This is really clever. And looks like it's easy to self-host via Docker.
> I wonder if there is a solution that would correctly deal with both situations (never requiring the other end to sign up or know anything about encryption).
I'm actually in the midst of building out a tool for that exact scenario[0]. You can send it to anyone so long as you know the email or username they use for a service that implements OAuth. If they successfully authenticate and the server sees their username / email match up, it sends back the encrypted data to the frontend, which then decrypts it with the key in the URI fragment. It's neither audited nor open source just yet, so I don't recommend using it for actual secret sharing until then.
There hopefully will be one soon! The backend was written haphazardly in a weekend, so once I clean it up to save myself some embarrassment, I'll open up the source on Github. I'll send you an email if that happens (using the one in your HN profile).
> I wonder if there is a solution that would correctly deal with both situations (never requiring the other end to sign up or know anything about encryption).
I'm actually in the midst of building out a tool for that exact scenario[0]. You can send it to anyone so long as you know the email or username they use for a service that implements OAuth. If they successfully authenticate and the server sees their username / email match up, it sends back the encrypted data to the frontend, which then decrypts it with the key in the URI fragment. It's neither audited nor open source just yet, so I don't recommend using it for actual secret sharing until then.
[0] https://intended.link/