source 2003 Stam "On Montgomery-like representations for elliptic curves over GF(2^k)", Section 3.2, plus a1 = 1, plus b8 = a6
compute X5 = Z1(X2^2 X3^2 + a6 Z2^2 Z3^2)
compute Z5 = X1((X2+Z2)(X3+Z3)-X2 X3-Z2 Z3)^2