[OTR-dev] Support for asynchronous communication

[Miro Kropáček]

Hello,

I've become another happy developer who has the pleasure to implement an
OTR-based software. I've managed to get a few simple use cases working
(persistent/verified key pairs in the key store, sending/receiving of
simple messages, smp authentication, ...) and I've been wondering ever
since whether I should use OTR also for ciphering or stick with
authentication only.

I've been working on a p2p communication client for a while and as you can
imagine, in such environment is hard to predict the order of delivery,
basically to predict delivery at all. According to this blog post:
https://whispersystems.org/blog/advanced-ratcheting it would seem that I'm
doomed. The author had implemented a "double ratchet" algorithm presented
here: https://github.com/trevp/double_ratchet/wiki but as far as I know, no
attempt has been made to "backport" it to libotr.

However, when I tried some basic use cases like "send (encrypt) messages 1,
2, 3" and "receive (decrypt) 3, 2, 1" or "receive (decrypt) 3, 1" within a
session, there wasn't any problem. Right now I don't care about forward
secrecy, I'm happy to have one session per user pair forever.

So my questions would be:

   1. In what scenarios OTR does fail when it comes to the order of
   delivery? Am I safe during session at least?
   2. If session is safe, what do I have to store to be able to renew the
   session later? Is there an API for this? (I'm using otr4j implementation,
   btw)
   3. Are there plans to include the double ratchet algorithm into OTR?
   4. And generally, is it a good idea to use OTR cipher in p2p
   environments?

Thank you for your insight.
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