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Updates from Gitea, step 5: the daily check's announcement, the docs, ADR 0009

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The announcement was cut at the notification's 48 bytes; it now reads
"v0.11.0 is out: see Settings > Firmware". README: Updates from Gitea
and its limits. ADR 0009: the device trusts the two ISRG roots. R1.md:
what was built and the checks on the device, with what wasn't checked.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT
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twisla
2026-10-06 16:16:32 +02:00
co-authored by Claude Sonnet 5.5
parent 4ef41347ab
commit d490a18b9a
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docs/adr/0009-the-device-trusts-the-isrg-roots.md
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# The device trusts the two ISRG roots for what it fetches
The firmware talks to the project's Gitea over HTTPS (issue #6, and #4 after it). A TLS client has to decide whose certificates it believes. Three ways were possible:
- **The framework's bundle**, about 130 certificate authorities (about 60 KB of flash). Any of them could vouch for `git.twis.la`.
- **Pin the server's certificate**, as the Gemini App does for capsules. The server's certificate is replaced every few months, so a pin would ask the question again at every renewal.
- **Carry the roots the server's chain ends in:** `ISRG Root X1` (RSA 4096) and `ISRG Root X2` (ECDSA P-384), the Let's Encrypt roots, about 2.7 KB of flash (`src/platform/ca_roots.h`).
We chose the third. The chain and the name are checked by mbedTLS during the handshake. It trusts one organisation's two roots, valid until 2035 and 2040, and a renewal changes nothing.
## What it costs
- **If the server moves to another CA, the device can no longer reach it**, and the next firmware, carrying that CA's root, has to come from the PC or the SD card. Both still work; they don't use TLS.
- The roots are public data checked against the published fingerprints (listed in the file), refreshed by hand if Let's Encrypt ever changes them.
## What it doesn't change
The Update File's own signature (ADR 0003) is what decides what gets installed. A hijacked connection could hide a release, or serve an older signed one, but never make the device install firmware that isn't ours. The TLS check matters more for #4, where a token will travel over it.
## Measured while building it
A TLS connection to this server peaks at about 52 KB of heap, **the same whether the certificate is checked or not**, so skipping the check would have saved nothing. The cost is the connection itself (record buffers and handshake), not the trust decision.