If $E_k$ is a CCA secure encryption scheme then why is $E′_{(k_1,k_2)}(M)=E_{k_2}(E_{k_1}(M))$, $D′_{(k_1,k_2)}(C)=D_{k_1}(D_{k_2}(C))$ not CCA secure when $k_2$ is known. Is there a way I can make a new cipher with keys $(k_1, k_2)$ such that its CCA secure regardless of which key the adversary finds?
My confusion here is how can I use the fact that key $k_2$ is known to break CCA security and why would that matter, because don't we treat these schemes as black-box when performing attack? What if I had $k_1$ instead, would that make any difference? And I'm quite clueless about the second part about new cipher using $k_1, k_2$ and still a secure CCA. Any idea or hint would be greatly appreciated.