Question

Show the first eight words of the AES key expansion for a 128-bit key of all ONES

Answer 1

W₀ = ff ff ff ff

W₁ = ff ff ff ff

W₂ = ff ff ff ff

W₃ = ff ff ff ff

calculating W₄

i = 4, temp = W_i-1 = W₃ = ffffffff

RotWord(temp) = ffffffff (rotation)

SubWord(RotWord(temp)) = 16161616 (S-box )

Rcon[4/4] = 01000000 (round constant table)

XORing SubWord withRcon[4/4] :

temp =16161616 XOR 01000000

= 17161616

XORing W_4-4 with temp we get W₄ ,

W₄ = W₀ XOR temp

= ffffffff XOR 17161616

= e8 e9 e9 e9

Next three keys are calculated like above by XORing with 0xffffffff.

=> W₅ = 17 16 16 16

=> W₆ = e8 e9 e9 e9

=> W₇ = 17 16 16 16

Calculating W₈

i = 8, temp = W_i-1 = W₇ = 17161616

RotWord(temp) = 16161617

SubWord(RotWord(temp)) = 474747f0 (S-box table)

Rcon[8/4] = 02000000 (round constant table)

XORing SubWord withRcon[8/4] :

temp = 474747f0 XOR 02000000

= 454747f0

XORing W_8-4 with temp we get W₈

W₈ = W₄ XOR temp

= e8e9e9e9 XOR 454747f0

= ad ae ae 19

Next three keys are calculated like above by XORing with appropriate previous values

=> W₉ = ba b8 b8 0f

=> W₁₀ = 52 51 51 e6

=> W₁₁ = 45 4747 f0