SS week4 Cryptography Begining

• use the same key to encrypt and decrypt

• Confusion : key 改變一個bit，cipher改變的程度

• Diffusion : plaintext 改變1個bit，cipher 改變的程度

• There are no shortcut attacks. Exhaustive key search required.

• It(the cryptography system) must not be required to be secret, and it must be able to fall into the hands of the enemy without inconvenience.
  (Kerckhoffs Principle)

1. Caeser Cipher

shift each char by n

2. Permutation Cipher

把原文的char一組做順序上的掉換

破解 : Frequency attack

3. Codebook Cipher

建表查表XDD

4. Double Transposition Cipher

5. One-time Pad Cipher

Encrypt : P ⊕ K = C

Decrypt : C ⊕ K = P

Note : when reusing the same key gives attackers additional information

∵ C1 ⊕ C2 = P1 ⊕ K ⊕ P2 ⊕ K = P1 ⊕ P2

當攻擊者得知你任一個plaintext，就能解出所有plaintext