Last edited one week ago

STM32 header for binary files

Applicable for STM32MP13x lines, STM32MP15x lines, STM32MP23x lines, STM32MP25x lines


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

1. Description[edit | edit source]

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 More info.png and STM32MP2 series) processes.


2. On STM32MP13x lines More info.png[edit | edit source]

2.1. Base header[edit | edit source]

STM32MP13 header base.png
Name Length Byte Offset [Note 1] 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 2]
Image checksum 32 bits 68 Checksum of the payload[Note 3]
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 4]
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 5]
Option flags 32 bits 100 b0=1: Authentication enabled[Note 6]
b1=1: Decryption enabled.[Note 7]
b31=1: Header padding enabled[Note 8]
Header extensions length 32 bits 104 Sizes of header extensions enabled by option flags.
Padding 20 Bytes 108 Reserved padding bytes[Note 9].
Must all be set to 0
  1. ↑ Unless expressed differently, the 32 bits values are stored in little endian.
  2. ↑ Signature is calculated from first byte of header version field to last byte of image given by image length field.
  3. ↑ 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).
  4. ↑ Length is the length of the built image, it does not include the length of the STM32 header.
  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 secured locked device. Authentication parameters are stored in "Authentication header extension".
  7. ↑ When decryption is enabled, authentication is mandatory. Decryption parameters are stored in "Decryption header extension".
  8. ↑ This header padding extension is always used to have a fixed size of 512 bytes for the whole size of header + its extensions.
  9. ↑ This padding forces STM32 base header size to 128 bytes (0x80).

2.2. Authentication header extension[edit | edit source]

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 (ECDSA Algorithm+Public key1).[Note 3].
... ... ... Hashes of (ECDSA Algorithm+Public key), for key2 up to key7
Public key8 hash[Note 4] 256 bits 308 Hash of (ECDSA Algorithm+Public key8)
  1. ↑ Offset is relative to header extension base. Unless expressed differently, the 32 bits values are stored in little endian.
  2. ↑ Public key is a point on the ECC curve. This field contains the two big numbers in big endian format corresponding to ECC Point coordinates x and y. (If these coordinates are extracted from a PEM public key file that only kept coordinates in a raw binary format, you shall remove the first byte "04" which is an asn1 tag ; for more info: RFC 5480).
  3. ↑ The Public Key hash computation is the SHA-256 of ECDSA Algorithm expressed in little endian, concatenated to the public key itself. The entry of the public key table referenced by the public key index is compared to the hash of the ECDSA algorithm and public key that are stored in the header.
  4. ↑ 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 | edit source]

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. Unless expressed differently, the 32 bits values are stored in little endian.
  2. ↑ Key size is fixed to 128 bits

2.4. Padding header extension[edit | edit source]

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. Unless expressed differently, the 32 bits values are stored in little endian.
  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 | edit source]

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][Note 8]
Padding 83 Bytes 172 Reserved padding bytes[Note 9]. 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 secured locked device.
  7. ↑ Public key is a point on the ECC curve. This field contains the two big numbers in big endian format corresponding to ECC Point coordinates x and y. (If these coordinates are extracted from a PEM public key file that only kept coordinates in a raw binary format, you shall remove the first byte "04" which is an asn1 tag ; for more info: RFC 5480).
  8. ↑ The Public Key hash computation is the SHA-256 of the public key itself, then the resulting value is compared to the Hash of pubKey that is stored in OTP.
  9. ↑ This padding forces STM32 header size to 256 bytes (0x100).

4. On STM32MP23x lines More info.png and STM32MP25x lines More info.png[edit | edit source]

4.1. Base header[edit | edit source]

STM32MP25 header base.png
Name Length Byte Offset [Note 1] Description Signed
Magic number 32 bits 0 4 bytes in big endian:
'S', 'T', 'M', 0x32 = 0x53544D32		 No
Image signature 512 bits 4 ECDSA signature for image authentication[Note 2] No
Image checksum 32 bits 68 Checksum of the payload[Note 3] No
Header version 32 bits 72 Header version v2.2 = 0x00020200
Byte0: reserved
Byte1: major version = 0x02
Byte2: minor version = 0x02
Byte3: reserved		 Yes
Image length 32 bits 76 Length of image in bytes[Note 4] Yes
Image entry Point 32 bits 80 Entry point of image Yes
Reserved 96 bits 84 Reserved Yes
Version number 32 bits 96 Image Version (monotonic number)[Note 5] Yes
Option flags 32 bits 100 b0=1: Authentication enabled[Note 6]
b1=1: Decryption enabled.[Note 7]
b31=1: Header padding enabled[Note 8]		 Yes
Header extensions length 32 bits 104 Sizes of header extensions enabled by option flags. Yes
Binary type 32 bits 108 Used to check binary type. Must be set to 0x30 for FSBLM. Yes
Padding 16 Bytes 112 Reserved padding bytes[Note 9].
Must all be set to 0		 Yes
  1. ↑ Unless expressed differently, the 32 bits values are stored in little endian.
  2. ↑ Signature is calculated from first byte of header version field to last byte of image given by image length field, excluding the last bytes following the padding field.
  3. ↑ 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).
  4. ↑ Length is the length of the built image, it does not include the length of the STM32 header.
  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 secured locked device. Authentication parameters are stored in "Authentication header extension".
  7. ↑ When decryption is enabled, authentication is mandatory. Decryption parameters are stored in "Decryption header extension".
  8. ↑ This header padding extension is always used to have a fixed size of 512 bytes for the whole size of header + its extensions.
  9. ↑ This padding forces STM32 base header size to 128 bytes (0x80).

4.2. Authentication header extension[edit | edit source]

Same as the authentication header extension on STM32MP13x lines.

4.3. Decryption header extension[edit | edit source]

Same as the decryption header extension on STM32MP13x lines

4.4. Padding header extension[edit | edit source]

Same as the padding header extension on STM32MP13x lines.


Elliptic curve digital signature algorithm

Read only memory

One time programmed

Error correction capability or Elliptic curve cryptography, meaning depends on context

Privacy enhanced mail (File format for storing and sending cryptographic keys, certificates, and other data)

Secure hash algorithm

Public key hash table hash, used for secure boot - NEW

First stage boot loader

Elliptic curve digital signature algorithm

Read only memory

One time programmed

Error correction capability or Elliptic curve cryptography, meaning depends on context

Privacy enhanced mail (File format for storing and sending cryptographic keys, certificates, and other data)

Secure hash algorithm

Public key hash table hash, used for secure boot - NEW

First stage boot loader

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