Predicting value of E(k,0) for Block cipher with random and secret key Some mode of operation of block ciphers rely on the fact that E(k,0) is an unpredictable value when k is random and secret (with 0 denoting the all-zero binary string). Why is this a reasonable assumption?
A block cipher, according to its design purpose, is assumed to be a pseudorandom permutation. This means when the key $k$ is chosen at random, $E(k,\cdot)$ can be viewed as a random permutation. Then, $E(k,0)$ is indistinguishable from a random value of the block length, hence unpredictable.