source 2007 Hisil--Carter--Dawson assume Z1 = 1 compute A = X1^2 compute B = Y1^2 compute D = A+B compute G = (X1+Y1)^2-D compute X3 = (2 Y1-G)(X1+A+1) compute Y3 = (G-2 X1)(Y1+B+1) compute Z3 = (X1-Y1)(G+2 D)