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Sidechannels in Signal is news to me. Can you provide some keywords to search for, or any links?

The contact discovery services leaks the fact that you are using Signal. It is not optional. It's clear why that is, so I won't spell it out.

They can get download records from Google or Apple to check for Signal downloads.

So there’s really no reason for Signal to try and hide one is merely using Signal. Best to focus on securing content.

You are missing the attack vector here.

It is optional isn’t it? I never gave Signal permission to read my contacts and yet I still use it.