Elementary Information Security, 3rd Edition

▶ 9.3 Block Cipher Modes

Block ciphers suffer from a built-in shortcoming. If we feed a block cipher the same plaintext and the same key, it always yields the same ciphertext. This is what block ciphers do. By itself, this isn’t a vulnerability. It becomes a vulnerability when we encrypt data that has block-sized patterns in it.

For example, look at FIGURE 9.4. On the left, we have a plaintext image, originally rendered in three colors. On the right, we have the ciphertext after it was encrypted with a block cipher. The block cipher transformed the areas of uniform color into distinctive corduroy patterns. Each color in the plaintext yielded its own pattern in the ciphertext. Although this doesn’t yield a quality copy of the plaintext image, ...