The big issue, as you note, is that you download normal software once, while with a website you download it every time you visit (modulo caching, of course). W

The big issue, as you note, is that you download normal software once, while with a website you download it every time you visit (modulo caching, of course). With free software, you could download it, audit it, and then feel secure using it for a decade or more; with in-browser crypto you're highly unlikely to load the site, audit the code, then use it every time you use it. Moreover, this constant redownloading enables user targeting: probably no-one will notice if Google serves a slightly different piece of JavaScript to one user. That's one of the beauties of signed free software: it's public, and many people can collaborate on ensuring that it's secure. I don't think that the appcache applies to this case because it's under the web server's partial control.