Which term describes adding bits to plaintext to fit block size in block cipher operations?

• A. Diffusion
• B. Padding
• C. DES
• D. Confusion

Answer: (B)

Padding is the process of adding extra bits or bytes to the end of plaintext so its length becomes a multiple of the block size used by a block cipher. Block ciphers operate on fixed-size blocks (for example, DES uses 64-bit blocks, AES uses 128-bit blocks), so if the message isn’t already a multiple of that size, padding fills the remaining space in the final block. This ensures every block can be encrypted cleanly and, when decrypting, the padding can be removed unambiguously.

A common padding method is PKCS#7 (used with AES and other modern ciphers): if N bytes are needed to complete the last block, you append N bytes, each with the value N. If the plaintext exactly fits a block, a full block of padding is added so the trailing padding can be detected and removed correctly during decryption. Other padding schemes exist, but the core idea remains the same—fit the plaintext to the required block size and include enough information to strip the padding after decryption.

The other terms mentioned—diffusion and confusion—describe properties of how ciphers transform plaintext into ciphertext, not how to align data to block boundaries. DES is a cipher algorithm itself, not the padding process.