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Comment by seemslegit | Hacker News Reader

A. You can design it in such a way so that sending a message to a non-user is indistinguishable from having a user see it and not reply/acknowledge it.

B. You can exchange identifiers with people whom you want to communicate it just like with any other non-phone-based system: "Hey I'm @username on signal", "Cool, I'm @username2" - composes well with method A.

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Solution A does not compose well with how Signal does encryption. In order to make this indistinguishable, Signal would basically have to man-in-the-middle all non-existent users. And if one of those users signed up for Signal it would have to stop man-in-the-middling them, causing all the people who were talking with their ghost to observe a key change. It's complicated at best, and sketchy at worst.

Solution B ties in to a bigger argument I won't address

There would be no key change because there would be no initial key, signal facilitates contacts anyway the only difference is that the sides have no ability to control with whom it takes place. Messages to non-contacts will not be sent because there would be noone in your contacts to send them to, hence indistinguishable.

I don't follow. I'm sending (or trying to send) messages to my contacts. If I know their phone number, I'm going to try to initiate a Signal conversation with them. So I ask the Signal server for a signed prekey. Your argument is that Signal should not respond with "I don't know this person" and should instead respond with something indistinguishable from a "real" response. So they must send me something that looks like a signed prekey, right? Well then I would use that in order to do a key exchange and now we're in the situation I described above.

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.

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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