source 1998 Cohen--Miyaji--Ono "Efficient elliptic curve exponentiation using mixed coordinates", formula (6) compute S = 4 X1 Y1^2 compute M = 3 X1^2+a Z1^4 compute T = M^2-2 S compute X3 = T compute Y3 = M (S-T)-8 Y1^4 compute Z3 = 2 Y1 Z1