XMPP Service Operators - 2017-09-27

Holger: hi, on check.messaging.one there's a thing not covered in faqs that I don't understand: in cipher section what means ""

Server does not rspect client cipher ordering?

random: it means that the server will choose the best cipher from the client's list, instead of the first one.

Both the server and the client advertise a list of supported ciphers. The client might advertise A|B|C and the server C|B|A. The question is whether A or C is chosen.

I see, can it be solved so the server is the one that decides?

Any way to force it

random: that's exactly what the message says.

Ackshhhhually, Client advertises, server picks one that is mutually supported. :) </pedantic>

Yes, the recommendation seems to be C, i.e. "don't respect the client's ordering". The assumption seems to be that the server knows more about ciphers than the client.

Zash: Right.

Server may take the clients ordering into consideration, or not.

Oooh i understand. So that's not a bad message

No.

It's just a message.

But yes it's worded in a somewhat negative way :-)

If the client has a terrible ordering then ignoring it is good. If the client has put thought into its ordering then ignoring it is .. less good.

I understand now, thanks everybody

xnyphs did some research into a the ordering of a bunch of clients and they weren't very nice, so having the server prefer its own order seemed like a sensible choice at the time

That situation may or may not have changed, and probably varies with clients.

I kinda wish TLS APIs had more flexibility than either-or ordering. I think some Google / Chrome people (agl?) looked into that, but I don't think any changes have made it into eg OpenSSL