Synapse: Implement cross-signing proposal

[uhoreg]

Implement server-side part of https://github.com/matrix-org/matrix-doc/issues/1756. Part of https://github.com/vector-im/riot-meta/issues/222.

• [x] manage master keys
• [x] store and retrieve local signatures
• [x] make sure signatures are properly filtered
• [x] notify users when new signatures need to be fetched
• [x] federate signatures
• [x] sytest everything
• [x] unit test everything

[uhoreg]

Main remaining work is cleaning up the code. We also need to decide whether to use the three-key system or the two-key system, but changing that should not take too long.

[Thatoo]

Do you think cross-signing will be implemented in synapse 1.0?

[neilisfragile]

@Thatoo While cross signing is nearly ready, we are not expecting it to be there in time for 1.0.

[Thatoo]

Good. Too bad for 1.0, it would have been awesome to be able to say "ok guys, matrix is mature for e2e with synapse 1.0".. :-)

[richvdh]

@uhoreg can we consider this closed if the synapse side is landed?

[richvdh]

see also https://github.com/vector-im/riot-web/issues/9631

[uhoreg]

Yes. Forgot there was an issue here.