We want to build a block cipher & = (E, D) from two block ciphers &1 = (E, D) and E2 = (E2, D2) so that if at some future time one of E or 2 is broken (but not both) then & is still secure. Suppose both & and E2 are defined over (K, X). Define & as: E ((k, k2), x):= E1 (k, E2 (k2, 2)) and Show that is secure if either E or E2 is secure. D ((k1, k2), y) = D2 (k2, D1 (k1, y))