source 2010 Hisil (thesis)
compute A = X1 Z2
compute B = Z1 Z2
compute C = Y1 X2
compute D = Y1 Y2
compute E = Z1 Y2
compute F = a X1 X2
compute G = (D+B)(A-C)
compute H = (D-B)(A+C)
compute J = (D+F)(A-E)
compute K = (D-F)(A+E)
compute X3 = G - H
compute Y3 = K - J
compute Z3 = J + K - G - H - 2(B-F)(C+E)