source 2001 Bernstein http://cr.yp.to/nistp224.html opt-idea53.c ecadd compute ZZ1 = Z1^2 compute ZZZ1 = Z1 ZZ1 compute ZZ2 = Z2^2 compute ZZZ2 = Z2 ZZ2 compute A = X1 ZZ2 compute B = X2 ZZ1 -A compute c = Y1 ZZZ2 compute d = Y2 ZZZ1 -c compute e = B^2 compute f = B e compute g = A e compute h = Z1 Z2 compute f2g = 2 g+f compute X3 = d^2-f2g compute Z3 = B h compute gx = g-X3 compute Y3 = d gx-c f