What are extra bits added to fill a block when plaintext is too short?

• A. ECB Mode
• B. Padding
• C. Confusion
• D. Diffusion

Answer: (B)

Padding is the process of adding extra bits so that the final block reaches the required fixed size for a block cipher. Block ciphers operate on data in fixed-length blocks, so if the last block isn’t full, padding fills it to make the total length a multiple of the block size. A common scheme, PKCS#7 (PKCS#5 for 8-byte blocks), uses padding bytes that indicate how many padding bytes were added, so the decryptor can remove them correctly. For example, with a 16-byte block and 10 bytes of actual data, six padding bytes are added, each with the value 0x06. Without padding, the final partial block can’t be encrypted properly.

ECB mode is a mode of operation for applying a block cipher, not a method for filling blocks. Confusion and diffusion are properties of encryption, not padding techniques.