source 2007 Meloni "New point addition formulae for ECC applications", page 192 assume Z1 = Z2 compute A = (X2-X1)^2 compute B = X1 A compute C = X2 A compute D = (Y2-Y1)^2 compute X3 = D-B-C compute Y3 = (Y2-Y1)(B-X3)-Y1(C-B) compute Z3 = Z1(X2-X1)