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Substitution Ciphers

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

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  1. Substitution Ciphers • Reference • Matt Bishop, Computer Security, Addison Wesley, 2003.

  2. Substitution Ciphers (p.2) • Substitution Cipher • Changes characters in the plaintext to produce the ciphertext. • Examples • Caesar Cipher • Vigenere Cipher • One Time Pad

  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.

  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.

  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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