One user story is that Alice uses Eve's webmail, and Bob uses PGP and mutt on his laptop. Before Eve's webmail supported PGP in JS, Bob had to send his emails to Alice unencrypted (and unsigned), which meant his mail provider could read the plaintext (even if he trusted that mail provider to always require a TLS connection when sending to Eve's servers).
From Alice's point of view, she is just using webmail as she always has, except now she has the assurance that no one (other than Eve) can spoof Bob's identity, and that Bob's mail server isn't reading the messages she sends him (unless Eve is deliberately leaking the plaintext somehow despite sending Bob the encrypted version).
Long term it would be nice if the W3C's SRI standard was extended to allow offline signing of JavaScript files:
> [Alice knows] no one (other than Eve) can spoof Bob's identity
If Carol or Chuck can spoof Eves "identity" they can spoof Bobs identity. This can be done in a multitude of technical or social ways.
Is it better to have this than nothing? The problem is that you have to trust your whole infrastructure if you want to do this kind of client side encrypting.
If your threat model says that Eve's webmail servers can be spoofed, then Alice can't use webmail at all, or possibly any websites. At that point, the security of PGP in JS is pretty much irrelevant.
I think that is one of the most obvious things that can happen, but no it only affects JS PGP that is integrated on a site you use. PGP in JS is still relevant because it makes it easier to download, verify and execute Javascript "anywhere", not as an integrated solution served by a third party. Sadly.
From Alice's point of view, she is just using webmail as she always has, except now she has the assurance that no one (other than Eve) can spoof Bob's identity, and that Bob's mail server isn't reading the messages she sends him (unless Eve is deliberately leaking the plaintext somehow despite sending Bob the encrypted version).
Long term it would be nice if the W3C's SRI standard was extended to allow offline signing of JavaScript files:
https://github.com/w3c/webappsec/issues/449
and for browsers to prompt you whether you wanted to run a new (offline signed, maybe independently audited) version of those files.