we should specify which servers must sign events

[richvdh]

We do not seem to specify which server(s) must have signed an event for it to be considered valid

[richvdh]

I'm not really sure if this belongs under Checks performed on receipt of a PDU, Validating hashes and signatures, or even Checking for a signature in the appendix, but it ought to be somewhere.

[turt2live]

I'd probably vote for the first two, maybe one referencing the other.

[richvdh]

The rules are something like:

• for all events except invite events in response to a 3pid invite, the event must be signed by the server whose server name is in the sender of the event
• for room v1-v2, the event must be signed by the server in the event_id, where that is different.

See https://github.com/matrix-org/synapse/blob/aa530e68005d041ad037b081fe748d301a11291a/synapse/federation/federation_base.py#L213.