The Key Lattice Framework for Concurrent Group Messaging

Today, two-party secure messaging is well-understood and widely adopted, e.g., Signal and WhatsApp. Multiparty protocols for secure group messaging are less mature and many protocols with different tradeoffs exist. Generally, such protocols require parties to first agree on a shared secret group key and then periodically update it while preserving forward secrecy (FS) and post compromise security (PCS). We present a new framework, called a key lattice, for managing keys in concurrent group messaging. Our framework can be seen as a “key management” layer that enables concurrent group messaging when secure pairwise channels are available. Security of group messaging protocols defined using the key lattice incorporates both FS and PCS simply and naturally. Our framework combines both FS and PCS into directional variants of the same abstraction, and additionally avoids dependence on time-based epochs.