STM32 header for binary files

The STM32 header is a STMicroelectronics header needed for binaries loaded by ROM code.

1 Description[edit]

Each binary image loaded by the ROM code needs to include a specific STM32 header added on top of the binary data. This header includes mandatory and optional information: this second one is only needed for the authentication (STM32MP1 Series) and decryption (STM32MP13x lines Warning.png) processes.

2 On STM32MP13x lines Warning.png[edit]

2.1 Base header[edit]

STM32MP13 header base.png
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 v2.0 = 0x00020000
Byte0: reserved
Byte1: major version = 0x02
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
Reserved 96 bits 84 Reserved
Version number 32 bits 96 Image Version (monotonic number)[Note 4]
Option flags 32 bits 100 b0=1: Authentication enabled[Note 5]
b1=1: Decryption enabled.[Note 6]
b31=1: Header padding enabled[Note 7]
Header extensions length 32 bits 104 Sizes of header extensions enabled by option flags.
Padding 20 Bytes 108 Reserved padding bytes[Note 8]. Must all be set to 0
  1. Signature is calculated from first byte of header version field to last byte of image given by image length field.
  2. 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=0 in Option flags).
  3. Length is the length of the built image, it does not include the length of the STM32 header.
  4. 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.
  5. Enabling signature verification is mandatory on secure closed chips. Authentication parameters are stored in "Authentication header extension".
  6. When decryption is enabled, authentication is mandatory . Decryption parameters are stored in "Decryption header extension".
  7. This header padding extension is always used to have a fixed size of 512 bytes for the whole size of header + its extensions.
  8. This padding forces STM32 base header size to 128 bytes (0x80).

2.2 Authentication header extension[edit]

This header extension contains parameters needed for authentication.

STM32MP13 header authentication.png
Name Length Byte Offset[Note 1] Description
Extension type 32 bits 0 4 bytes in big endian:
'S', 'T', 0x00, 0x02 = 0x53540002
Extension length 32 bits 4 Number of bytes of header extension = 340
Public key index 32 bits 8 Index of the public key to be used.
Public key number 32 bits 12 Number of public keys in table = 8
ECDSA algorithm 32 bits 16 1: P-256 NIST ; 2: brainpool 256
ECDSA public key 512 bits 20 ECDSA public key to be used to verify the signature.[Note 2]
Public key1 hash 256 bits 84 Hash of (Algorithm+Public key1)
... ... ... Hashes of (Algorithm+Public key), for key2 up to key7
Public key8 hash[Note 3] 256 bits 308 Hash of (Algorithm+Public key8)
  1. Offset is relative to header extension base
  2. 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 the entry of the public key table referenced by the public key index.
  3. The table of public key hashes will be hashed with SHA-256 by the ROM code and compared to the Hash of Public Keys Hashes Table (PKHTH) that is stored in OTP.

2.3 Decryption header extension[edit]

STM32MP13 header decryption.png

This header extension contains parameters needed for decryption.

Name Length Byte Offset[Note 1] Description
Extension type 32 bits 0 4 bytes in big endian:
'S', 'T', 0x00, 0x01 = 0x53540001
Extension length 32 bits 4 Number of bytes of header extension = 32
Key size 32 bits 8 Size of extension key (128 bits)[Note 2].
Derivation constant 32 bits 12 Constant used to derive decryption key from master key stored in OTP.
Plain hash 128 bits 16 128 msb bits of plain payload SHA256.
  1. Offset is relative to header extension base
  2. Key size is fixed to 128 bits

2.4 Padding header extension[edit]

STM32MP13 header padding.png
Name Length Byte Offset[Note 1] Description
Extension type 32 bits 0 4 bytes in big endian:
'S', 'T', 0xFF, 0xFF = 0x5354FFFF
Extension length 32 bits 4 Number of bytes of header extension = N + 8
Padding bytes N bytes 8 Padding bytes[Note 2].
  1. Offset is relative to header extension base
  2. N shall be calculated by signing tool so that the size of whole header plus its extension is equal to 512 bytes

3 On STM32MP15x lines More info.png[edit]

STM32 header.png
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
0x10-0x1F: FSBL
0x30: Copro
  1. Signature is calculated from first byte of header version field to last byte of image given by image length field.
  2. 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).
  3. Length is the length of the built image, it does not include the length of the STM32 header.
  4. This field is not used by ROM code.
  5. 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.
  6. Enabling signature verification is mandatory on secure closed chips.
  7. 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.
  8. This padding forces STM32 header size to 256 bytes (0x100).