Write proof-of-concept hidden service chat, email, etc applications
Hidden services don't have to be just for publishing. They are arbitrary communication endpoints.
We could prototype hidden service-based chat and email by bundling local XMPP and SMTP servers for use with Tails.
Client software developed for #6583 (moved) could then be used almost as-is to communicate with other hidden service endpoints.
The benefits of hidden service communications are that you do not reveal your social graph to the network or even to any centralized chat or email infrastructure, and you get end-to-end encryption and authentication for free.
The key things that makes this a large project are that we need solid integration with client software, and the XMPP and SMTP servers have to "just work" right out of the box. If this integration and deployment is not seamless, almost no one will use hidden services this way, and the few who manage to do so will not have anyone to communicate with.