trezor-firmware/docs/core/misc/boot.md

Trezor Core Boot Stages

Trezor T initialization is split into two stages. See Memory Layout for info about in which sectors each stage is stored.

First stage (boardloader) is stored in write-protected area, which means it is non-upgradable. Only second stage (bootloader) update is allowed.

First Stage - Boardloader

First stage checks the integrity and signatures of the second stage and runs it if everything is OK.

If first stage boardloader finds a valid second stage bootloader image on the SD card (in raw format, no filesystem), it will replace the internal second stage, allowing a second stage update via SD card.

The boardloader is special in that it is the device's write protected embedded code. The primary purpose for write protecting the boardloader is to make it the immutable portion that can defend against code-based attacks (e.g.- BadUSB) and bugs that would reprogram any/all of the embedded code. It assures that only verified signed embedded code is run on the device (and that the intended code is run, and not skipped). The write protection also provides some defense against attacks where the attacker has physical control of the device.

The boardloader must include an update mechanism for later stage code because if it did not, then a corruption/erasure of later stage flash memory would leave the device unusable (only the boardloader could run and it would not pass execution to a later stage that fails signature validation).

Developer note:

A microSD card can be prepared with the following. Note that the bootloader is allocated 128 KiB.

WARNING: Ensure that you want to overwrite and destroy the contents of /dev/mmcblk0 before running these commands. Likewise, /dev/mmcblk0 may be replaced by your own specific destination.

1. sudo dd if=/dev/zero of=/dev/mmcblk0 bs=512 count=256 conv=fsync

2. sudo dd if=build/bootloader/bootloader.bin of=/dev/mmcblk0 bs=512 conv=fsync

Second Stage - Bootloader

Second stage checks the integrity and signatures of the firmware and runs it if everything is OK.

If second stage bootloader detects a pressed finger on the display or there is no firmware loaded in the device, it will start in a firmware update mode, allowing a firmware update via USB.

Common notes

• Hash function used for computing data digest for signatures is BLAKE2s.
• Signature system is Ed25519 (allows combining signatures by multiple keys into one).
• All multibyte integer values are little endian.
• There is a tool called headertool.py which checks validity of the bootloader/firmware images including their headers.

Bootloader Format

Trezor Core (second stage) bootloader consists of 2 parts:

1. bootloader header
2. bootloader code

Bootloader Header

Total length of bootloader header is always 1024 bytes.

offset	length	name	description
0x0000	4	magic	firmware magic TRZB
0x0004	4	hdrlen	length of the bootloader header
0x0008	4	expiry	valid until timestamp (0=infinity)
0x000C	4	codelen	length of the bootloader code (without the header)
0x0010	1	vmajor	version (major)
0x0011	1	vminor	version (minor)
0x0012	1	vpatch	version (patch)
0x0013	1	vbuild	version (build)
0x0014	1	fix_vmajor	version of last critical bugfix (major)
0x0015	1	fix_vminor	version of last critical bugfix (minor)
0x0016	1	fix_vpatch	version of last critical bugfix (patch)
0x0017	1	fix_vbuild	version of last critical bugfix (build)
0x0018	8	reserved	not used yet (zeroed)
0x0020	32	hash1	hash of the first code chunk (128 - 1 KiB), this excludes the header
0x0040	32	hash2	hash of the second code chunk (128 KiB), zeroed if unused
...	...	...	...
0x0200	32	hash16	hash of the last possible code chunk (128 KiB), zeroed if unused
0x0220	415	reserved	not used yet (zeroed)
0x03BF	1	sigmask	SatoshiLabs signature indexes (bitmap)
0x03C0	64	sig	SatoshiLabs aggregated signature of the bootloader header

Firmware Format

Trezor Core firmware consists of 3 parts:

1. vendor header
2. firmware header
3. firmware code

Vendor Header

Total length of vendor header is 84 + 32 * (number of pubkeys) + (length of vendor string rounded up to multiple of 4) + (length of vendor image) bytes rounded up to the closest multiple of 512 bytes.

offset	length	name	description
0x0000	4	magic	firmware magic TRZV
0x0004	4	hdrlen	length of the vendor header (multiple of 512)
0x0008	4	expiry	valid until timestamp (0=infinity)
0x000C	1	vmajor	version (major)
0x000D	1	vminor	version (minor)
0x000E	1	vsig_m	number of signatures needed to run the firmware from this vendor
0x000F	1	vsig_n	number of different pubkeys vendor provides for signing
0x0010	2	vtrust	level of vendor trust (bitmap)
0x0012	14	reserved	not used yet (zeroed)
0x0020	32	vpub1	vendor pubkey 1
...	...	...	...
?	32	vpubn	vendor pubkey n
?	1	vstr_len	vendor string length
?	?	vstr	vendor string
?	?	vstrpad	padding to a multiple of 4 bytes
?	?	vimg	vendor image (120x120 pixels in TOIf format)
?	?	reserved	padding to an address that is -65 modulo 512 (zeroed)
?	1	sigmask	SatoshiLabs signature indexes (bitmap)
?	64	sig	SatoshiLabs aggregated signature of the vendor header

Vendor Trust

Vendor trust is stored as bitmap where unset bit means the feature is active.

bit	hex	meaning
0	0x0001	wait 1 second
1	0x0002	wait 2 seconds
2	0x0004	wait 4 seconds
3	0x0008	wait 8 seconds
4	0x0010	use red background instead of black one
5	0x0020	require user click to continue
6	0x0040	show vendor string (not just the logo)
7	0x0080	allow access to pairing secret
8	0x0100	disable access to pairing secret

Bits 0 to 6 represent vendor screen settings. The wait times are additive.

Two bits are used for access to the pairing secret for historical reasons. On T2B1 only bit 7 is evaluated. On newer models, both bits 7 and 8 are evaluated.

Firmware Header

Total length of firmware header is always 1024 bytes.

offset	length	name	description
0x0000	4	magic	firmware magic TRZF
0x0004	4	hdrlen	length of the firmware header
0x0008	4	expiry	valid until timestamp (0=infinity)
0x000C	4	codelen	length of the firmware code (without the header)
0x0010	1	vmajor	version (major)
0x0011	1	vminor	version (minor)
0x0012	1	vpatch	version (patch)
0x0013	1	vbuild	version (build)
0x0014	1	fix_vmajor	version of last critical bugfix (major)
0x0015	1	fix_vminor	version of last critical bugfix (minor)
0x0016	1	fix_vpatch	version of last critical bugfix (patch)
0x0017	1	fix_vbuild	version of last critical bugfix (build)
0x0018	8	reserved	not used yet (zeroed)
0x0020	32	hash1	hash of the first code chunk excluding both the firmware and the vendor header (128 - 1 - [vendor header length] KiB)
0x0040	32	hash2	hash of the second code chunk (128 KiB), zeroed if unused
...	...	...	...
0x0200	32	hash16	hash of the last possible code chunk (128 KiB), zeroed if unused
0x0220	415	reserved	not used yet (zeroed)
0x03BF	1	sigmask	vendor signature indexes (bitmap)
0x03C0	64	sig	vendor aggregated signature of the firmware header