source 1998 Cohen--Miyaji--Ono "Efficient elliptic curve exponentiation using mixed coordinates", formula (4) compute w = a Z1^2+3 X1^2 compute s = Y1 Z1 compute B = X1 Y1 s compute h = w^2-8 B compute X3 = 2 h s compute Y3 = w(4 B-h)-8 Y1^2 s^2 compute Z3 = 8 s^3