Limitations on Transformations from Composite-Order to Prime-Order Groups: The Case of Round-Optimal Blind Signatures

Beginning with the work of Groth and Sahai, there has been much interest in transforming pairing-based schemes in composite-order groups to equivalent ones in prime-order groups. A method for achieving such transformations has recently been proposed by Freeman, who identified two properties of pairings using composite-order groups — “cancelling” and “projecting” — on which many schemes rely, and showed how either of these properties can be obtained using prime-order groups. In this paper, we give evidence for the existence of limits to such transformations. Specifically, we show that a pairing generated in a natural way from the Decision Linear assumption in prime-order groups can be simultaneously cancelling and projecting only with negligible probability. As evidence that these properties can be helpful together as well as individually, we present a cryptosystem whose proof of security makes use of a pairing that is both cancelling and projecting. Our example cryptosystem is a simple round-optimal blind signature scheme that is secure in the common reference string model, without random oracles, and based on mild assumptions; it is of independent interest.