Matrix: bot device shows as 'untrusted' in Element — no cross-signing or bot-initiated SAS verification path

[orrinwitt]

Problem

When nanobot connects to a Matrix homeserver with e2eeEnabled: true and sasVerification: true, the bot device appears as "Untrusted" in Element (Web, Desktop, and Element X Android). There is no clean way to clear this warning.

Root Cause

Modern Element clients use cross-signing to determine device trust. Per-device SAS verification alone no longer clears the "Untrusted" label — Element checks for cross-signing key signatures, not individual device verification.

matrix-nio (the library nanobot uses for Matrix E2EE) does not support cross-signing (tracked at matrix-nio/matrix-nio#229, open since 2020). This means:

• The bot cannot upload cross-signing keys
• Even after successful SAS verification, Element still shows "Untrusted"
• Modern Element versions have also removed the UI for manually verifying another user's individual devices

Additional Gap: Bot-initiated verification

The built-in SAS handler (added in v0.2.2 via sasVerification: true) only handles KeyVerificationStart, KeyVerificationKey, KeyVerificationMac, and KeyVerificationCancel events. It does not handle:

• m.key.verification.request (sent by Element to initiate verification) — arrives as UnknownToDeviceEvent
• m.key.verification.ready (Element's response accepting the request) — arrives as UnknownToDeviceEvent
• m.key.verification.done (final confirmation after MAC exchange) — never sent

Without handling request/ready, the SAS flow never starts when a user initiates verification from Element. The bot can only respond to start events, but Element no longer sends start directly — it sends request first.

The bot also has no mechanism to initiate verification toward a user's devices, which is needed since Element removed the manual device verification UI.

Current Workarounds

1. Accept the cosmetic warning — encryption works (megolm sessions are shared, messages decrypt), the "Untrusted" label is just Element being strict about cross-signing
2. Disable E2EE (e2eeEnabled: false) — removes encryption and the warning, acceptable for bots on private homeservers but not ideal

Feature Request

1. Add cross-signing support (depends on Support cross-signing matrix-nio/matrix-nio#229, or implement directly using Olm)
2. Handle m.key.verification.request and m.key.verification.ready events in the SAS verification flow so Element-initiated verification completes
3. Send m.key.verification.done after successful MAC exchange
4. Add bot-initiated verification — allow the bot to start SAS verification toward a user's devices (e.g., a CLI command or API endpoint)

Environment

• nanobot v0.2.2
• matrix-nio 0.25.x
• Homeserver: Continuwuity (also tested with Synapse)
• Element Web, Element Desktop, Element X Android — all show "Untrusted"
• e2eeEnabled: true, sasVerification: true