Public Access
twislaandClaude Sonnet 5.5
d490a18b9a
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
23 lines
1.8 KiB
Markdown
23 lines
1.8 KiB
Markdown
# The device trusts the two ISRG roots for what it fetches
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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:
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- **The framework's bundle**, about 130 certificate authorities (about 60 KB of flash). Any of them could vouch for `git.twis.la`.
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- **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.
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- **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`).
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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.
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## What it costs
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- **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.
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- The roots are public data checked against the published fingerprints (listed in the file), refreshed by hand if Let's Encrypt ever changes them.
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## What it doesn't change
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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.
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## Measured while building it
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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.
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