English
Menu

Applied Cryptography

Which of the following is the correct combination of block length and key length for AES, the block cipher widely used today?

  1. AA block length of 128 bits and a key length of 128, 192 or 256 bits
  2. BA block length of 64 bits and a key length of 56 bits
  3. CBoth the block length and the key length can be chosen in any combination from 128, 192 and 256 bits
Show answer and explanation

Answer: A. A block length of 128 bits and a key length of 128, 192 or 256 bits

AES is a block cipher standardized by the US NIST in 2001 as FIPS 197; its block length is fixed at 128 bits and its key length is one of three: 128, 192 or 256 bits. Depending on the key length, the number of rounds (repetitions of the transformation) is 10, 12 or 14. Rijndael, on which it is based, was designed to allow different block lengths too, but only the 128-bit block was standardized as AES. A 64-bit block and a 56-bit key are the values of DES, the standard before AES.

  • A. Correct. The numbers in AES-128, AES-192 and AES-256 are the key lengths.
  • B. These are the values of DES, the standard before AES. A 56-bit key became breakable by brute force, and DES fell out of use.
  • C. Selectable block lengths were part of the original Rijndael design. The block length of AES is 128 bits only.
Practice this topic Back to topic

Books on Applied Cryptography Ad

As an Amazon Associate I earn from qualifying purchases.

More Applied Cryptography questions

  1. Image data were encrypted with AES in ECB mode, and the outline of the original picture could still be recognized in the encrypted image. Which of the following is the cause?
  2. Which of the following is an appropriate way to handle the initialization vector (IV) when using AES in CBC mode?
  3. AES (block length 16 bytes) is used in CBC mode with PKCS#7 padding. In PKCS#7, if n bytes are missing, n bytes of value n are added, and even when the plaintext length is exactly a multiple of 16, 16 bytes of value 16 are added. Which of the following is the length of the ciphertext (not including the IV) when a 96-byte plaintext is encrypted?
  4. A server receiving data encrypted in CBC mode returned different errors when the padding after decryption was incorrect and when the padding was correct but the check of the contents failed. Which of the following correctly describes this situation?
  5. Which of the following correctly describes a feature of CTR mode (counter mode) of a block cipher?

Comparisons in this topic