source 1998 Cohen--Miyaji--Ono "Efficient elliptic curve exponentiation using mixed coordinates", formula (5) compute U1 = X1 Z2^2 compute U2 = X2 Z1^2 compute S1 = Y1 Z2^3 compute S2 = Y2 Z1^3 compute H = U2-U1 compute r = S2-S1 compute X3 = r^2-H^3-2 U1 H^2 compute Y3 = r (U1 H^2-X3)-S1 H^3 compute Z3 = Z1 Z2 H