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Substitution Ciphers. Reference Matt Bishop, Computer Security , Addison Wesley, 2003. Substitution Ciphers (p.2). Substitution Cipher Changes characters in the plaintext to produce the ciphertext. Examples Caesar Cipher Vigenere Cipher One Time Pad. Substitution Ciphers (p.3).

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Substitution ciphers
Substitution Ciphers

  • Reference

    • Matt Bishop, Computer Security, Addison Wesley, 2003.


Substitution ciphers p 2
Substitution Ciphers (p.2)

  • Substitution Cipher

    • Changes characters in the plaintext to produce the ciphertext.

  • Examples

    • Caesar Cipher

    • Vigenere Cipher

    • One Time Pad


Substitution ciphers p 3
Substitution Ciphers (p.3)

  • Caesar Cipher

    • Consider the plaintext to be the letters A,B,C,...,Z.

    • Now shift the sequence, say, by 3 to get D,E,F,...Z,A,B,C.

    • Then the cipher text becomes D for A, E for B, and so on.

    • If each letter is represented by integers 0,1,...,25, we can describe this process as C=(M + K) mod 26, where the key is K=3.


Substitution ciphers p 4
Substitution Ciphers (p.4)

  • Vigenère Cipher

    • The Vigenère cipher chooses a sequence of keys, represented by a string.

    • Key letters are applied to successive plaintext.

    • When the end of the key sequence is reached, the key starts over again.

    • The length of the key is called the period of the cipher.


Substitution ciphers p 5
Substitution Ciphers (p.5)

  • One-Time Pad

    • A variant of the Vigenère cipher.

    • The key is chosen at random.

    • The length of the key is at least as long as that of the message, and so it does not repeat.


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