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 wil...