compute xy mod m, you can first calculate xR and yR, multiply
them together to get xyR^2, and do a Montgomery reduction to get
xyR^2 R^{-1} mod m. Then the R^{-1} cancels one of the Rs and
compute xy mod m, you can first calculate xR and yR, multiply
them together to get xyR^2, and do a Montgomery reduction to get
xyR^2 R^{-1} mod m. Then the R^{-1} cancels one of the Rs and