1 Purpose[edit source]
In this article, the stm32key U-Boot command is used to illustrate and experiment the steps to provision the keys in the correct OTP needed to activate secure boot features: authentication and encryption.
It also allows setting the device directly to the CLOSED state.
1.1 Prerequisite[edit source]
All the required keys must be generated to provision the OTP.
The OTP write support must be activated in OP-TEE STM32MP BSEC PTA with CFG_STM32_BSEC_WRITE.
In ecosystem release ≤ v4.1.0 , this configuration is activated only on OP-TEE debug release with:
CFG_STM32_BSEC_WRITE ?= $(CFG_TEE_CORE_DEBUG)
The command stm32key is not functional by default with the release version of OP-TEE.
2 stm32key command[edit source]
U-Boot in OpenSTLinux embeds a stm32key command that can be called from U-Boot command-line interface to manage the keys in OTPs.
stm32key help stm32key - Manage key on STM32 Usage: stm32key list: list the supported key with description stm32key select [<key>]: Select the key identified by <key> or display the key used for read/fuse command stm32key read [<addr> | -a ]: Read the curent key at <addr> or current / all (-a) key in OTP stm32key fuse [-y] <addr>: Fuse the current key at addr in OTP stm32key close [-y]: Close the device, force use of PKH stored in OTP
The optional option -y is used to skip the confirmation message.
The name of the used <key> is
- for STM32MP13x lines
: PKHTH for authentication and EDMK for encryption
stm32key list PKHTH: Hash of the 8 ECC Public Keys Hashes Table (ECDSA is the authentication algorithm) OTP24..32 EDMK: Encryption/Decryption Master Key" OTP92..95
- for STM32MP15x lines : PKH for authentication
stm32key list PKH: Hash of the ECC Public Key (ECDSA is the authentication algorithm) OTP24..32
3 Authentication keys provisioning[edit source]
The key provisioning is the first step to enable the authentication: burn the keys in OTPs with the key hash output file from STM32 KeyGen.
3.1 Select keys[edit source]
Key is selected with the command sm32key select <key>, with <key>=
- PKHTH for STM32MP13x lines
- PKHTH for STM32MP13x lines
sm32key select PKHTH PKHTH selected
- PKH for STM32MP15x lines
- PKH for STM32MP15x lines
sm32key select PKH PKH selected
3.2 Load keys file in DDR[edit source]
The keys hash file, output file from STM32 KeyGen, must be available in DDR before proceeding with the stm32key command;
this file is loaded at 0xc0000000 in the next examples.
The file publicKeysHash.bin can be loaded from a filesystem partition on a storage device by using the load command.
For example, the file publicKeysHash.bin is in the bootfs (partition 7) on SD™ card (mmc0):
load mmc 0:7 0xc0000000 publicKeysHash.bin 32 bytes read in 50 ms (0 Bytes/s)
3.3 Verify keys file in DDR[edit source]
Once the publicKeysHash.bin file is loaded in DDR, you can verify the content of the file with the command:
stm32key read 0xc0000000
Example for STM32MP13x lines with PKHTH
stm32key read 0xC0000000 Read PKHTH at 0xc0000000 PKHTH OTP 24: [c0000000] 27051956 PKHTH OTP 25: [c0000004] b56aef2d PKHTH OTP 26: [c0000008] 6215263c PKHTH OTP 27: [c000000c] 00000439 PKHTH OTP 28: [c0000010] 00000000 PKHTH OTP 29: [c0000014] 00000000 PKHTH OTP 30: [c0000018] 72429173 PKHTH OTP 31: [c000001c] 05020600
Example for STM32MP15x lines with PKH
stm32key read 0xC0000000 Read PKH at 0xc0000000 PKH OTP 24: [c0000000] 27051956 PKH OTP 25: [c0000004] b56aef2d PKH OTP 26: [c0000008] 6215263c PKH OTP 27: [c000000c] 00000439 PKH OTP 28: [c0000010] 00000000 PKH OTP 29: [c0000014] 00000000 PKH OTP 30: [c0000018] 72429173 PKH OTP 31: [c000001c] 05020600
3.4 Key provisioning[edit source]
To write and lock the keys in OTP, you use the command:
stm32key fuse 0xc0000000
3.5 Verify keys file in OTP[edit source]
After the previous command, the device contains the keys to authenticate images and it can be verified with the command:
stm32key read
Result for STM32MP13x lines with PKHTH
stm32key read PKHTH OTP 24: 27051956 lock : 50000000 PKHTH OTP 25: b56aef2d lock : 50000000 PKHTH OTP 26: 6215263c lock : 50000000 PKHTH OTP 27: 00000439 lock : 50000000 PKHTH OTP 28: 00000000 lock : 50000000 PKHTH OTP 29: 00000000 lock : 50000000 PKHTH OTP 30: 72429173 lock : 50000000 PKHTH OTP 31: 05020600 lock : 50000000
Result for STM32MP15x lines with PKH
stm32key read PKH OTP 24: 27051956 lock : 50000000 PKH OTP 25: b56aef2d lock : 50000000 PKH OTP 26: 6215263c lock : 50000000 PKH OTP 27: 00000439 lock : 50000000 PKH OTP 28: 00000000 lock : 50000000 PKH OTP 29: 00000000 lock : 50000000 PKH OTP 30: 72429173 lock : 50000000 PKH OTP 31: 05020600 lock : 50000000
4 Encryption Decryption Master Key provisioning[edit source]
The EDMK key provisioning is the first step to enable the image decryption.
It is only available on STM32MP13x lines .
4.1 Select EDMK[edit source]
Key is selected with the command sm32key select <key>, with <key>=
- EDMK for STM32MP13x lines
- EDMK for STM32MP13x lines
stm32key select EDMK EDMK selected
4.2 Load EDMK file in DDR[edit source]
The keys file must be available in DDR before proceeding the stm32key command;
this file is loaded at 0xc0000000 in the next examples.
The file edmk.bin can be loaded from a filesystem partition on a storage device by using the load command.
For example, the file edmk.bin is in the bootfs (partition 7) on SD™ card (mmc0):
load mmc 0:7 0xc0000000 edmk.bin 32 bytes read in 50 ms (0 Bytes/s)
4.3 Verify EDMK in DDR[edit source]
Then you can verify the content of keys files loaded in DDR with the command: sm32key read <addr>
Result for STM32MP13x lines with EDMK and <addr>=0xc0000000
stm32key read 0xc0000000 Read EDMK at 0xc0000000 EDMK OTP 92: [c0000000] 27051956 EDMK OTP 93: [c0000004] b56aef2d EDMK OTP 94: [c0000008] 6215263c EDMK OTP 95: [c000000c] 00000439
4.4 EDMK provisioning[edit source]
To write and lock the EDMK in OTP, you use the command with the same address:
stm32key fuse 0xc0000000
4.5 Verify EDMK in OTP[edit source]
After the previous command, the device contains the keys to decrypt the images and it can be verified with the command:
stm32key read
Result for STM32MP13x lines with EDMK
stm32key read EDMK OTP 92: 00000000 lock : 50000000 EDMK OTP 93: 00000000 lock : 50000000 EDMK OTP 94: 00000000 lock : 50000000 EDMK OTP 95: 00000000 lock : 50000000
Warning: the content of the key cannot be read. It is masked, but the lock property can be verified to ensure that the key has been written.
5 Closing the device[edit source]
Once the authentication process is confirmed in ROM code and in TF-A, the device can be closed to ensure that only signed images can be used.
This operation is performed with the U-Boot command:
stm32key close
Arm® is a registered trademark of Arm Limited (or its subsidiaries) in the US and/or elsewhere. 