Which block cipher mode is INSECURE and should NEVER be used because it leaks PATTERNS in the plaintext?

1. ECB MODE: each block of plaintext is independently encrypted with the same key. 2. THE WEAKNESS: identical plaintext blocks produce identical ciphertext blocks. 3. FAMOUS DEMO: ECB-encrypting an image of the Linux Tux penguin still shows the penguin outline in the encrypted image. Patterns survive. 4. SECURE modes use an IV/nonce to randomise: • CBC: chains blocks (each block XOR'd with previous ciphertext) — still needs MAC for integrity. • GCM: authenticated encryption (AES-GCM), provides both confidentiality + integrity. Modern default. • CTR: turns block cipher into a stream cipher with a counter. 5. ECB is the canonical 'don't ever use this' example in any crypto course. _Source: Boneh & Shoup, §4.6 (Cipher modes; ECB warning)._