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trezor-firmware/docs/bootloader.md
2017-04-12 14:45:58 +02:00

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# TREZOR Core Bootloader
TREZOR initialization in split into two stages.
See [Memory Layout](memory.md) 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.
## 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 [binctl](../tools/binctl) 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 512 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 | 427 | reserved | not used yet (zeroed) |
| 0x01BF | 1 | sigmask | SatoshiLabs signature indexes (bitmap) |
| 0x01C0 | 64 | sig | SatoshiLabs aggregated signature |
## 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 | 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 (in [TOIf format](toif.md)) |
| ? | ? | reserved | padding to an address that is -65 modulo 512 |
| ? | 1 | sigmask | SatoshiLabs signature indexes (bitmap) |
| ? | 64 | sig | SatoshiLabs aggregated signature |
### Firmware Header
Total length of firmware header is always 512 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 | 427 | reserved | not used yet (zeroed) |
| 0x01BF | 1 | sigmask | vendor signature indexes (bitmap) |
| 0x01C0 | 64 | sig | vendor aggregated signature |
## 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.