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matrix: auto-bootstrap cross-signing on first startup

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Without this, every Matrix bot started under hermes-agent shows the
"Encrypted by a device not verified by its owner" badge in Element
indefinitely, because the cross-signing chain (master → SSK → device)
was never published. Operators currently have to write their own
bootstrap script and remember to run it once per bot — and it's easy
to get wrong (the obvious base64.b64encode().decode() produces padded
keyids that matrix-rust-sdk silently rejects in /keys/query, so even
correctly-signed keys fail to load identity in Element).

mautrix already has the right primitive: generate_recovery_key() does
the full flow — generate seeds, upload privates to SSSS, publish
publics to the homeserver, sign the current device with the new SSK,
and return the human-readable recovery key. We invoke it once on
startup if the bot has no existing cross-signing identity, and log
the recovery key with a clear instruction to save it for future
restarts via MATRIX_RECOVERY_KEY (which the existing recovery-key
path already consumes).

Skipped when MATRIX_RECOVERY_KEY is set (existing path takes over)
or when the bot already has cross-signing keys on the homeserver
(get_own_cross_signing_public_keys returns non-None).

Bootstrap failure is non-fatal — logged with hint about UIA; the bot
continues without cross-signing and Element will show the warning
that prompted this PR. That matches the existing soft-fail pattern
for verify_with_recovery_key.

Tested against Continuwuity 0.5.7 (no UIA required). Synapse with
UIA enabled will need a follow-up PR to thread MATRIX_PASSWORD
through to /keys/device_signing/upload.
This commit is contained in:
Andrew Miller
2026-04-23 22:52:46 -04:00
committed by Teknium
parent 32d4048c6b
commit d497387cec
1 changed files with 38 additions and 0 deletions
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38
gateway/platforms/matrix.py
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@@ -698,6 +698,44 @@ class MatrixAdapter(BasePlatformAdapter):
logger.warning(
"Matrix: recovery key verification failed: %s", exc
)
else:
# No recovery key — bootstrap cross-signing if the bot
# has none yet. Without this, Element shows "Encrypted
# by a device not verified by its owner" on every
# message from this bot, indefinitely. mautrix's
# generate_recovery_key does the full flow: generates
# MSK/SSK/USK, uploads private keys to SSSS, publishes
# public keys to the homeserver, and signs the current
# device with the new SSK. Some homeservers require UIA
# for /keys/device_signing/upload — those will need an
# alternate path; Continuwuity and Synapse-with-shared-
# secret accept the unauthenticated upload.
try:
own_xsign = await olm.get_own_cross_signing_public_keys()
except Exception as exc:
own_xsign = None
logger.warning(
"Matrix: cross-signing key lookup failed: %s", exc
)
if own_xsign is None:
try:
new_recovery_key = await olm.generate_recovery_key()
logger.warning(
"Matrix: bootstrapped cross-signing for %s. "
"SAVE THIS RECOVERY KEY — set "
"MATRIX_RECOVERY_KEY for future restarts so "
"the bot can re-sign its device after key "
"rotation: %s",
client.mxid,
new_recovery_key,
)
except Exception as exc:
logger.warning(
"Matrix: cross-signing bootstrap failed "
"(non-fatal — Element will show 'not "
"verified by its owner'): %s",
exc,
)
client.crypto = olm
logger.info(