Earlier quoted context omitted.
It's just really clear that people in this thread are trying to approach this from first principles without any engagement in the field that they're discussing. That's a fun thing to do as, like, a game or a way to pass the time, and I guess that's what HN is, but it's still crazymaking, because essentially every paper written about messaging cryptography refutes this comment. Cryptographers would like to move people…
Ok, my GPG example was wrong. And yes, you got me, I'm not a professional cryptographer. But can you address the point? You said "once counterparties have authenticated each other's messages, the legitimate need for authentication is gone". I provided a counter-example to demonstrate that your statement was an exaggeration. You clearly dispute some part of this, but it's unclear to me what the disputed part is. Edit:…
I think the person you're talking to thinks this is very obvious and thus isn't stating it explicitly, but in the special case where you want an email to include non-repudiation, such as for a purchase receipt, the sender should just add non-repudiation to it in the form of a signature that's intended for that. Simple.