Assume that Alice has a file F which she is going to send, in encrypted form, to Bob. Alice possesses F and the public encryption key K of Bob in form of an X509-certificate. She generates the file E using the encryption algorithm CNT_3DES, BitsInKey = 192 (RSA PKCS#1 encryption method). She sends to Bob the file E through a certified mail system which will get a digital signature and a time stamp from an external authentication authority. Therefore it is possible to include in the body of the mail any information, including hash codes, that will be accepted as well known before the creating time of the mail.
After some time Alice has to proof to Mike that the file E send to Bob has the same content of the file F. But Mike does not have the private key of Bob for decrypting the file. And encrypting the file F now produces a different file than E since encryption adds a random padding on encryption stage. How can this proof be given from Alice to Mike? As explained, it is possible to include in the body of the mail any useful information that will be necessary for this proof.
I hope that I explained my problem in a sufficiently clear way. And sorry for my poor English.