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[liberationtech] Surespot? Re: Feedback on Threema - Seriously secure mobile messaging.

Nathan of Guardian nathan at guardianproject.info
Mon Jul 15 17:02:38 PDT 2013


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On 07/15/2013 05:55 PM, Pavol Luptak wrote:
> Any idea how to have offline secure messaging (when Jabber+OTR is
> not possible to use)?

Gibberbot already partially implements this, and we are working with
ChatSecure and others to move forward with a standards-based solution
to this. While we already have OpenPGP support on Android, we don't
think PGP is the way to go for efficient mobile chat, and also are not
ready to abandon forward secrecy as a feature.

And so, our OTR offline solution is comprised of the following pieces:

1) Client/App supports XMPP extension for message delivery receipts
and auto-retry:
http://xmpp.org/extensions/xep-0184.html

While the person you are trying to send a message to may be offline at
the very moment you want to message them, it is very likely that they
will be online at some point in the near future. If your chat client
is always running in the background on your device, it can sense when
they come online, and deliver the message you wrote earlier. By
supporting delivery receipts, you can confirm it did indeed get
through to them, and if not, continue auto-retrying until it does.

2) XMPP server that supports offline messages storage and delivery:
http://xmpp.org/extensions/xep-0160.html
http://xmpp.org/extensions/xep-0091.html
ejabberd support: http://www.process-one.net/en/ejabberd/protocols/
http://prosody.im/doc/modules/mod_offline

XMPP already has a well-established mechanism for this, and many
open-source servers like ejabberd and prosody support it well.

3) Client that supports long-lived OTR sessions. If a chat
conversation is held open, then the same OTR session key should be
used as long as possible, i.e. until one of the participants request a
session restart.

4) Optionally: use the OTR v3 shared "extra" symmetric key generation
feature to encrypt offline messages and send those via a mobile push
service.

Would love comments on this. Anything else we might have missed?

+n


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