That only prevents attackers who read the current symmetric key from reading past messages. That won't protect you against the disclosure of a private key invo

That only prevents attackers who read the current symmetric key from reading past messages. That won't protect you against the disclosure of a private key involved in the prior key exchange. https://doc.libsodium.org/key_exchange https://doc.libsodium.org/key_exchange The only way to achieve true forward secrecy is to throw away the private half of the involved key pairs. You need ephemeral keys. Which Libsodium does not provide out of the box. You'd need something like Noise. https://noiseprotocol.org/ https://noiseprotocol.org/