be a free key block from BLOCK_KE0 to BLOCK_KEY4.

Enable Flash Encryption

espefuse.py --port $ESPPORT --chip esp32c3 burn_efuse SPI_BOOT_CRYPT_CNT 7

At this point the Flash Encryption feature is enabled for the device. The necessary security eFuses shall be enabled by the security_features firmware.

Encrypting the partitions

After the application is built (Later in the workflow), all partitions that need encryption can be encrypted with the following command:

espsecure.py encrypt_flash_data --aes_xts --keyfile my_flash_encryption_key.bin --address /* Placeholder for partition offset */ --output /* Placeholder for Output File */ /* Placeholder for File to encrypt */

The bootloader offset for esp32c3 is 0x0. The partition table offset for the example has been set to 0xD000 which can be changed through menuconfig. The partition offset for other partitions can be obtained by running idf.py partition-table

For this example we need to encrypt only the following 3 partitions: bootloader.bin, partition-table.bin, security_features.bin.

It can be done with following commands:

espsecure.py encrypt_flash_data --aes_xts --keyfile my_flash_encryption_key.bin --address 0x0 --output encrypted_data/bootloader-enc.bin build/bootloader/bootloader.bin

espsecure.py encrypt_flash_data --aes_xts --keyfile my_flash_encryption_key.bin --address 0xD000 --output encrypted_data/partition-table-enc.bin build/partition_table/partition-table.bin

espsecure.py encrypt_flash_data --aes_xts --keyfile my_flash_encryption_key.bin --address 0x20000 --output encrypted_data/security_features-enc.bin build/security_features_app.bin

Please refer to Encrypted Partition to check which partitions must be encrypted by default.

Enabling NVS Encryption

We shall use the HMAC based NVS encryption scheme, Please find more details in the NVS encryption documentation

For generation of NVS encryption keys and NVS partition, we shall use NVS partition generator We shall use the nvs_partition_gen.py script for the operations.

Generate HMAC key and NVS encryption key

python3 $IDF_PATH/components/nvs_flash/nvs_partition_generator/nvs_partition_gen.py generate-key --keyfile nvs_encr_key.bin --key_protect_hmac --kp_hmac_keygen --kp_hmac_keyfile hmac_key.bin

Burn the HMAC key in eFuse
```
espefuse.py --port $ESPPORT burn_key BLOCK_KEY2 keys/hmac_key.bin HMAC_UP
```
We have used BLOCK_KEY2 here to store the HMAC key. Generally, BLOCK can be a free keyblock between BLOCK_KEY0 and BLOCK_KEY5.

If you want to change the value of the eFuse key block for this example, make sure to update the same value in menuconfig → Component config → NVS Security Provider → eFuse key ID storing the HMAC key.
Generate encrypted NVS partition.

If you dont want to put external data in the NVS partition then you may skip this step. See Generating NVS partition for detailed information on generating the encrypted NVS partition. Execute following command to generate the encrypted NVS partition.
```
python3 nvs_partition_gen.py encrypt /* CSV placeholder */ nvs_encr_partition.bin /* NVS partition offset */ --inputkey keys/nvs_encr_key.bin
```
- CSV placeholder: CSV file which contains data of the NVS partition. See CSV file format for more details.
- NVS partition offset: NVS partition offset. Can be found out by executing idf.py partition-table
This shall generate nvs_encr_partition.bin which we shall flash later at the Flash stage of the workflow.

Enabling Secure JTAG Return Material Access (RMA)

The target provides an ability to disable JTAG access in the device for the software. Which can be re-enabled in future after authentication using a unique token generated beforehand. This way the module can be opened up by bypassing security features after authentication for debugging purposes after it has returned back to the manufacturer due to some issue. This way when a security wise locked device comes back to the ODM/OEM due to some issue, the module can be opened up by bypassing security features after successful authentication.

Generate the HMAC key
```
python3 ../hmac_soft_jtag/jtag_example_helper.py generate_hmac_key secure_jtag_hmac_key.bin
```
This key needs to be stored at a secure place in order to re-generate the secure token afterwards.
Generate the secure token
```
python3 ../hmac_soft_jtag/jtag_example_helper.py generate_token secure_jtag_hmac_key.bin secure_jtag_token.bin
```
The example directly consumes this token data and re-enables the software disabled JTAG interface. The re-enablement can be tested by attempting a JTAG connection with the device after JTAG is enabled by the firmware. More details about JTAG debugging can be found here If this is not generated, the example uses a test-only token which is present in the folder.
Burn the key in the eFuse
```
espefuse.py --port $ESPPORT --chip esp32c3 burn_key BLOCK_KEY3 secure_jtag_hmac_key.bin HMAC_DOWN_JTAG 
```
We have used BLOCK_KEY3 here to store the HMAC key. Generally, BLOCK can be a free keyblock between BLOCK_KEY0 and BLOCK_KEY5.
Disable software access for JTAG
```
espefuse.py --port $ESPPORT burn_efuse SOFT_DIS_JTAG 7
```
After this the JTAG cannot be accessed before it is re-enabled through software with correct token.
Configuring appropriate JTAG interface

By default esp32c3 is set to use the built-in JTAG interface. Please follow the steps given here to configure the alternative JTAG interface. Please note that JTAG configuration cannot be done once the application firmware is flashed on the device.

Build

Set the target

The target can be set with following command:

idf.py set-target esp32c3

Build the example

The example can be built with following command

idf.py build

Secure Boot V2 signing

The example is configured to build the signed binaries for the bootloader.bin and security_features.bin (application). These shall be signed with the first secure boot key.

If you want to use multiple Secure Boot V2 signing keys for the image then please perform the step of Signing with multiple Secure Boot V2 keys.

Encrypting partitions

At this point, we shall encrypt all the necessary partitions. Please perform Encrypting the partitions step to do the same.

Flash

At this point we shall have all the necessary partitions which are ready to flash.

The offsets at which the partitions need to be flashed can be found out by executing idf.py partition-table.

The partitions can be flashed with help of the esptool utility.

esptool.py -p $ESPPORT write_flash /* Placeholder for offset */ /* Placeholder for file name */

Along with these, esptool command may need some additional options. Please check the output of idf.py build command executed earlier for all the necessary options that need to be provided with esptool.

For this example the following command can be used

esptool.py --chip esp32c3 -b 115200 --before default_reset --after no_reset --no-stub -p $ESPPORT write_flash 0x0 encrypted_data/bootloader-enc.bin 0xd000 encrypted_data/partition-table-enc.bin 0x20000 encrypted_data/security_features-enc.bin --force

Generate flash image for qemu

In case of qemu the flash image can be generated with help of the following command:

esptool.py --chip esp32c3 merge_bin --fill-flash-size 4MB -o qemu/security_features_flash_image.bin @qemu/qemu_flash_args

The same file shall be used by the command to execute the image on qemu.

Monitor the output

Run the monitor tool to view serial output

idf.py -p $ESPPORT monitor

Example Output

On the first boot-up, there would be prints about firmware not being secure. Please ignore the prints as we shall enable all necessary security eFuses in our application. On the Second boot onwards, you shall not see any such prints.

ROM bootloader verifying software bootloader.

ESP-ROM:esp32c3-api1-20210207
Build:Feb  7 2021
rst:0x1 (POWERON),boot:0x8 (SPI_FAST_FLASH_BOOT)
SPIWP:0xee
mode:DIO, clock div:1
Valid secure boot key blocks: 0
secure boot verification succeeded
load:0x3fcd5990,len:0x3b94
load:0x403cc710,len:0xb9c
load:0x403ce710,len:0x5ba8
entry 0x403cc71a

Early logs for Secure Boot V2 and Flash Encryption

I (101) esp_image: Verifying image signature...
I (106) secure_boot_v2: Verifying with RSA-PSS...
I (107) secure_boot_v2: Signature verified successfully!
I (108) boot: Loaded app from partition at offset 0x20000
I (109) secure_boot_v2: enabling secure boot v2...
I (109) secure_boot_v2: secure boot v2 is already enabled, continuing..
I (110) boot: Checking flash encryption...
I (110) flash_encrypt: flash encryption is enabled (0 plaintext flashes left)
I (111) boot: Disabling RNG early entropy source...

Flash Encryption warning on first boot (should be ignored)

W (156) spi_flash: Detected size(4096k) larger than the size in the binary image header(2048k). Using the size in the binary image header.
E (157) flash_encrypt: Flash encryption settings error: app is configured for RELEASE but efuses are set for DEVELOPMENT
E (160) flash_encrypt: Mismatch found in security options in bootloader menuconfig and efuse settings. Device is not secure.

Unused Secure Boot V2 Digests getting revoked

I (162) efuse: Batch mode of writing fields is enabled
W (163) secure_boot: Unused SECURE_BOOT_DIGEST1 should be revoked. Fixing..
W (164) secure_boot: Unused SECURE_BOOT_DIGEST2 should be revoked. Fixing..
I (165) efuse: BURN BLOCK0
I (175) efuse: BURN BLOCK0 - OK (all write block bits are set)
I (177) efuse: Batch mode. Prepared fields are committed
I (177) secure_boot: Fixed
I (179) efuse: BURN BLOCK0
I (189) efuse: BURN BLOCK0 - OK (all write block bits are set)

Enablement of relevant security eFuses

W (199) flash_encrypt: Not disabled UART bootloader encryption (set DIS_DOWNLOAD_MANUAL_ENCRYPT->1)
W (199) flash_encrypt: Not disabled UART bootloader cache (set DIS_DOWNLOAD_ICACHE->1)
W (199) flash_encrypt: Not disabled JTAG PADs (set DIS_PAD_JTAG->1)
W (199) flash_encrypt: Not disabled USB JTAG (set DIS_USB_JTAG->1)
W (199) flash_encrypt: Not disabled direct boot mode (set DIS_DIRECT_BOOT->1)
W (199) flash_encrypt: Not write-protected DIS_ICACHE (set WR_DIS_DIS_ICACHE->1)
I (199) flash_encrypt: Disable UART bootloader encryption...
I (199) efuse: BURN BLOCK0
I (209) efuse: BURN BLOCK0 - OK (all write block bits are set)
I (219) flash_encrypt: Disable UART bootloader cache...
I (219) efuse: BURN BLOCK0
I (229) efuse: BURN BLOCK0 - OK (all write block bits are set)
I (229) flash_encrypt: Disable JTAG...
I (229) efuse: BURN BLOCK0
I (239) efuse: BURN BLOCK0 - OK (all write block bits are set)
I (249) efuse: BURN BLOCK0
I (249) efuse: BURN BLOCK0 - OK (all write block bits are set)
I (259) efuse: BURN BLOCK0
I (269) efuse: BURN BLOCK0 - OK (all write block bits are set)
I (269) efuse: BURN BLOCK0
I (279) efuse: BURN BLOCK0 - OK (all write block bits are set)
I (289) efuse: BURN BLOCK0
I (299) efuse: BURN BLOCK0 - OK (all write block bits are set)
W (299) secure_boot: Not disabled JTAG in the soft way (set SOFT_DIS_JTAG->max)
W (299) secure_boot: Not enabled AGGRESSIVE KEY REVOKE (set SECURE_BOOT_AGGRESSIVE_REVOKE->1)
I (299) secure_boot: Enabling Security download mode...
I (299) secure_boot: Disable hardware & software JTAG...
I (299) efuse: BURN BLOCK0
I (309) efuse: BURN BLOCK0 - OK (all write block bits are set)
I (319) efuse: BURN BLOCK0
I (329) efuse: BURN BLOCK0 - OK (all write block bits are set)
I (329) secure_boot: Prevent read disabling of additional efuses...

Final status of Secure Boot V2 and Flash Encryption

I (329) security_features_app: Flash Encryption is enabled in Release Mode
I (329) security_features_app: Secure Boot is enabled in Release Mode

Flash Encryption demo

Erasing partition "storage" (0x1000 bytes)
Writing data with esp_partition_write:
I (339) security_features_app: 0x3fc8fa40   00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e 0f  |................|
I (339) security_features_app: 0x3fc8fa50   10 11 12 13 14 15 16 17  18 19 1a 1b 1c 1d 1e 1f  |................|
Reading with esp_partition_read:
I (339) security_features_app: 0x3fc8fa60   00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e 0f  |................|
I (339) security_features_app: 0x3fc8fa70   10 11 12 13 14 15 16 17  18 19 1a 1b 1c 1d 1e 1f  |................|
Reading with esp_flash_read:
I (339) security_features_app: 0x3fc8fa60   b3 e8 57 98 45 1f 33 de  71 30 71 9b 48 a2 d7 71  |..W.E.3.q0q.H..q|
I (339) security_features_app: 0x3fc8fa70   28 fd fb 54 39 fb 4f 47  be cd 7a e1 55 70 09 6d  |(..T9.OG..z.Up.m|

NVS encryption

These logs show that, the initialisation of the encrypted NVS partitions is successful.

I (349) nvs: NVS partition "nvs" is encrypted.
I (359) security_features_app: NVS partition "custom_nvs" is encrypted.

Enable Security Features with help of QEMU

Espressif fork of QEMU offers the ability to emulate esp32c3 target on the host machine with help of QEMU. That way all of the above security features can be enabled on the esp32c3 that is emulated on the host machine. A major advantage of this is that no hardware is lost while trying out the security features.

Below are the commands that can be used to to emulate the esp32c3 device on host machine.

Configure eFuse for QEMU

Generate the qemu eFuse file: A hex file containing the eFuse configuration of ESP32C3 v0.3 (ECO3) has been kept in the same folder. This file can be used to generate the qemu eFuse binary with help of following command.
```
xxd -r -p qemu/efuse_esp32c3.hex qemu/efuse_esp32c3.bin
```
Setup serial connection to interact with QEMU

The below command shall enable a serial connection to the esp32c3 emulated using qemu. The espefuse utility when invoked in other terminal can interact with this for performing eFuse related operations.
```
qemu-system-riscv32 -nographic \
    -machine esp32c3 \
    -drive file=qemu/flash_image.bin,if=mtd,format=raw \
    -global driver=esp32c3.gpio,property=strap_mode,value=0x02 \
    -drive file=qemu/efuse_esp32c3.bin,if=none,format=raw,id=efuse \
    -global driver=nvram.esp32c3.efuse,property=drive,value=efuse \
    -serial tcp::5555,server,nowait
```
This command shall start a serial connection with QEMU. Keep this running in one terminal and execute espefuse commands in an alternate terminal.

After espefuse commands are used to update the eFuses of the emulated esp the eFuse file generated in Step 1 shall get ovwewritten. To revert to the original state, execute the command provided in Step 1 once again.
Execute the commands to enable security features

At this point you can execute all the commands mentioned above for enabling security features. Please keep the above qemu instance running in one terminal and execute all esptool/espefuse related commands in a different terminal.

For qemu, before=no_reset option needs to be provided additionally to every espefuse command. Please make sure you add this just after -p $ESPPORT in the command

After all the esptool/espefuse operations are completed you can close this terminal by pressing q + Enter in the same terminal.
Build the example

Perform the Build step and all necessary substeps (e.g. encrypting partition). Please make sure the file names of newly generated files and their locations in the commands are not changed.

Build qemu image

The qemu image can be built with following command

esptool.py --chip esp32c3 merge_bin --fill-flash-size 4MB -o qemu/security_features_flash_image.bin @qemu/qemu_flash_args

Run example on QEMU

The following command can be used to run example on qemu

qemu-system-riscv32 -nographic \
    -machine esp32c3 \
    -drive file=qemu/security_features_flash_image.bin,if=mtd,format=raw \
    -drive file=qemu/efuse_esp32c3.bin,if=none,format=raw,id=efuse \
    -global driver=nvram.esp32c3.efuse,property=drive,value=efuse \
    -serial mon:stdio

The qemu session can be closed by pressing CTRL+ a and then immediately pressing x.

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