I just tried it and it seems like a pretty good start!

It doesn't seem to have launched publically yet but I hope they will have a landing page with utterly clear copy in a large font written with the mindset that for example this sentence

"Once you have entered your email address, SC4 will automatically provision you with a set of random keys."

is terrifyingly incomprehensible gobbledygook.

...actually I might be confused right now but even I (a total smartypants guy) can't seem to figure out how to paste in somebody else's public key in there. The default option is to encrypt for myself, and I see no way to input another key, only a text box for the message...

Edit: oh, maybe the text box is for the keys, and the thing I want to encrypt should be drag and dropped? Either way, the UI as it is right now is difficult to understand, but I suppose it is a prototype.

Thanks! "They" is me :-) And yes, you can paste anything into the text box and the app should automagically do the "right thing" with it.

Keys are stored in local browser storage. How does this work when browser cache is cleared?

Cache is separate from localStorage. Clearing your cache should have no effect on your SC4 keys. (Also, if you're running one of the two standalone versions, your key is stored in a file.)

At least in Firefox, clearing cookies (not an unreasonable thing for users to do once in a while) will also clear localStorage.

That's true. There are limits to what one can do when running in a browser. If you have a better idea for where to store keys, I'm all ears.