The main difficulty in group messaging is consensus. How does a group of people agree on what key to encrypt the next message too. If people disagree, someone can't decrypt the messages of someone else. That's a fork in the group state, it's quite a similar problem to Bitcoin. Contrary to Bitcoin, in most group messaging systems you know exactly who the group members are, which makes things easier. The main hurdle for marmot specifically is that there is no central server whom you can trust with ordering messages properly. Every client has to be a "fully validating node" and create "the blockchain" of messages so that they're in the exact same order as everyone else. Cordn takes a different approach, each group has one central coordinator who is trusted with message ordering. This massively simplifies the consensus problem, and reduces the amount of complexity in the client. This likely means cordn is relatively more stable than marmot if the coordinator is honest. And cordn uses lots of fancy tricks to make this central coordinator as blind and trust minimized as possible, and easy to run even in a browser tab. This is somewhat similar to Bitcoin CoinJoin, almost impossible to do fully decentralized (only coin shuffle achieved it in theory) so everyone built centralized coordination models (even joinmarket) and then Wasabi optimized the coordinator to be blind and trust minimized. So to some extend Marmot is trying to achieve the impossible, decentralized encrypted group messaging, many wizards keep telling us this is too difficult and probably impossible to achieve. But let's see, it smells to me like we're close on actually making it work...