trezor-firmware/docs/bootloader.md

#TREZOR Core Bootloader

Bootloader is split into two stages. See [Memory Layout](memory.md) for info about in which sectors each stage is stored.

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

##First Stage Bootloader

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

If first stage bootloader 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.

##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 below is SHA-256 and signature system is Ed25519 (allows combining signatures by multiple keys into one).
* All multibyte integer values are little endian.

##Bootloader Format

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

1. bootloader header
2. bootloader code

There is a tool called [check_bootloader](../tools/check_bootloader) which parses and checks validity of the bootloader including its header.

###Bootloader Header

Total length of bootloader header is 256 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      | sigidx | SatoshiLabs signature indexes (bitmap) |
| 0x0015 | 64     | sig | SatoshiLabs signature |
| 0x0055 | 171    | reserved | not used yet (zeroed) |

##Firmware Format

TREZOR Core firmware consists of 3 parts:

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

There is a tool called [check_firmware](../tools/check_firmware) which parses and checks validity of the firmware including the both headers.

###Vendor Header

Total length of vendor header is 82 + 32 * (number of pubkeys) + (length of vendor string) + (length of vendor image) bytes rounded up to the closest multiply of 256 bytes.

| offset | length | name | description |
|-------:|-------:|------|-------------|
| 0x0000 | 4      | magic | firmware magic `TRZV` |
| 0x0004 | 4      | hdrlen | length of the vendor header |
| 0x0008 | 4      | expiry | valid until timestamp (0=infinity) |
| 0x000C | 1      | vsig_m | number of signatures needed to run the firmware from this vendor |
| 0x000D | 1      | vsig_n | number of pubkeys vendor wants to use for signing |
| 0x000E | 2      | reserved | not used yet (zeroed) |
| 0x0010 | 32     | vpub1 | vendor pubkey 1 |
| ...    | ...    | ... | ... |
| ?      | 32     | vpubn | vendor pubkey n |
| ?      | 1      | vstr_len | vendor string length |
| ?      | ?      | vstr | vendor string |
| ?      | ?      | vimg | vendor image (in [TOIf format](toif.md)) |
| ?      | 1      | sigidx | SatoshiLabs signature indexes (bitmap) |
| ?      | 64     | sig | SatoshiLabs signature |

###Firmware Header

Total length of firmware header is 256 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      | sigidx | vendor signature indexes (bitmap) |
| 0x0015 | 64     | sig | vendor signature |
| 0x0055 | 171    | reserved | not used yet (zeroed) |

##Various ideas

* Bootloader should be able to read vendor+firmware header and send info about FW to client in features message.
* Bootloader should not try to run firmware if there is not any.
* Storage wiping rule: Don't erase storage when old FW and new FW are signed using the same key set. Otherwise erase.
* Bootloader should send error to client when firmware update fails and allow client to try one more time. This prevents storage area erasure by accident.