We present two constructions of short signature schemes based on the polynomial hardness decisional Diffie Hellman. Our simplest scheme guarantees selective security (i.e. the adversary has to commit to the forged message ahead of time) while the second one realizes full fledged existential unforgeability. Remarkably, our schemes can be implemented over standard prime order groups (no pairings needed) and can be proven secure without resorting to the random oracle heuristic.