oauth-1.0a VS webext-signed-pages

A nonce is just a random string. Timestamp is Math.floor(new Date().getTime()/1000). Auth signature will be some sort of hash. You could get an idea of how it's done by looking at a js lib

There is "Signed Pages" by the debeloper of EteSync. It is a browser extension, that checks webapps based on signatures in the html file. The addon then warns the user if the signature is not correct or - if I remember correctly - the source changed. This allows you to be sure what webapp code was delivered. But it seems like it did not really get used outside of his own projects. https://github.com/tasn/webext-signed-pages

EteSync has implemented something called Signed Pages, this might be worth looking closer at. This uses PGP keys which is preloaded into the browser; but I suspect that will be a barrier too high for most non-tech users.

There are also projects like signed web pages which can also help increasing the trust level to some degree. But that requires that you can download the source code and regenerate the verification hash locally - or have other trusted methods to verify the hash value hasn't been modified as well. The current concept is reasonably sane, but it requires too much from users currently to make it widely used.

> The server can at any time start serving malicious payloads

True, and I call this threat model "Beware Each and Every Fetch" (BEEF) in contrast to the more common TOFU model (although if you trust a desktop app to auto-update itself then these two models might not be all that different).

In any case, I think you're being a little quick to dismiss the idea of server-hosted applications. It's true that browsers don't natively have a nice way of pinning specific versions of a web app, but there is the clever hack of SecureBookmarks[0] (if you're prepared to sacrifice the UX), or, more realistically, you can pin the web app version using some sort of browser extension.

Examples of the latter include the Signed Pages extension[1], and Code Verify[2], which is the result of a collaboration between Meta and Cloudflare (for securing the WhatsApp Web code, currently, but should eventually support other sites like Proton's too). Of course, it would be much better if this capability was natively included in browsers themselves, but hopefully adoption of this technology will pressure browsers and standards bodies to take ownership of this.

[0] https://coins.github.io/secure-bookmark/

[1] https://github.com/tasn/webext-signed-pages

[2] https://github.com/facebookincubator/meta-code-verify

Something like a browser extension for this does already exist, fortunately:

https://github.com/tasn/webext-signed-pages

In regards to untrusted webapp, yes, that is a reasonable attack vector. That said, I've heard from ProtonMail they have been considering to implement Signed Pages to help mitigate (at least some of the) issues with this attack vector.

Which is why it is important to get proper E2E encryption on e-mail, where the source is open source and can be audited. And then that there are verify mechanisms to verify that the source code has not been manipulated. For web services there are signed-pages which is quite interesting.