Cryptography Foundations Solution Exercise 2 2 . 1 Block Ciphers in ECB and CBC Mode

a) If a block cipher is used in ECB mode, the encryption of two equal n-bit blocks (aligned at n-bit boundaries) of the plaintext will yield the same n-bit blocks in the ciphertext. This can be hazardous for security in applications: If the plaintext encodes a bitmap with a schematic image (think of Mickey Mouse ears), then most of the n-bit blocks will either be 0n or 1n, so the ciphertext will mostly consist of the corresponding blocks F (0n, k) and F (1n, k). In fact, if the resulting ciphertext is drawn as a bitmap, then the schematic structure will still be visible.1