you don't. You assume that the key distribution mechanism is secure, so that whoever has the key _is_ who they say they are. This doesn't prevent adversarial i

you don't. You assume that the key distribution mechanism is secure, so that whoever has the key _is_ who they say they are. This doesn't prevent adversarial impersonation (where you cannot trust the party that want themselves impersonated). E.g., if you are an employer, and interviewing via video, you cannot tell that the person authenticated and performing on video is indeed the person you're hiring. I dont think this is a problem that _should_ be solved tbh.