Advanced Encryption Standard

Question 1

What are the key features of AES?

Answer 1

• Uses a symmetric block cipher
• Operates on 128 bit blocks
• Uses 128, 192 or 256 bit keys
• Uses 10, 12 or 14 rounds depending on key size

Question 2

How does AES arrange its blocks?

Answer 2

It stores it within a state matrix, where each value in the matrix is 1 byte i.e. 8 bits

Question 3

What are the 4 stages of AES?

Answer 3

SubBytes - Applies a non-linear S-Box to each byte in the matrix
ShiftRows - Cyclically shifts rows of the state to the left
MixColumns - Each column is treated as a polynomial over GF(2^8)
AddRoundKey - XORs each byte in the state with the corresponding byte of the round key

Question 4

What does the SubBytes stage do in AES?

Answer 4

• Applies a non-linear S-Box to each byte in the matrix.
• The S-Box is based on multiplicative inverses in GF(2^8), where the inverse undergoes an affine transformation to produce the final S-Box
• Aims to introduce confusion to break linear relationships

Question 5

What does the ShiftRows stage do in AES?

Answer 5

Cyclically shifts rows of the state to the left, which ensures diffusion across rows.
Each row shifts an additional byte along, corresponding to the row number e.g. Row 0 = no byte shift, Row 1 = 1 byte shift, Row 2 = 2 byte shift, Row 3 = 3 byte shift

Question 6

What does the MixColumns stage do in AES?

Answer 6

• Each column is treated as a polynomial over GF(2^8)
• It is then multiplied by a fixed matrix (modulo an irreducible polynomial)

Question 7

What does the stage AddRoundKey do in AES?

Answer 7

• XORs each byte in the state with the corresponding byte of the round key
• Integrates key material at every round

Question 8

What are some of the key properties of the S-Box in AES?

Answer 8

• Has no fixed points
• Has no inverse fixed points
• Minimisation of the largest non-trivial correlation between linear combinations of input bits and linear combination of output bits
• Minimisation of the largest non-trivial value in the EXOR table

Question 9

How does the Key Schedule work in AES?

Answer 9

It starts with the original key
Divides the key into 4-byte words
It then generates new words/keys using previous ones
Depending on the size of the AES, this could generate enough keys for 11, 12 or 14 rounds, corresponding to the three types of AES.