# Substitution Ciphers - PowerPoint PPT Presentation

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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

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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

### 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)

• 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.