Cryptography Lec 2(Historical Ciphers) - Notes

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Cryptography Lec 2(Historical Ciphers) - Notes

tags: Cryptography NTU

:::spoiler 資訊安全筆記2 The Vigenere Cipher

The Vigenere Cipher是一種”polyalphabetic cipher”用一串的 Key 不斷地重複！例如 abcabcabcabc 計算方法： 例如key = d 代表a會變成d所以 w 會變成 Z

課本上的例子： :::

:::spoiler Playfair Cipher

The Playfair Cipher Encryption Algorithm: The Algorithm consists of 2 steps:

  1. Generate the key Square(5×5):
  2. Algorithm to encrypt the plain text: The plaintext is split into pairs of two letters (digraphs). If there is an odd number of letters, a Z is added to the last letter.

    For example:

    1
    2
    		 
     PlainText: "instruments"
 After Split: 'in' 'st' 'ru' 'me' 'nt' 'sz'

:::info Note that: Pair cannot be made with same letter. Break the letter in single and add a bogus letter to the previous letter. :::

  1. Plain Text: “hello” After Split: ‘he’ ‘lx’ ‘lo’ Here ‘x’ is the bogus letter.

  2. If the letter is standing alone in the process of pairing, then add an extra bogus letter with the alone letter Plain Text: “helloe” After Split: ‘he’ ‘lx’ ‘lo’ ‘ez’ Here ‘z’ is the bogus letter.

Exception

:::

Introduce the encryption flow of Enigma WWII納粹德國密碼機Enigma