Answer Happy

a. For any block cipher, the fact that it is a nonlinear function is crucial to its security. To see this, suppose that we have a linear block cipher EL that encrypts 128-bit blocks of plaintext into 128-bit blocks of ciphertext. Let EL(k, m) denote the encryption of a 128-bit message m under a key k (the actual bit length of k is irrelevant). Thus, EL(k, [m1 XOR m2]) = EL(k, m1) XOR EL(k, m2) for all 128-bit patterns m1, m2. Describe (mathematically) how, with 128 chosen ciphertexts, an adversary can decrypt any ciphertext without knowledge of the secret key k. Prove that your method works for any 128-bit ciphertext. Note that a "chosen ciphertext" means that an adversary has the ability to choose a ciphertext and then obtain its decryption. Here, you have 128 plaintext/ciphertext pairs to work with and you have the ability to choose the value of the ciphertexts.