Easy decision-Diffie-Hellman groups

The decision-Diffie-Hellman problem (DDH) is a central computational problem in cryptography. It is already known that the Weil and Tate pairings can be used to solve many DDH problems on elliptic curves. A natural question is whether all DDH problems are easy on supersingular curves. To answer this question it is necessary to have suitable distortion maps. Verheul states that such maps exist, and this paper gives an algorithm to construct them. The paper therefore shows that all DDH problems on the supersingular elliptic curves used in practice are easy. We also discuss the issue of which DDH problems on ordinary curves are easy.