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STM32MP15 ROM code secure boot
Secure boot is a key feature to guarantee a secure platform.
STM32MP1 boot sequence supports a trusted boot chain that ensures that the loaded images are authenticated and checked in integrity before being used.
|The secure boot feature availability is indicated in the security field of the chip part number.|
2 Authentication processing
Two algorithms are supported for ECDSA calculation:
- P-256 NIST
- Brainpool 256
The algorithm selection is done via the signed binary header, as shown in STM32 header (subchapter in this same article).
The EDCSA verification follows the process below:
2.1 Key generation
First step is to generate the ECC pair of keys with STM32 KeyGen tool. This is the key pair that will be used to sign the images.
The tool also generates a third file containing the public key hash (PKH) that will be used to authenticate the public key on the target.
2.2 Key registration
|Make sure that a device with Secure boot enabled is used: this is mentioned in the part number, otherwise the device will become permanently unusable.|
2.2.1 Register hash public key
First step to enable the authentication is to burn the OTP WORD 24 to 31 in BSEC with the corresponding public key hash (PKH, output file from STM32 KeyGen).
OpenSTLinux embeds a stm32key tool that can be called from U-Boot command line interface to program the PKH into the OTP.
Make sure that a trusted image was programmed on your board, because below operation will not be possible with optee boot.
PKH file (publicKeyhash.bin) must be available in a filesystem partition (like bootfs) on a storage device (like sdcard) before proceeding.
Board $> ext4load mmc 0:4 0xc0000000 publicKeyhash.bin Load hash file from mmc 0 partition 4 (ext4) in DDR 32 bytes read in 50 ms (0 Bytes/s)
Board $> stm32key read 0xc0000000 Read loaded key from DDR to confirm it is valid (without writing it in OTP) OTP value 24: 12345678 OTP value 25: 12345678 OTP value 26: 12345678 OTP value 27: 12345678 OTP value 28: 12345678 OTP value 29: 12345678 OTP value 30: 12345678 OTP value 31: 12345678
|If hash key is ok, the key in OTP can be fused|
Board $> stm32key fuse -y 0xc0000000 Write the key in OTP
The device now contains the hash to authenticate images. To read back the OTP, you can use NVMEM framework.
2.3 Image signing
In a second step, FSBL binary must be signed. STM32 Signing tool allows to fill the STM32 binary header that is parsed by the embedded software to authenticate each binary.
2.3.1 STM32 Header
Each binary image (signed or not) loaded by ROM code needs to include a specific STM32 header added on top of the binary data. The header includes the authentication information.
|Magic number||32 bits||0||4 bytes in big endian:|
'S', 'T', 'M', 0x32 = 0x53544D32
|Image signature||512 bits||4||ECDSA signature for image authentication[Note 1]|
|Image checksum||32 bits||68||Checksum of the payload[Note 2]|
|Header version||32 bits||72||Header version v1.0 = 0x00010000|
Byte1:major version = 0x01
Byte2: minor version = 0x00
|Image length||32 bits||76||Length of image in bytes[Note 3]|
|Image entry Point||32 bits||80||Entry point of image|
|Load address||32 bits||88||Load address of image[Note 4]|
|Version number||32 bits||96||Image Version (monotonic number)[Note 5]|
|Option flags||32 bits||100||b0=1: no signature verification[Note 6]|
|ECDSA algorithm||32 bits||104||1: P-256 NIST ; 2: brainpool 256|
|ECDSA public key||512 bits||108||ECDSA public key to be used to verify the signature.[Note 7]|
|Padding||83 Bytes||172||Reserved padding bytes[Note 8]. Must all be set to 0|
|Binary type||1 Byte||255||Used to check the binary type |
- Signature is calculated from first byte of header version field to last byte of image given by image length field.
- 32-bit sum of all payload bytes accessed as 8-bit unsigned numbers, discarding any overflow bits. Used to check the downloaded image integrity when signature is not used (if b0=1 in Option flags).
- Length is the length of the built image, it does not include the length of the STM32 header.
- This field is not used by ROM code.
- Image version number is an anti rollback monotonic counter. The ROM code checks that it is higher or equal to the monotonic counter stored in OTP.
- Enabling signature verification is mandatory on secure closed chips.
- This field is an extract of PEM public key file that only kept the ECC Point coordinates x and y in a raw binary format (RFC 5480). This field will be hashed with SHA-256 and compared to the Hash of pubKey that is stored in OTP.
- This padding forces STM32 header size to 256 bytes (0x100).
2.4 Image programming
Once the image is signed, it can be programmed into the flash on the target board with STM32CubeProgrammer.
2.5 PKH check
Before really starting the authentication process, the ROM code compares the hash of the public key carried in the STM32 header with the one that was provisioned in OTP.
2.6.1 ROM code authentication
Using a signed binary, the ROM code authenticates and starts the FSBL.
If the authentication fails, the ROM code enters into a serial boot loop indicated by the blinking Error LED (cf ROM code common debug and error cases)
The ROM code provides secure services to the FSBL for image authentication with the same ECC pair of keys, so there is no need to support ECDSA algorithm in FSBL.
2.7 Closing the device
Notice that this last step is not shown in the diagram above.
Without any other modification, the device is able to perform image authentication but non authenticated images can still be used and executed: the device is still opened, let's see this as a kind of test mode to check that the PKH is properly set.
As soon as the authentication process is confirmed, the device can be closed and the user forced to use signed images.
OTP WORD0 bit 6 is the OTP bit that closes the device. Burning this bit will lock authentication processing and force authentication from the Boot ROM. Non signed binaries will not be supported anymore on the target.
Here is how to proceed with U-Boot:
Board $> fuse prog 0 0x0 0x40
|This must not be done on STM32MP15 part numbers without Secure boot enabled, otherwise the chip will be bricked and could not be used anymore|