shafemtol on Nostr: Apart from the shared secret issue, one thing not addressed here is sender ...
Apart from the shared secret issue, one thing not addressed here is sender authentication.
Assuming the encryption key is `S' = w * Q2`, since the sender signs with the ephemeral key `w`, unless the recipient can somehow verify that `w` belongs to the intended counterparty, the recipient has no way of authenticating the sender.
Anyone who correctly guesses one of the participants of the silent inbox (e.g. a relay operator would be able to trivially figure this out in many cases) can send a DM to that participant with an ephemeral private key of their own, pretending to be the other participant.
Assuming the encryption key is `S' = w * Q2`, since the sender signs with the ephemeral key `w`, unless the recipient can somehow verify that `w` belongs to the intended counterparty, the recipient has no way of authenticating the sender.
Anyone who correctly guesses one of the participants of the silent inbox (e.g. a relay operator would be able to trivially figure this out in many cases) can send a DM to that participant with an ephemeral private key of their own, pretending to be the other participant.