E2EE spectacularly fails to recover after running out of OTKs

[ara4n]

I'm assuming that after being offline for a week, eleweb ran out of OTKs. However, since coming back online, i'm still getting UISIs from everyone - it looks like they've given up trying to reestablish new Olm sessions.

Meanwhile, surely we should be providing fallback keys to prevent this failure mode?

[jryans]

I have confirmed that at least for two web users, we do successfully claim fallback keys when OTKs run out, and the target user is able to read those messages (without trying a room key request), so the core functionality of fallback keys is working here.

This may require deeper analysis by the crypto team down the road.

[ara4n]

any idea whether Element Mobile handles sending to an OTK-exhausted Element Web well? As this might explain my symptoms.

[ara4n]

@lampholder and @Philip-Roy confirm seeing this recently too - i've asked tom to rageshake given his is in progress right now.