Establishing trust is always a complicated task but especially difficult in a multi-device environment where every device has its own identity. Concepts like Trust On First Use (TOFU) don’t work very well because a malicious server could sneak a ‘surveillance device’ into every device announcement. Furthermore Trust On First Use doesn’t have an answer to users switching devices or otherwise reinstalling the App which would generate a new key. If you consider that an App or an identity is being used for 10+ years you can assume that every user will reinstall the App or get a new device at least once. Subsequently TOFU only prolongs the problem of having to make a manual trust decision. Signal and WhatsApp have recognized this and are moving to a trust-everything approach and merely warn users when fingerprints change. Train somebody long enough to ignore those warnings and you’ve essentially made End-to-End-Encryption completely useless. Imagine if we only used the Web with self signed certificates; Even cautious users would quickly adopt a behaviour of simply ignoring the warning. Of course you could drive to the bank and compare the certificate’s fingerprint in person, but in reality nobody is going to do that, especially if they keep changing every couple of months.
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Establishing trust is always a complicated task but especially difficult in a multi-device environment where every device has its own identity. Concepts like Trust On First Use (TOFU) don’t work very well because a malicious server could sneak a ‘surveillance device’ into every device announcement. Furthermore Trust On First Use doesn’t have an answer to users switching devices or otherwise…
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