source 2002 Billet--Joye "The Jacobi model of an elliptic curve and side-channel analysis", formula (11) compute X3 = X1 Z1 Y2+Y1 X2 Z2 compute Y3 = ((Z1 Z2)^2+(X1 X2)^2) (Y1 Y2+2 a X1 X2 Z1 Z2)+2 X1 X2 Z1 Z2 (X1^2 Z2^2+Z1^2 X2^2) compute Z3 = (Z1 Z2)^2-(X1 X2)^2