Substitution Ciphers

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# Substitution Ciphers - PowerPoint PPT Presentation

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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Presentation Transcript
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.
Substitution Ciphers (p.5)