Leak-proof protocol: is such a thing possible?
It doesn't work without further restrictions.

For simplification, let's assume Alice encrypts a bunch of files but doesn't have to send all of them to Bob. She can still decide to re-order the files or leave out some.

Then she can encode the secret shared key between Alice and Bob in the least significant bits of the ciphertexts without changing the files. She is only rearranging them so that Eve can extract the key. But if Bob checks the integrity of each message, it holds.

If more files are transmitted than the keylength, this works. Otherwise, more than one bit has to be encoded in less messages. And in order to get enough decryptable ciphertexts more files might have to be encrypted.