source 2002 Izu--Takagi "A fast parallel elliptic curve multiplication resistant against side channel attacks", page 296, Formula 2, plus assumption Z1 = 1 assume Z1 = 1 compute T1 = X2 X3 compute T2 = Z2 Z3 compute T3 = X2 Z3 compute T4 = X3 Z2 compute T5 = T3 + T4 compute T6 = a T2 compute T7 = T1 + T6 compute T8 = T5 T7 compute T9 = 2 T8 compute T10 = T2^2 compute T11 = b T10 compute T12 = 4 T11 compute T13 = T9 + T12 compute T14 = T3 - T4 compute T15 = T14^2 compute T16 = T13 compute T17 = X1 T15 compute X5 = T16 - T17 compute Z5 = T15