[qrmn]

Automatic contact discovery is tricky, but the beginnings of one potential solution is I think explored in agl's Pond, using pairings on BN curves?: https://pond.imperialviolet.org/

To a point, so is offline messaging. Distributed datastores in general do that kind of thing - there are old implementations over Freenet in particular, and GNUnet as well and other things I think. The robust part is harder: if the client is offline, where's the disk space coming from? Volunteers' (nodes') cache? That needs to be opaque, and so do lookups. What's to stop spammers flooding the network? (Spam, and denial-of-service in general, is a Hard problem to deal with in general even in distributed systems.)

They're not among the hardest problems. Pseudonymity is a huge challenge as the latency gets lower against traffic correlation attacks, and a secure messenger is most definitely up against the kind of threat model that can and will try to pull those off in the wild. Perhaps mixing high and low-latency traffic may help, although it's going to need to be very carefully analysed how much (I believe I2P's design can do that, although I'm not sure if jrand0m ever implemented it in the router).

[lewisl9029]

Yes those are definitely not the hardest problems in this space. I was only referring to UX-related features in my original post (i.e. rather than the much more tricky privacy and security related ones), ones that I couldn't figure out how to actually implement in the context of a client-side web app. Impossible was probably too strong of a word to express that.

Thank you for all the interesting ideas and resources though! They'll definitely come in handy when I eventually revisit this space when I have more time later on.