That makes no sense to me. There must be some mechanism for accessing messages. Whatever that is, it's those credentials that you'll need to produce. Or rot in jail indeterminately.
Somehow, an attacker can have your whole conversation log (encrypted), including the key exchange, and be unable to retrieve the key used, even if he has the credentials you used at the time the key was generated.
The real crux of the question may be, "How does Diffie-Hellman work?" (Well known key exchange method.)