Cryptography and Network Security-ch7

Ch7 Advanced Encryption Standard

●3 version

1. AES 128(10 round, cipher key:128 bit)

2. AES 192(12 round, cipher key:192 bit)

3. AES 256(14 round, cipher key:256 bit)

●steps

1. Pre-round
    和key做XOR

2. Subbyte(Substation)
    

3. Shiftrow(Permutation)

  每個row做left shift 

 Row1: x

 Row2: 1 byte

 Row3: 2 byte

 Row4: 3 byte

4. Mix column 

 每個column獨立做(column乘constant matrix)

 最後一個column不做

5. Add round key

 每個column獨立做, plaintext 和 roundkey 做 XOR

6. 2, 3, 4, 5為一個round 依照不同version 做不同round

AES總流程圖

AES每個round的流程圖

● key expansion

1. AES 128：每個round需要4個word(w_i, w_i+1, w_i+2, w_i+3)
  (1). pre-round：w₀, w₁, w₂, w₃ 為cipher key
  (2). 當i mod 4 = 0：
    w_i = t + w_i-4
    t = Sub(Rot(w_i-1)) + Rcon[i/4]
  (3). 當i mod 4 ≠ 0：
    w_i = w_i-1 + w_i-4

AES key產生圖

2. AES 192：每次產生6個word(w_i, w_i+1, w_i+2, w_i+3, w_i+4, w_i+5)
  和AES 128同，但4均改為6
  (w_i = t + w_i-6)
  ( w_i = w_i-1 + w_i-6)
3. AES 256：每次產生8個word(w_i, w_i+1, w_i+2, w_i+3, w_i+4, w_i+5, w_i+6, w_i+7)
  (1). pre-round：w₀, w₁, w₂, w₃ 為cipher key
  (2). 當i mod 8 = 0：
    w_i = t + w_i-8
    t = Sub(Rot(w_i-1)) + Rcon
  (3). 當i mod 4 = 0, 且i mod 8 ≠ 0：
    w_i = SubWord(w_i-1) + w_i-8
  (4). 當i mod 8 ≠ 0：
    w_i = w_i-1 + w_i-8