Thanks a lot for your comment! Although I think it's out of scope for this basic tutorial, I've had this same question before. I believe It's not something that can be solved cryptographically. You need some approach like handing out the public key physically by the person (much like the QR code approach you described in another comment), and/or at least using TOFU (Trust on First Use) which is the approach partially taken by Signal (they just display a warning): en.wikipedia.org/wiki/Trust_on_fir...
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Thanks a lot for your comment! Although I think it's out of scope for this basic tutorial, I've had this same question before. I believe It's not something that can be solved cryptographically. You need some approach like handing out the public key physically by the person (much like the QR code approach you described in another comment), and/or at least using TOFU (Trust on First Use) which is the approach partially taken by Signal (they just display a warning): en.wikipedia.org/wiki/Trust_on_fir...