ARArathorn·Sep 13, 2021
This is a great example of why key-sharing exists today... and why it shouldn't. In theory, you should be able to pick up and old client and decrypt new messages and history on it absolutely fine. However, in practice, Matrix's E2EE publishes 100 one-time keys (OTKs) on your server to let other devices establish secure 1:1 channels with you if you're offline - and if you go offline and that pool of OTKs exhausts, then new sessions won't get set up and you won't receive keys for new messages... giving the misbehaviour you're seeing.
Now, you're right that key-sharing is a useful way to fudge around that failure mode.
But an even better way to fix it would be to find a way to stop the OTK pool exhausting - and that's precisely what MSC2732 is: https://github.com/uhoreg/matrix-doc/blob/fallback_keys/prop.... This provides a last-ditch key
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which can be used to set up 1:1 sessions even if you run out of OTKs, which is marginally inferior to using a different OTK every time, but in practice really isn't a disaster (see the MSC for details).
However, fallback keys are relatively new and aren't implemented on all clients yet (matrix-js-sdk has them, but matrix-ios-sdk is implementing this coincidentally this week)... and so until they land, we still need keyshare requests to paper over this limitation.
But in future, hopefully it will be almost unheard-of to need a keyshare request, and we can change them to be an entirely manual or out-of-hand mechanism of some kind, and avoid classes of bugs like the vuln in question here in future.