This seems like a straightforward application of zero knowledge techniques, but an answer eludes me. Alice and Bob claim to have devised an encryption scheme: specifically, they claim to possess functions $E()$ and $D()$ to encrypt and decrypt arbitrary $n$-bit messages. Charlie wishes to verify this claim, but Alice and Bob don't want Charlie to learn anything about the method. Can Charlie verify that Alice and Bob can indeed encrypt and decrypt in a zero knowledge fashion ? 
Zero knowledge verification of an encryption protocol
cr.crypto security;zero knowledge