source 2002 Izu--Takagi "A fast parallel elliptic curve multiplication resistant against side channel attacks", formula (9), plus assumption Z1 = 1 assume Z1 = 1 compute R = 2(X2 Z3 + X3 Z2)(X2 X3 + a Z2 Z3) + 4 b Z2^2 Z3^2 compute S = (X2 Z3 - X3 Z2)^2 compute X5 = R - S X1 compute Z5 = S