Consider a Feistel cipher composed of 16 rounds with blocklength 128 bits and key length 128 bits. Suppose that, for a givenk, the key scheduling algorithm determines values forthe first 8 round keys, k1,k2, ..., k8, andthen sets
k9 = k8,k10 = k7,k11 = k6, ...,k16 = k1
Suppose you have a ciphertext c. Explain how, withaccess to an encryption oracle, you can decrypt c anddetermine m using just a single oracle query. Thisshows that such a cipher is vulnerable to a chosen plaintextattack. (An encryption oracle can be thought of as a device that,when given a plaintext, returns the corresponding ciphertext. Theinternal details of the device are not known to you and you cannotbreak open the device. You can only gain information from theoracle by making queries to it and observing its responses.)