A string over a finite set ∑ is a finite sequence of elements from ∑. Show that the following procedure defines a cryptosystem.
Let w be a string over {A,B, . . . ,Z}. Choose two shift cipher keys k1 and k2. Encrypt the elements of w in odd-numbered positions with k1 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.