source 2008 Hisil--Wong--Carter--Dawson, http://eprint.iacr.org/2008/522, Section 3.2 compute A = X1 X2 compute B = Y1 Y2 compute C = Z1 T2 compute D = T1 Z2 compute E = D + C compute F = (X1-Y1)(X2+Y2)+B-A compute G = B + a A compute H = D - C compute X3 = E F compute Y3 = G H compute T3 = E H compute Z3 = F G