The beauty of it is that Scramble encrypts and signs whenever the recipient is also a Scramble address--automatically. The goal is to protect even users who don't know what a "PGP key" is.
(Don't get me wrong, we're planning improvements for power users as well--for example, we want to make it easy to use Scramble over Tor. But the goal really is "Encrypted email for everyone".)
> So then why am I trusting you with my PGP keys?
No, as explained in the writeup, the server stores your private key encrypted with your passphrase. The server never sees your passphrase, or your private key.
We've used a good key derivation function (scrypt)---this makes it difficult to brute-force the password.
In short: Yes, the server stores things so that you get the Gmail experience, sit down at any computer and it just works. No, the server never sees your plaintext private key.
Thanks for the feedback! If I get a chance, I'll paraphrase your questions and add them to the upcoming FAQ.
The only place a private key belongs to is the users machine (preferably somewhere without internet access).
As a user, why would I want to delegate my key management to you? Encrypting the email I want to send to a person with their public key is what gives me a sense of trust that what I am sending is encrypted and only readable to that person.
However--as the example of Tormail and others show us--the server cannot be trusted to serve uncompromised client-side code, even if the organization behind it is well intentioned.
So then why am I trusting you with my PGP keys?