This is the question. Any keys can be chosen.

5. A string over a finite set E is a finite sequence of elements from E. Show that the following procedure defines a cryptosystem. Let w be a string over {A, B, ...,Z}. Choose two shift cipher keys { ki and k2. Encrypt the elements of w in odd-numbered positions with ki and those in even-numbered positions with k2. Then reverse the order of the encrypted string. Determine the plaintext space, the ciphertext space, and the key space