Which mode is vulnerable because it does not chain blocks and thus can reveal patterns in plaintext across blocks?

• A. ECB Mode
• B. CFB Mode
• C. AES
• D. CBC Mode

Answer: (A)

Pattern leakage occurs when a block cipher mode encrypts blocks without linking them together. In ECB mode, each block of plaintext is encrypted independently with the same key, so identical plaintext blocks produce identical ciphertext blocks. Because there’s no chaining between blocks, repeating patterns in the plaintext across blocks become visible in the ciphertext. This makes it easy to see structure or even infer the content in many cases (for example, recognizable blocks in image data). Other modes incorporate chaining, so the encryption of one block depends on previous data, which spreads information and hides those patterns. That’s why this mode is considered vulnerable. The other option is a cipher algorithm, not a mode, so it doesn’t describe how blocks are processed.