Which block cipher mode encrypts each block independently (insecure)?

• A. CFB Mode
• B. ECB Mode
• C. Block Cipher
• D. CBC Mode

Answer: (B)

In this mode, each block is encrypted without any dependence on previous blocks, so identical plaintext blocks produce identical ciphertext blocks. That determinism leaks structure and patterns in the data, making it easy to recognize repeated information and perform traffic or frequency analysis. This lack of semantic security is why it’s considered insecure for most uses. The mode that embodies this independent-block approach by applying the block cipher to every block with the same key is Electronic Codebook. By contrast, other modes introduce chaining or feedback between blocks, which mixes the ciphertext and hides patterns: CBC uses the previous ciphertext block, and CFB uses prior output as input to the next step. The option Block Cipher isn’t a mode at all; it’s the underlying primitive. So the independent-block approach described is characteristic of Electronic Codebook, which is why it’s generally discouraged for secure encryption.