source 2004 Hankerson--Menezes--Vanstone, page 91 assume Z2=1 compute T1 = Z1^2 compute T2 = T1 Z1 compute T1 = T1 X2 compute T2 = T2 Y2 compute T1 = T1-X1 compute T2 = T2-Y1 compute Z3 = Z1 T1 compute T3 = T1^2 compute T4 = T3 T1 compute T3 = T3 X1 compute T1 = 2 T3 compute X3 = T2^2 compute X3 = X3-T1 compute X3 = X3-T4 compute T3 = T3-X3 compute T3 = T3 T2 compute T4 = T4 Y1 compute Y3 = T3-T4