Group to group commitments do not shrink

We investigate commitment schemes whose messages, keys, commitments, and decommitments are elements of bilinear groups, and whose openings are verified by pairing product equations. Such commitments facilitate efficient zero-knowledge proofs of knowledge of a correct opening. We show two lower bounds on such schemes: a commitment cannot be shorter than the message and verifying the opening in a symmetric bilinear group setting requires evaluating at least two independent pairing product equations. We also present optimal constructions that match the lower bounds in symmetric and asymmetric bilinear group settings.