The STM32 header is a STMicroelectronics header needed for binaries loaded by ROM code and by TF-A.
1. Description[edit source]
Each binary image (signed or not) loaded by ROM code and by TF-A need to include a specific STM32 header added on top of the binary data. The header includes the authentication information.
| Name | Length | Byte Offset | Description |
|---|---|---|---|
| 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 Byte0: reserved Byte1:major version = 0x01 Byte2: minor version = 0x00 Byte3: reserved |
| Image length | 32 bits | 76 | Length of image in bytes[Note 3] |
| Image entry Point | 32 bits | 80 | Entry point of image |
| Reserved1 | 32 bits | 84 | Reserved |
| Load address | 32 bits | 88 | Load address of image[Note 4] |
| Reserved2 | 32 bits | 92 | Reserved |
| 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 0x00: U-Boot 0x10-0x1F: TF-A 0x20-0X2F: OPTEE 0x30: Copro |
- ↑ 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).
The OTP used for the Version number and the Hash of pubKey are defined in the chapter “OTP configuration“ of the ROM code overview.
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