SMMP is a secure, multi-party, synchronous communication protocol. The protocol follows a peer-to-peer model and provides perfect forward secrecy and perfect future secrecy against a computationally-bounded adversary, as well as information-theoretic plausible deniability for participants in a multi-party conversation. The protocol uses a three-round, authenticated Burmester-Desmedt group key agreement protocol to generate a shared secret between a group of N participants. Conversation participants authenticate to each other during key agreement via a triple Diffie-Hellman mechanism. Individual message keys are updated after receipt of each message by incorporating new key material distributed by the sending participant. No security requirements are imposed on the underlying transport layer, and our protocol leaks no metadata beyond that exposed by the transport layer. Conversation transcript universality is assured by a conversation digest that is updated upon receipt of each message, and all conversation messages are signed to verify proof of origin. All setup operations prior to group key agreement take place over an insecure channel. SMMP represents a significant improvement in security and efficiency over current secure, multi-party messaging protocols including GOTR, mpOTR, and improved GOTR.
See the paper in the smmp/spec directory for full details on the protocol.
This repository includes a reference implementation of SMMP written in python. It also inclues an ncurses-based secure multiparty chat client.
Notes:
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The curve25519 repository is not in PyPI, so you will need to install it by hand. To do that:
git clone https://github.com/agl/curve25519-donna cd curve25519-donna sudo python setup.py install
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There is a bug in the python-gnupg 0.3.6 module that throws an exception if gpg emits an "UNEXPECTED" message. This happens when two messages collide on receipt, and are mixed.
While the exception messes up the smmpchat screen a bit, it causes no lasting harm. I submitted a patch to python-gnupg to fix the bug. It was accepted and is in the official mercurial repository at bitbucket. Presumably the PyPI repo will be updated at some point, probably when the python-gnupg module version is bumped to 0.3.7.
Should you wish to install the patched python-gnupg module, you can clone the official repository with the following command:
hg clone https://bitbucket.org/vinay.sajip/python-gnupg
Then do the usual sudo python setup.py install to install it.
You may need to remove the old version first.