Cryptography Begining

Symmetric Key Cryptography

  • use the same key to encrypt and decrypt

Confusion and Diffusion of Information

  • 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

What makes a cipher “secure” ?

  • 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)

SK-Cryptography

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