That's what I meant with a common understanding of what the ciphertext is. I don't think this is possible if all you have is your own recording of the cipherte

That's what I meant with a common understanding of what the ciphertext is. I don't think this is possible if all you have is your own recording of the ciphertext, with no proof that it was actually transferred over the wire. This is the typical situation with packet capturing at one endpoint only.