What is the block size used by AES?

• A. 64 bits
• B. 128 bits
• C. 256 bits
• D. 512 bits

Answer: (B)

AES processes data in fixed-size blocks, which sets the unit of data that is encrypted at a time. For AES, that block size is 128 bits, so each encryption handles 128 bits of plaintext to produce 128 bits of ciphertext. The key length can be 128, 192, or 256 bits, but the block size stays the same at 128 bits. This consistent block size also affects how modes of operation behave, such as how many blocks you can safely encrypt with one key or how a counter in CTR mode advances. The other options don’t fit AES: 64 bits is associated with DES, while 256 or 512 bits would imply different block sizes not used by AES.