原本要把數論篇分為上、中和下，三篇。但好像大家都不喜歡看數論 ＱＡＱ

古典密碼學

加法加密法/位移加密法/凱薩加密法

• 加密

  • 將明文向右位移n位(n為金鑰)
  • 函式:C_i = (P_i + n) mod 26

• 解密

  • 將明文向左位移n位(n為金鑰)
  • 函式:P_i = (C_i - n) mod 26

• 舉例

  • Key = 3
    C = vhfxulwb

  • C_1 = v = 21 -> (21 - 3) mod 26 = 18 = s
    C_2 = h = 7 -> (7 - 3) mod 26 = 4 = e
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    .
    .
    C_8 = b = 1 -> (1 - 3) mod 26 = -2 mod 26 = 24 = y

  • P = security

Vigenere 加密法

• P = P_1P_2...P_n
  K = [(k_1k_2..k_m), (k_1k_2..k_m), ...] (n不一定等於m)

• 加密

  • C_i = (P_i + k_i)mod26

• 解密

  • P_i = (C_i - k_i)mod26

• 範例

  • P = hackersir, K = try
    
  • P_1 = h = 7, K_1 = t = 19
    • C_1 = (17+9) mod 26 = 0 = a
  • P_2 = a = 0, K_2 = r = 17
    • C_2 = (0+17) mod 26 = 17 = r
      .
      .
      .
  • P_9 = r = 17, K_3 = y = 24
    • C_9 = (17+24) mod 26 = 15 = p

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今天線這樣吧！今天終於辦完活動了，過程意外的順利。好累！先來睡個覺再說吧！