XSF Discussion - 2019-02-06

I'm having this discussion on E2E encryption with michael on Twitter, but what he describes about key exchange sounds like a bug. https://twitter.com/mickael/status/1093128804728557570

mickael

This isn't inherent to the protocol, is it?

does it use multi-item PEP?

No, the key exchange stuff is pretty rubbish.

PEP and MAM on login adds a bit, not sure how significant it is

This isn't helped by the key service really needing more smarts than just a big PEP blob anyway.

Multi-item was less commonly supported by servers before, so would have been a pain to deploy if it was required

dwd: but is it as bad as mickael describes?

ralphm, bundles are only needed on session init

So they get fetched at that time

But session init doesn't happen often

ralphm, Well, I don't know about how loaded blatting out a big blob actually makes a server.

pep.: Yeah, that was my thought, too.

I think the increased message size in MAM is probably more worrisome

Well that's an issue with e2ee at all right?

that affects bandwidths (memory, disk and network) and disk storage

pep., yes, that’s kind of the point

jonas’: well, I am not surprised about encrypted stuff being bigger in general

k

Depends if it's hitting disk for PEP each time or something, I imagine.

The real problem is that everything is hitting the disk for way more stanzas now than in the past

Kev, And if not, the memory pressure is probably greatly increased.

dwd: Quite.

But this isn't a property of which protocol we use for e2e, is it?

Message size? No. Key exchange details? Yes.

The increase in MAM storage, I mean.

(Or ln -s No Not\ Entirely)

Right, if the key exchange is suboptimal, then that's a thing we can look into. I'm surprised that mickael says the impact would be 'huge'.

ralphm, Yes, it can be. MLS, for instance, would be hugely more efficient in bandwidth terms where OMEMO is used within MUCs, for example.

ralphm, But I would personally just treat OMEMO as the experiment it is, and await MLS patiently, and do things properly then.

I don't think we are talking about group chats at this point, though.

If you don't have to reencrypt for each device, certainly yes :)

dwd: I forgot to ask you about that by the way, is there any progress and where can we follow them?

ralphm, for 1:1 you still encrypt for each of your recipient's devices

(mls)

With OMEMO

goffi, MLS has an IETF working group, and is taking time - lots of effort in the low-level crypto.

pep.: sure, but what's this about 100s of PreKeys?

ralphm, That's a property of most of these algorithms, including MLS. Each device must upload a bunch of prekeys, and a new session takes one.

I'm not exactly sure about prekeys, maybe Syndace can chime in, and why we need 100. I guess that's when people want to start a session with you, and how often you need to renew them

(renew them > the prekeys)

What happens if two sessions are started using the same prekey?

ralphm, My sketch of a XEP for MLS, for instance, says that servers provide a keying service and hand out a single prekey for each device on demand.

Although even 'having 100 prekeys' doesn't necessitate our current mechanism.

Right, that'd do it.

Zash, Nothing - in principle. But it makes the cryptanalysis much harder.

Zash, that happens regularly, because we don't have reliable message delivery. So some clients will use the same prekey until they get an ACK from the other side

(for reference, the WG is here: https://datatracker.ietf.org/wg/mls/)

ralphm, Thanks.

ralphm, Note that for each session, the requester will grab all the prekeys, pick a set (one from each device), and then use them. On successful session start, the called device then removes the set from the prekey blob and re-uploads. This new bundle is then pulled by each device.

> ralphm, My sketch of a XEP for MLS, for instance, says that servers provide a keying service and hand out a single prekey for each device on demand. This is probably the 'proper' solution. For omemo we deliberately decided against an omemo component in favor of faster adoption. But learning about the limitations of PEP and how fast we can role out components if we need to (http upload) this is one of the regrettable features of omemo.

Or not that much regrettable but rather 'lessons learned'

Daniel, For sure - and I would note that this is a fine design if treated as an experiment.

dwd: sure, but how often does one have a new session? That's still not a common thing, is it?

ralphm, Every new conversation.

dwd, really?

Is it some security thing I don't understand, or is it a limitation, or?

pep.: What thing?

what what

pep., I would think so. The ratcheting means you can re-use a session for lengthy periods, but you'd need to store a fair chunk of cryptographically-sensitive state, so I'd think you'd discard that after a period of inactivity in a conversation and start anew later.

Daniel: when do sessions expire in Conversations?

pep., That said, I've not looked at what OMEMO implementations actually do. The spec more or less says "use libsignal", so it's hard to tell.

I would assume in OMEMO session initiation is not something that happens often, judging by how easy it is to lose messages when negociating a new one. If the other side starts a new session and drops the old one, and I don't, I'll send a message and they won't be able to read it.

pep.: Cryptosystems usually need something like a 3-way handshake. As I understand it, prekeys are the first step in that. (Is that the "what"?)

ralphm: they don't

Daniel: so in Conversations, a session between two users doesn't change keys, until you add/remove devices, or explicitly want to use a new key?

A session is something that exists between devices. So adding a new device will create an additional session and not change something about the existing ones

But the ratchet for each session is of course moved forward on every received message

right, I'm trying to understand what the actual traffic behaviour is, with respect to our current key exchange, and whether this is worse in some implementations than others

Ugh not reading for a day and 500 messages with a lot about OMEMO >.<

ralphm: regarding the Twitter?

yeah

Syndace: Enjoy your FOMO :)

Syndace, stop slacking off!

This is I assume about the prekeys

I Slack off all the time, I call it market research, or work, depending who's asking.

Haha

:D

So on login due to broken pep a client doesn't know if the prekey list on the server is up to date and will update it

Which as I just mentioned on Twitter is something one could probably fix now that pep isn't broken

so without session resumption (XMPP session, not OMEMO), this might be often

That often

ralphm: yes

I wonder why people insist on PFS, deniability and per-device keys. If you just have a per-account key that is shared between your devices, you can have such a simple e2ee scheme.

Ge0rG: Because PFS has more marketing behind it and looks better for ültraparanoid cryptonerds :)

then why wasn't xep-27 widely adopted/used ?

OpenPGP is Hard and '27 is awkward

it's just a different use-case, PFS has valuable properties, it also has downsides

It's got the word "Perfect" in it!!!1

*cough* https://op-co.de/tmp/SEX.html *cough*

moparisthebest: Also beacuse crypto wasn't hip and cool and buzzword-compatible enough back then

plus it doesn't have signing or replay prevention

Snowden and Facebook scandals does a lot of marketing for magic crypto dust and these last years seem to have been a good time to push this stuff

moparisthebest: As I said, '27 is awkward.

moparisthebest, eSessions ticked most boxes, but wasn't ever deployed either.

Gajim had it (removed now?)

moparisthebest, But also, there's RFC 3923, of course.

Zash, Yes, indeed. MattJ and I used to fiddle constantly to stop it encrypting as I recall.

It looks like xmpp.org is flapping ipv6 again.

IPv4 is also affected.

Sometimes it works, sometimes it fails.

my monitoring agrees

But now also my IRCnet fails, so maybe it's a larger issue somewhere in Yurop

the Brexit has strarted

bad network weather

Dun dun duuuun

Council meeting about to start over there: --> council@muc.xmpp.org

/correct Council meeting about to start over there: --> xmpp:council@muc.xmpp.org?join

/sudo -u dwd /correct ...

I can haz sudo XEP?

SASL sorta has it already actually

yeah, without reconnecting pls

authz vs authn something something

But yeah, overwriting other peoples messages ... might have its uses

Tho attaching or somesuch might be fine for most of them

So fun question, is the 'by' attribute of stanza errors (RFC 6120 § 8.3.2) guaranteed to be an JID, or can it be an arbitrary string?

I am missing the "the JID of" before "the entity" in the paragraph specifying it, just like it is done everywhere else

flow: Is there any text that suggests that "entity" is equivalent to JID?

Zash, yes and no, guess it depends on the type of the entity

flow: you don't want to enforce this in the type system, do you?

I thought it was a JID

I'm really excited to get my client crashed by type system exceptions caused by remote entities!

Maybe(JID | String)

Have fun

Someone is going to mess up my bookmarks with a robot face MUC and I'll end up in an endless connect loop.

It seems there is an error in https://xmpp.org/extensions/xep-0261.html examples 9, 10. New SID has to be different for second stream but it has to be the same for for <transport> and <open>

rion, can you post that to the standards@ list?

I'll send a pull request with fix

rion, do you mean example 10 has the wrong sid

lovetox: yes

should be bt8a71h6 instead

yeah seems like a copy/paste error

https://github.com/xsf/xeps/pull/755

Daniel, could you add IPv6 support to conference.siacs.eu please?

I don’t currently have an IPv4 on my server, and it prevents me from joining.

Link Mauve: it's on the todo list

Great, thanks. :)

Kev, could you do the same with rooms.swift.im please?

another error https://xmpp.org/extensions/xep-0167.html example 50. wrong action.

rion, please PR :)

https://github.com/xsf/xeps/pull/756

thanks o/

xep-0167 example 48 probably should include some candidates to <transport> or at least refer to consequent transport-info actions for transport negotiation.