It is completely clear how desktop and mobile platforms for sync.com allow zero knowledge. However, it blows my mind when I try to understand how "Zero Knowledge" could be theoretically possible when using a browser, i.e. web application upload.
So I login to sync.com with Chrome. Then I hit
upload a file button, and a file from my PC is getting uploaded and encrypted on the fly by the browser? Then the browser must know my encryption key, i.e. my login password! Does this mean the browser (and the sync.com) is keeping my login credentials while I'm logged in? As far as I know the credentials should not be kept like this, the modern practice is JWT token or something.
Anyway, does a browser even have such a complicated encryption capability (comparable with the desktop app), or are my files being uploaded to the server and encrypted there? Either way that would not be a zero knowledge.