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古典密碼： Caesar Cipher

古典密碼： Caesar Cipher. 中興資科詹進科 (jkjan@cs.nchu.edu.tw) 中興資管陳育毅 (chenyuyi@nchu.edu.tw ). Caesar Cipher 課程模組大綱. 對稱式加密 ( Symmetric Encryption ) 凱撒加密法 ( Caesar Cipher) 凱撒加密法 ( Caesar Cipher ) - 以數字表示 凱撒加密法 ( Caesar Cipher ) – 加解密公式 凱撒加密的破解 – 暴力破解法. 對稱式加密 ( Symmetric Encryption ).

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古典密碼： Caesar Cipher

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  1. 古典密碼：Caesar Cipher 中興資科詹進科(jkjan@cs.nchu.edu.tw) 中興資管陳育毅(chenyuyi@nchu.edu.tw)

  2. Caesar Cipher課程模組大綱 • 對稱式加密(Symmetric Encryption) • 凱撒加密法(Caesar Cipher) • 凱撒加密法(Caesar Cipher) - 以數字表示 • 凱撒加密法(Caesar Cipher) – 加解密公式 • 凱撒加密的破解 – 暴力破解法

  3. 對稱式加密(Symmetric Encryption) 又稱為Conventional / Private-key / Single-key Encryption 1.共用一把key 2.安全管道(Secure Channel) key key K K 3.強固的加解密演算法、必須假設公開的 4.以數學式子表示 M M C 明文Plaintext 明文Plaintext E( ) D( ) 密文Ciphertext 解密演算法Decryption Algorithm 加密演算法Encryption Algorithm 2.安全管道 1.共用一把key 3.強固加密演算法、必須假設是公開的 4.以數學公式表示之 C=EK(M), M=DK(C)

  4. a a a a z z b b z z b b y y c c y y c c x x d d x x d d e e e e f f f f w p w p m m t t 明文：meet me after the toga party 密文：phhw ph diwhu wkh wrjd sduwb 凱撒加密法(Caesar Cipher) m m e e e e … … t t p h h … w h h 凱撒大帝 前方軍士

  5. 00 00 00 00 25 25 01 01 25 25 01 01 24 24 02 02 24 24 02 02 23 23 03 03 23 23 03 03 04 04 04 04 05 05 05 05 15 22 22 15 19 12 12 19 明文：meet me after the toga party 密文：phhw ph diwhu wkh wrjd sduwb 凱撒加密法(Caesar Cipher) 若把英文字母改成以 00~25 數字表示 12 12 04 04 04 04 … … 19 19 15 07 07 … 22 07 07 凱撒大帝 前方軍士

  6. 12 12 04 04 04 04 … … 19 19 00 00 00 00 25 25 01 01 25 25 01 01 24 24 02 02 24 24 02 02 15 07 07 … 22 23 23 03 03 23 23 03 03 04 04 04 04 05 05 05 05 明文：meet me after the toga party 密文：phhw ph diwhu wkh wrjd sduwb 凱撒加密法(Caesar Cipher) 加密(Encryption)的公式C=(M+K) mod 26 解密(Decryption)的公式M=(C-K) mod 26 h

  7. 凱撒加密的破解 – 暴力破解法 M M C 明文Plaintext 明文Plaintext 密文Ciphertext 解密演算法Decryption Algorithm 加密演算法Encryption Algorithm

  8. oggu og chvgt vjg vqic rctva - 1 a a a a a a b b b b b b z z z z z z c c c y y y c c c y y y d d d x x x d d d x x x nffu nf bgufs uif uphb qbsuz - 2 e e e e e e f f f f f f meet me after the toga party - 3 Bingo! 凱撒加密的破解 – 暴力破解法 phhw ph diwhu wkh wrjd sduwb

  9. 對凱撒加密做暴力破解法(Brute-force Cryptanalysis) • 25種的可能，逐一測試 • 測試出來是明文，一看便知 凱撒加密的破解 – 暴力破解法

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