Key exchange would be great though as the ability to securely share secret keys could then allow switching to a symmetric key encryption algorithm (these are us

Key exchange would be great though as the ability to securely share secret keys could then allow switching to a symmetric key encryption algorithm (these are usually simpler and more efficient). I believe RSA is mostly only used for key exchange anyway (due to bad performance) in popular protocols such as TLS. I'm not sure about the second part regarding the MitM attack; doesn't BB84's requirement of "the existence of an authenticated public classical channel." mitigate this? What kind of authentication is required there? Would signing a message using a secret key (e.g. Lamport OTS or some other quantum-resistant/hash-based signature algorithm) suffice to establish an 'authenticated channel'? The main problem I can see based on my limited understanding is that if an MitM attacker managed to intercept every key exchange attempt over a channel, they might not be able to intercept private messages, but they could prevent the encrypted channel from being formed.