AES is a round cipher based on the Rijndael Algorithm that uses a 128-bit…
2012
AES is a round cipher based on the Rijndael Algorithm that uses a 128-bit block of data. AES has three different configurations: ______ rounds with a key size of 128 bits, ______ rounds with a key size of 192 bits and ______ rounds with a key size of 256 bits.
Answer: B. 10, 12, 14 — Concept. AES (the Rijndael cipher as standardised in FIPS 197) always encrypts a fixed 128-bit block, but its number of rounds is not fixed: it is determined…
- A.
5, 7, 15
- B.
10, 12, 14
- C.
5, 6, 7
- D.
20, 12, 14
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Correct answer: B
Concept. AES (the Rijndael cipher as standardised in FIPS 197) always encrypts a fixed 128-bit block, but its number of rounds is not fixed: it is determined by the key length. Writing Nk for the key length measured in 32-bit words and Nr for the number of rounds, the standard fixes the relation Nr = Nk + 6. Every additional 64 bits of key therefore buys two more rounds.
Application. Convert each of the three key sizes into Nk and apply Nr = Nk + 6:
128-bit key: Nk = 128 / 32 = 4 words, so Nr = 4 + 6 = 10 rounds.
192-bit key: Nk = 192 / 32 = 6 words, so Nr = 6 + 6 = 12 rounds.
256-bit key: Nk = 256 / 32 = 8 words, so Nr = 8 + 6 = 14 rounds.
The three blanks therefore read 10, 12 and 14 rounds respectively.
Cross-check. The key schedule gives an independent confirmation. AES expands the cipher key into 4 x (Nr + 1) words of 32 bits each - one 128-bit round key for the initial AddRoundKey step plus one for every round:
Key size | Nk (words) | Nr = Nk + 6 | Expanded key = 4(Nr + 1) words |
|---|---|---|---|
128 bits | 4 | 10 | 44 |
192 bits | 6 | 12 | 52 |
256 bits | 8 | 14 | 60 |
The 44-, 52- and 60-word expansions are exactly the key-schedule sizes specified in FIPS 197, so the round counts are consistent with the standard. One further structural point worth remembering: in every configuration the last round omits the MixColumns step, so an Nr-round AES performs Nr - 1 full rounds followed by one reduced final round.