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Signal server should respond with "I either don't know this person or they have not approved to be contacted by you". You should only get a prekey if they are in fact a signal user and have opted in to be discoverable by everyone or only by select people including you.

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Ah, ok. So in that case, people would probably be have to be uncontactable by default (otherwise we'd be back to the current universe). Now you'll have to explicitly opt in to being contacted by every new person you meet. This is fine for some, but it's a massive usability tradeoff. Think about all the non-tehcnical people that Signal is intended to be used by, and imagine trying to teach them about Safety Numbers and a necessarily-opt-in contacting scheme.

Nothing necessary about it - those people (also all people) would be presented with a screen upon installation asking for their preferred discoverability policy - do they want to be contactable by: everyone in their phone contacts, only specific people from their contacts, only people they approve ad-hoc.

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