[messaging] MSc thesis on group messaging and group key exchange protocols

[George Kadianakis]

Greetings!

I just finished my MSc thesis which you can find here:

  https://people.torproject.org/~asn/thesis_msc_20150615.pdf

It all started as an attempt to design a secure messaging protocol,
but because of the nature of my degree it became more theoretical and
cryptographic so I ended up looking at group key exchange protocols
and provable security.

Still, the document contains some information about group messaging
and of previous work on this space. However the content is incomplete
and a bit outdated; if you are looking for a robust survey on
messaging protocols I would suggest the recent paper by Bonneau et al.[0]

We also present a key integrity attack on the group key exchange
protocol from the paper "Flexible group key exchange with on-demand
computation of subgroup keys". That's the paper that the (n+1)sec key
exchange protocol is based on, however the (n+1)sec protocol itself is
*not* affected.

Finally, we present a toy new key exchange protocol based on a paper
by Bresson et al., slightly modified to make it nicer, and then prove
it secure using a sequence-of-games proof. Finally, in Appendix B,
there is some *research-level* simulation source code of the "Deniable
Group Key Agreement" by Bohli et al.

It's worth mentioning that halfway into writing my thesis, I got
convinced that key exchanges might not be the best way to do
cryptography in the group messaging setting. Discussions with Trevor
and Ximin helped me realize that pairwise crypto and some kind of
group key (a la sender-keys) might be the way forward since it's more
asynchronous, making it more useful for mobile platforms.

That said, it was a fun thesis to write and I learned lots about
provable security. I believe that key exchange security models and
proof techniques are still very useful for demonstrating security even
on pairwise sender-key type of constructions.

Big thanks to my supervisor Kenny Paterson and to the various friends
that talked group messaging with me over the past years!

Sorry for the noise and don't forget to enjoy life!

PS: I recently realized that I completely lost my undergrad thesis and
    I got very sad.  By uploading this one to the Internet it will
    always stick around so that I can make fun of myself in the
    future!

[0]:  "Sok: Secure Messaging"
      http://cacr.uwaterloo.ca/techreports/2015/cacr2015-02.pdf