Symmetric Key Cryptography
- use the same key to encrypt and decrypt
Confusion and Diffusion of Information
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Confusion : key 改變一個bit,cipher改變的程度
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Diffusion : plaintext 改變1個bit,cipher 改變的程度
What makes a cipher “secure” ?
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There are no shortcut attacks. Exhaustive key search required.
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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
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