source 2008 Hisil--Wong--Carter--Dawson, http://eprint.iacr.org/2008/522, Section 3.1 compute A = X1 X2 compute B = Y1 Y2 compute C = T1 d T2 compute D = Z1 Z2 compute E = (X1+Y1)(X2+Y2)-A-B compute F = D - C compute G = D + C compute H = B - a A compute X3 = E F compute Y3 = G H compute T3 = E H compute Z3 = F G