개요

Symmetric Cryptography에 대해 알아본다.

plain text cipher text shared key encryption cipher decryption cipher

What is strong encrythion algorithm?

Diffusion : plain text 를 조금 마꾸면 cipher text는 마구 바뀌어야한다. Confusion : key 와 cipher text와의 관계가 추정하기 어려워야한다.

How make strong encrytion algorithm

• Substitution cipher (대치 암호)

  one or more characters replace to ciphertext by fixed mapping system monoalphabetic polyalphabetic

  modulo operation

  Caesar cipher

  frequncy analysis

• Permutation/transportation cipher (전치 암호)

  순서를 바꿈

  columnar cipher

• exclusive-or cipher

  X-OR addition modulo 2

DES and AES

DES

64 bit -> 64 bit key 56 bit + 8 parity

AES

128 bit -> 128 ->

key 128, 192, 256 rount 10, 12 ,14

16bytes -> 4 x 4 matrix

SubBytes ShiftRows MixColumns AddRoundKey