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.
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