source 2001 Liardet--Smart "Preventing SPA/DPA in ECC systems using the Jacobi form"
compute Z1C2 = Z1 C2
compute S1D2 = S1 D2
compute D1S2 = D1 S2
compute C1Z2 = C1 Z2
compute U = Z1C2 C1Z2
compute V = D1S2 S1D2
compute S3 = (Z1C2+D1S2)(C1Z2+S1D2)-U-V
compute C3 = U-V
compute D3 = Z1 D1 Z2 D2-a S1 C1 S2 C2
compute Z3 = Z1C2^2+D1S2^2