Is it possible to have an encryption and decryption scheme so that decryption is a 2-party process?
For instance, Alice sends an encrypted message to Bob along with the key for that message. Bob, by himself, cannot feasibly decrypt this message. However, with some interactive help from Alice, he can decrypt it. If Bob manages to lose the plaintext after this process, the information gained during the decryption process cannot help him decrypt it again. He must ask Alice for help again.
I could see this being useful in access-control systems or DRM.