Agreed that E2E is mandatory for privacy. The federated nature of Matrix doesn't make it any harder to implement though - after all, TextSecure/Signal has basic federation too. The one subtlety we have is that we want to give the users the option of selecting between PFS and replayable epochs of history on a per-room basis - the latter making it easier to sync history between devices; the former being for the privacy paranoid.

Now, a more interesting privacy concern is that Matrix doesn't and can't protect metadata currently - see https://matrix.org/~matthew/2015-06-26%20Matrix%20Jardin%20E... for details. So if you want both encrypted contents as well as obfuscated metadata, go check out Ricochet or Vuvuzela or Pond or something for now :)

Yes metadata is probably going to remain an unsolved problem for Matrix because it's not fully distributed, but I agree with the tradeoff Matrix is making here.

For now, building a great federated chat protocol that allows users to fully control their own data should take priority over just about everything else, because once users are in control of their own data, they're no longer locked to a single platform. So if another protocol comes along that does everything Matrix gets right and adds metadata protection, users can simply export their data and import it to the next great app/protocol (which is hopefully also federated). Until we have a great federated protocol that everybody is using, the friction associated with moving between chat apps/protocols is going to continuously inhibit real innovation in the messaging apps space by favoring apps that stand on the strength of their network effects over apps that stand on their own merits.

On the topic of fully distributed apps, I really like what replikativ is doing in this space:

https://github.com/replikativ/replikativ

It's a CRDT-based data replication library for building fully distributed applications. They envision replikativ being used as an open data exchange whereby users fully control their own data, and can specify any number of applications to have access to that data, instead of the status quo where user data is kept in closed, per-application silos. Their README has a much more comprehensive overview of the library and their vision.

Have you heard much about the Signal server federation? It's been more than a year since it was mentioned (that I've seen) by the Open Whisper Systems folks.

I'm hoping it's still on some sort of roadmap.

I'm not sure it's a huge priority there currently, but we don't have any internal knowledge. We asked to federate Signal with Matrix (which is a concrete possibility given we share the same E2E spec) and were politely turned down, as they'd prefer Signal to be a silo whose security they control rather than risking interop with 3rd parties. Hopefully they will change their mind in future once Matrix's E2E is more proven :)