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Fourth week of GSoC and OMEMO thoughts

GSoC

Evaluation phase is there! Time went by faster than I expected. That’s a good sign, I really enjoy working on Smack (besides when my code does not work :D ).

I spent this week to work on the next iteration of my Jingle code. IBB once again works, but SOCKS5 still misses a tiny bit, which I struggle to find. For some reason the sending thread hangs up and blocks just before sending begins. Its probably a minor issue, which can be fixed by changing one line of code, nevertheless I’m struggeling to find the solution.

Apart from that it turned out, that a bug with smack-omemo which I was earlier (unsuccessfully) trying to solve resurrected from the land of closed bug reports. Under very special conditions pubsub results seem to only arrive in Smack exactly after the result listener timed out. This keeps smack-omemo from fetching device bundles on some servers. I originally thought this was a configuration issue with my server, but it turned out that aTalk (a Jitsi for Android fork which is working on including OMEMO support) faces the exact same issue. Looks like I’ll have to investigate this issue once more.

OMEMO – Reflections

It appears that soon the council will decide on the future of OMEMO. I really hope, that the decision will make everyone happy and that the XMPP community (and – perhaps more important – the users) will benefit from the outcome.

There are multiple options for the direction, which the development of OMEMO can take. While everybody aggrees, that something should happen, it is not clear what.

OMEMO is already rather well deployed in the wild, so it is obviously not a good idea to break compatibility with existing implementations. A few months ago, OMEMO underwent a very minor protocol change to mitigate a vulnerability and even though Conversations (birthplace to OMEMO) made a smooth transition over multiple versions, things broke and confused users. While things like this probably cannot be avoided in a protocol which is alive and changing, it is desirable to avoid breaking stuff for users. Technology is made for users. Keeping them happy should be highest priority.

However, at the moment not every user can benefit from OMEMO since the current specification is based on libsignal, which is licensed under the GPLv3. This makes the integration in premissively licensed software either expensive (buying a license from OWS), or laborious (substitute libsignal with other double ratchet library). While the first option is probably unrealistic, the second option has been further investigated during discussions on the standards mailing list.

Already the “official” specification of OMEMO is based on the Olm library instead of libsignal. Olm more or less resembles OWSs double ratchet specification, which has been published by OWS roughly half a year ago. Both Olm and libsignal got audited, while libsignal got significantly more attention both in the media and among experts. One key difference between both libraries is, that libsignal uses the Extended Triple Diffie Hellman (X3DH) key exchange using the XEdDSA signature scheme. The later allows a signature key to be derived from an encryption key, which spares one key. In order to use this functionality, a special conversion algorithm is used. While apparently this algorithm is not too hard to implement, there is no permissive implementation available in any established, trusty crypto library.

In order to work around this issue, one proposal suggests to replace X3DH completely and switch to another form of key exchange. Personally I’m not sure, whether it is desirable to change a whole (audited) protocol in order to replace a single conversion algorithm. Given the huge echo of the Signal protocol in the media, it is only a matter of time until the conversion algorithm makes its way into approved libraries and frameworks. Software follows the principle of supply and demand and as we can conclude from the mailing list discussion, there is quite a lot of demand even alone in the world of XMPP.

I guess everybody aggrees that it is inconvenient, that the “official” OMEMO XEP does not represent, what is currently implemented in the real world (siacs OMEMO). It is open to debate now, how to continue from here on. One suggestion is to document the current state of siacs OMEMO in a historical XEP and continue development of the protocol in a new one. While starting from scratch offers a lot of new possibilities and allows for a quicker development, it also most definitely implies to completely break compatibility to siacs OMEMO at users expense. XMPP is nearly 20 years old now. I do not believe that we are in a hurry :) .

I think siacs OMEMO should not get frozen in time in favor of OMEMO-NEXT. Users are easily confused with names, so I fear that two competing but incompatible OMEMOs are definitely not the way to go. On the other hand, a new standard with a new name that serves the exact same use case is also a bad idea. OMEMO works. Why sould users switch? Instead I’d like to see a smooth transition to a more developer friendly, permissively implementable protocol with broad currency. Already there are OTR and OpenPGP as competitors. We don’t need even more segmentation (let alone the same name for different protocols). I propose to ditch the current official OMEMO XEP in favor of the siacs XEP and from there on go with Andreas’ suggestion, which allows both libsignal as well as Olm to be used simultaneously. This allows permissive implementations of OMEMO, drives development of a permissively licensed conversion algorithm and does not keep users standing in the rain.

To conclude my reflections: Users who use OMEMO will use OMEMO in two or three years. It is up to the community to decide, whether it will be frozen “historical” siacs OMEMO, or collectively developed, smoothly transitioned and unified OMEMO.

Thank you for your time :)

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