This makes me a bit nervous for the participants though. If I send you a private message, the message is encrypted along that private channel - but it's also s

This makes me a bit nervous for the participants though. If I send you a private message, the message is encrypted along that private channel - but it's also stored permanently on the blockchain unless I'm mistaken. For something like WhatsApp or Signal any possible future technology is not a concern - as long as our encrypted channel is secure right now that's good enough for what we need (TLS is similarly fine if it's fine right now). With this design, private conversations will be held encrypted forever in the chain, waiting for a day when the encryption is either broken or vulnerable to brute force (e.g. by a future quantum processor). I realise this sounds incredibly paranoid, but with Google's Sycamore and whatever else is being cooked up in other labs, we know the tech is coming eventually. The only question mark is really over the time horizon of getting there. We assume that a functionally useful implementation of Shor's algorithm for breaking RSA cryptography is beyond reach in the near term, but that's only an assumption, and ignores the possibility that an orders-of-magnitude-better classical or quantum algorithm for factorisation remains undiscovered.