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Kevin's Log

created at 2021-03-10

AES

has symmetric key, symmetric block cypher

  1. Key generation
  2. Round

Key generation *** if we want to ‘HWANGBO!HWANGBO!’ 16 words into cypher text

H -> 48(Hexadecimal) -> 8 bit

‘HWANGBO!HWANGBO!’ -> 8 bit * 16 = 128 bit

  1. Change words into hexadecimal

Result

| 48 | 57 | 41 | 4e | 47 | 42 | 4f | 21 | 48 | 57 | 41 | 4e | 47 | 42 | 4f | 21 | ——————– ——————- ——————- ——————- 1st column 2nd column 3rd column 4th column

  1. hexadecimal to key state

Key state

‘HWAN’ : 1st column of key state

‘GBO!’ : 2nd column of key state

‘HWAN’ : 3rd column of key state

‘GBO!’ : 4th column of key state

Result

key state

48 47 48 47
57 72 57 72
41 4f 41 4f
4e 21 4e 21
  1. rotate last column and alter value with S-Box S-Box 스크린샷 2022-06-04 오후 4 44 42
47=>72=>40
72=>4f=>84
4f=>21=>fd
21=>47=>a0

Result

sub byte column

40
84
fd
a0
  1. [1st column key state] XOR [last sub byte column] XOR [1st column RCON] = [1st column KEY]

RCON 스크린샷 2022-06-04 오후 4 59 04

48 40 01

| 57 | | 84 | | 00 | XOR XOR | 41 | | fd | | 00 |

4e a0 00

Result

79
d3
bc
ee
  1. [1st column KEY] XOR [2nd column key state] = [2nd column KEY]

Thus, result 4 * 4 metrix => KEY 1

KEY 1->key state and loop back to 3 until 10 times

  1. FINALLY!

We now have CYPHER KEY 1~10

Rounds ***

  1. Initial Round

[Pain Text(128 bit)] XOR [CYPHER KEY 1] = initial STATE

  1. Main Round

8.1 initial STATE -> Sub-byte

8.2 Sub-byte -> Shift rows

8.3 Shift rows -> Mix column

8.4 Mix column -> XOR with CYPHER KEY 1

8.5 Loop to 8.1 until 10 times

  1. Last Round

After loop main round 10 times XOR caculation with CYPHER KEY, we now get CYPHER TEXT!!!

Overall procedure

Pain Text(128 bit) [ENCRYPTION] CYPHER KEY 1~10(128/192/256 bit) => Cypher Text(128 bit)