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 EnLinear that encrypts $256-$bit blocks of plaintext into

$256-$bit blocks of ciphertext. Let EnLinear $(k,m)$ denote the encryption of a $256-$bit message $m$

under a key $k($the actual bit length of $k$ is irrelevant$).$ Thus,

EnLinear$(k,[m1o+m2])=$EnLinear$(k,m1)o+$EnLinear$(k,m2)$ for all $128-$bit patterns

$m1,m2.$

Describe how, with $256$ chosen ciphertexts, an adversary can decrypt any ciphertext without

knowledge of the secret key $k.($A "chosen ciphertext" means that an adversary has the ability to

choose a ciphertext and then obtain its decryption. Here, you have $256$ plaintext$/$ciphertext pairs

to work with, and you have the ability to choose the value of the ciphertexts.$)(4$ points$)$