Joining rooms and backfill are broken if you change your signing key and use the old ID

[richvdh]

If you create a bunch of state events, and then change the key and use the same ID, it can cause difficulty for new servers trying to join or backfill such rooms, because the signatures appear invalid

[richvdh]

this is somewhat related to https://github.com/matrix-org/matrix-doc/issues/1685.

[richvdh]

also related: https://github.com/matrix-org/matrix-doc/issues/831

[richvdh]

The solution to this is to use a better identifier as the key ID (such as a hash of the public key, or the key itself for ed25519), so that servers can't claim that two different keys are both called ed25519:auto.

[richvdh]

dup matrix-org/matrix-spec#807

[richvdh]

matrix-org/matrix-spec#807 got resolved by #8350, but I'm not sure joining rooms got the same treatment.

[richvdh]

Unverifiable events no longer stop you joining a room altogether (though they can still cause inconsistent state, obviously).

[richvdh]

dup matrix-org/matrix-spec#807

I wish I knew what this was actually trying to link to. I'm pretty sure it wasn't that.

[turt2live]

dup matrix-org/matrix-spec#807

I wish I knew what this was actually trying to link to. I'm pretty sure it wasn't that.

Thanks to email history, we've discovered that it's actually #3121

Best guess is github guessed wrong on the numbered reference when 3121 moved from matrix-doc to matrix-spec (evidenced by https://github.com/matrix-org/matrix-doc/issues/3121 becoming matrix-org/matrix-spec#807 )