Which mode leaks plaintext patterns?

• A. CTR
• B. CBC
• C. ECB
• D. GCM

Answer: (C)

The tendency to leak plaintext patterns comes from how the mode handles blocks. In a mode that encrypts each block independently with the same key and without chaining, identical plaintext blocks produce identical ciphertext blocks. That means you can see repeating patterns and structure in the ciphertext, which reveals information about the plaintext.

Other modes avoid this. CBC chains each block with the previous ciphertext, so the same plaintext block will yield different ciphertext blocks. CTR uses a keystream based on a counter and a nonce, producing ciphertext that depends on that unique keystream instead of repeating blocks. GCM also provides authentication and uses a similar keystream approach with integrity protection. Because of these chaining or randomized elements, plaintext patterns aren’t directly visible.

So the mode that leaks plaintext patterns is the one that encrypts blocks independently without chaining: ECB.