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trezor-firmware/docs/misc/update-wipes.md
Tomas Susanka 38599e3364 docs: document when firmware wipes storage
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Firmware update and device wipe

This document describes under which circumstances the device gets wiped during a firmware update.

Trezor 1

The device gets wiped:

  • If the firmware to be installed is unsigned.
  • If the present firmware is unsigned.
  • If the firmware to be installed has lower version than the current firmware's fix_version [1].

The device gets wiped on every reboot:

  • If the firmware's debug mode is turned on.

Trezor T

In Trezor T this works a bit differently, we have introduced so-called vendors headers. Each firmware has its vendor header and this vendor header is signed by SatoshiLabs. The actual firmware is signed by the vendor header's key. That means that all firmwares are signed by someone to be able to run on Trezor T.

We currently have two vendors:

  1. SatoshiLabs
  2. UNSAFE DO NOT USE

As the names suggest, the first one is the official SatoshiLabs vendor header and all public firmwares are signed with that. The second one is meant for generic audience; if you build firmware this vendor header is automatically applied and the firmware is signed with it (see tools/headertool.py).

The device gets wiped:

  • If the firmware to be installed is from different vendor than the present firmware [2].
  • If the firmware to be installed has lower version than the current firmware's fix_version [1].

The device gets wiped on every reboot:

  • If the firmware's debug mode is turned on.

[1] Firmware contains a fix_version, which is the lowest version to which that particular firmware can be downgraded without wiping storage. This is typically used in case the internal storage format is changed. For example, in version 2.2.0, we have introduced Wipe Code, which introduced some changes to storage that the older firmwares (e.g. 2.1.8) would not understand. It can also be used to enforce security fixes.

[2] The most common example is if you have a device with the official firmware (SatoshiLabs) and you install the unofficial (UNSIGNED) firmware -> the device gets wiped. Same thing vice versa.