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For the secure messaging example, it can be isolated per contact, and handling things like audio / video decoding for video calls can be isolated, as can cryptography, etc. Finer grained isolation than a group of applications for a certain identity / task is very important.
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It's fine-grained isolation of different components. Improving that involves having minimal attack surface exposed between the components, simple data formats and a focus on hardening the code most exposed at the boundaries with safe tools, etc.
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Isolating per-contact in a messaging client, per-site in a browser, etc. is applying the same principle of QubesOS at a fine-grained level using existing privacy/security boundaries. Since they're existing boundaries, it doesn't require the user to do anything or be aware of it.
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There are implementations of fine-grained isolation within applications using different mechanisms than OS sandboxing. Architecture-level virtualization is one possible approach and has pros / cons, as do other approaches like a higher-level virtual machine, etc.
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The isolation between sites in a browser or contacts in a messaging app are good examples of existing fine-grained trust boundaries to reinforce. There are a lot of other examples and reinforcing those can improve security for a billion users with no more work on their part.
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IDK if that's literally true. The move to https was itself a form of site isolation & industry was tinkering with hiding URLs altogether when they did about-face out of necessity. We might not have Chrome internal isolation today if not for trend reinforcing site identity in UI.
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The domain is the site identity. Marking EV identity is on the way out because it isn't actually truly meaningful / helpful for users. They don't know where companies are registered and those names aren't unique while domain names are unique.
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