source 2007(?) Kim--Kim "A New Method for Speeding Up Arithmetic on Elliptic Curves over Binary Fields"
assume Z2 = 1
compute A = X1 + X2 Z1
compute B = Y1 + Y2 ZZ1
compute C = A Z1
compute D = C (B + C)
compute Z3 = C^2
compute ZZ3 = Z3^2
compute X3 = B^2+ C A^ 2 + D
compute Y3 = (X3 + X2 Z3) D + (X2 + Y2) ZZ3