RTSL Enablement and Bootloader Signature

WARNING Even though below steps are separated, it is important to note that SECURE_BOOT and/or SECURE_BOOT_PAGE_LOCK are set together

The Security config can be provisioned only ONCE per device

RTSL Enablement and Bootloader Page Lock (optional)

This will mostly be a reuse of AN1218 section 3, using Simplcity Commander

Provision OTP with signing key

NOTE : Always provide –serialno and the right –device OPN

Generate your key pair in PEM format (we will use commander but other options are available)

commander util genkey --type ecc-p256 --privkey sign_key.pem --pubkey sign_pubkey.pem
  

Flash the public key into the EFR32 MG24's OTP dedicated slot

commander security writekey --sign sign_pubkey.pem --device EFR32MG24B210F1536 --serialno 440048205
  

Read key back :

commander security readkey --sign --device EFR32MG24B210F1536 --serialno 440048205
  

Provision OTP with security configuration (No Page Lock, anti rollback enabled)

Generate the security configuration JSON Structure, make sure it is done for the right OPN

commander security genconfig --nostore --outfile user_configuration.json --device EFR32MG24B210F1536 --serialno 440263650
  

Edit the security configuration according to below

"mcu_flags": {
     "SECURE_BOOT_ANTI_ROLLBACK": true,
     "SECURE_BOOT_ENABLE": true,
     "SECURE_BOOT_PAGE_LOCK_FULL": false,
     "SECURE_BOOT_PAGE_LOCK_NARROW": false,
     "SECURE_BOOT_VERIFY_CERTIFICATE": false
 },
  

Provision the device with the generated configuration

commander security writeconfig --configfile user_configuration.json --device EFR32MG24B210F1536 --serialno 440263650
  

Verify the current OTP configuration

commander security readconfig --device EFR32MG24B210F1536 --serialno 440263650

Reset the device

commander device reset --device EFR32MG24B210F1536 --serialno 440263650

Provision OTP with security configuration (Flash Page Lock enabled)

It is possible to lock secure booted flash pages via the Secure Engine

This is covered by the OTP flags SECURE_BOOT_PAGE_LOCK_FULL and SECURE_BOOT_PAGE_LOCK_NARROW

The key difference between those is that FULL will protect all the secure booted application (here bootloader), INCLUDING the space where App Properties is

NARROW will avoid locking the page where the app properties is, allowing for certificate use and revocation.

To enable page lock simply change the OTP configuration :

   "mcu_flags": {
      "SECURE_BOOT_ANTI_ROLLBACK": true,
      "SECURE_BOOT_ENABLE": true,
      "SECURE_BOOT_PAGE_LOCK_FULL": true,
      "SECURE_BOOT_PAGE_LOCK_NARROW": false,
      "SECURE_BOOT_VERIFY_CERTIFICATE": false
   },
  

And provision this new config :

   commander security writeconfig --configfile user_configuration.json --device EFR32MG24B210F1536 --serialno 440263650
  

Verify the current OTP configuration

commander security readconfig --device EFR32MG24B210F1536 --serialno 440263650

Reset the device

commander device reset --device EFR32MG24B210F1536 --serialno 440263650

Signing And flashing the bootloader

Start by signing the bootloader using the below command :
```
commander convert bootloader-storage-internal-single-1536k-ROMed.s37 --secureboot --keyfile sign_key.pem --verify sign_pubkey.pem --outfile bootloader-storage-internal-single-1536k-ROMed_signed.s37
```
NOTE : Once secure boot is enabled, if no valid image is present in the main flash, the Cortex-M33 will hang up on reset state (See AN1218) It is therefore more stable to unlock it prior to flashing a signed bootloader or image

Check the device status using :

commander security status --device EFR32MG24B210F1536 --serialno 440263650

The device should be in a failed boot state, with signing key present and secure boot enabled :

Secure boot           : Enabled
Boot status           : 0x14 - Failed: Error while finding host application properties pointer
Sign key installed    : True
  

Unlock the device using :

commander device unlock --device EFR32MG24B210F1536 --serialno 440263650

This will trigger a mass erase.

Then flash your signed image using :

commander flash bootloader-storage-internal-single-1536k-ROMed_signed.s37 --masserase --device EFR32MG24B210F1536 --serialno 440263650
  

Check the device status again :

commander security status --device EFR32MG24B210F1536 --serialno 440263650

The device should be in boot state OK, with signing key present and secure boot enabled :

Secure boot           : Enabled
Boot status           : 0x20 - OK
Sign key installed    : True
  

Permanently locking Bootloader flash space (Not recommended as prevents bootloader updates)

Once Page lock is enabled, Cortex-M33 becomes unable to write/erase the dedicated space to the bootloader

However, the Secure Engine still can edit these 2 pages of flash over either SWD or SE Mailbox

To completely block the state of these pages two other Secure Engine flags must be written as per configuration 2 below.

Remember this is the most risky setting if device is not provisioned with the right binaries in the first place.

This could result in bricking you chipset if no binary or bootloadeer with upgrade capability is flashed prior to locking

Available flags are :

Debug Lock (Debug port kept locked on boot) – Defaults to false
Device Erase (Device Erase command is availble after boot) – Defaults to True
Secure Debug (Secure Debug Unlock is made available) – Defaults to False Allows for an external authenticated connnection via DCI (Debug Challenge) interface

Default state is DL = 0; DE = 1; SD = 0

Possible locking configurations are :

Standard Debug Lock Unlock possible via mass erase DL = 1 ; DE = 1 ; SD = 0

commander security lock --device EFR32MG24B210F1536 --serialno 440263650

Permanent Debug Lock - ROMs already flashed bootloader, prevents from writing an app without DFU capable bootloader Risk of brick, –disabledeviceerase should be the last step in production DL = 1 ; DE = 0 ; SD = 0
```
commander security lock --device EFR32MG24B210F1536 --serialno 440263650
commander security --disabledeviceerase --device EFR32MG24B210F1536 --serialno 440263650
```

Secure Debug Lock Unlock possible, mass erase disabled Risk of brick, –disabledeviceerase should be the last step in production DL = 1 ; DE = 0 ; SD =1

commander security lockconfig --secure-debug-unlock enable --device EFR32MG24B210F1536 --serialno 440263650
commander security lock --device EFR32MG24B210F1536 --serialno 440263650 
or
commander security lock --trustzone 1100 --device EFR32MG24B210F1536 --serialno 440263650
commander security disabledeviceerase --device EFR32MG24B210F1536 --serialno 440263650
  

Secure Debug Lock / Unlock

Using Secure Debug Lock / Unlock it is however possible to securely regain access to the SWD interface

This allows for updating the bootloader from SWD

Below will mostly follow AN1190 section 6.3.3

The first action to perfor is to generate a key pair for securing debug access

This is referred to as "Command Key"

Generate your key pair in PEM format (we will use commander but other options are available)

commander util genkey --type ecc-p256 --privkey command_key.pem --pubkey command_pubkey.pem
  

Provision your command key to the OTP

commander security writekey --command command_pubkey.pem --device EFR32MG24B210F1536 --serialno 440263650
  

Read back the command key

commander security readkey --command --device EFR32MG24B210F1536 --serialno 440263650
  

Enable Secure Debug

commander security lockconfig --secure-debug-unlock enable --device EFR32MG24B210F1536 --serialno 440263650
  

Locking the device depends on whether the application will be TrustZone aware or not

For TrustZone unaware application use the below command
```
commander security lock --device EFR32MG24B210F1536 --serialno 440263650
```
For TrustZone aware application use the below command
```
commander security lock --trustzone 1100 --device EFR32MG21A010F1024 --serialno 440048205
```

Finally disable device erase

commander security disabledeviceerase --device EFR32MG24B210F1536 --serialno 440263650
  

Check the device security status

commander security status --device EFR32MG24B210F1536 --serialno 440263650
or
commander security status --trustzone --device EFR32MG24B210F1536 --serialno 440263650
  

Unlock debug access To securely unlock debug access a whole flow with tokenized challenge is put in place.

Unlock should follow section 6.4.2 of AN1190

The initial idea was that third parties can get debug access provided by the device manufatcurer, without sharing the private command key

It is possible to simplify section 6.4.2 by using the command private key as follows (RISK OF PRIVATE KEY LEAK):
```
commander security unlock --command-key command_key.pem --unlock-param 1111 --device EFR32MG24B210F1536 --serialno 440263650
```