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Kasiski Method. Reference Matt Bishop, Computer Security , Addison Wesley, 2003. Kasiski Method. The Cesar Cipher is a monoalphabetic-key cipher and is susceptable to statistical analysis of ciphertext. Consider plaintext using the English language.

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kasiski method
Kasiski Method
  • Reference
    • Matt Bishop, Computer Security, Addison Wesley, 2003.
kasiski method2
Kasiski Method
  • The Cesar Cipher is a monoalphabetic-key cipher and is susceptable to statistical analysis of ciphertext.
    • Consider plaintext using the English language.
    • The frequency of occurrence of the letters a through z is well documented.
    • When the Cesar Cipher is used, the plaintext becomes ciphertext, but the ciphertext still retains the frequency of occurrence of the plaintext and the code can be broken.
kasiski method p 2
Kasiski Method (p.2)
  • In the Vigenère Cipher, the longer key might obscure the statistics.
  • Kasiski, however, noticed that repetitions occur when characters of the key appear over the same characters in the ciphertext.
  • The number of characters between repetitions must be a multiple of the length of the key.
kasiski method p 3
Kasiski Method (p.3)
  • The index of coincidence (IC)measures the difference in the frequencies of letters in the ciphertext.
  • A frequency analysis of the ciphertext could provide estimates of Fi, where Fi is the relative frequency of the ith ciphertext characters.
  • Then IC = {1/N(N-1)}i=0,25 Fi(Fi-1).
kasiski method p 4
Kasiski Method (p.4)
  • Now the index of coincidence (IC) will decrease as the length of the key (or period) increases.
  • This means by estimating the IC, we have an estimate of the period.
  • This can be used to check the observed repetitions.
kasiski method p 5
Kasiski Method (p.5)
  • How does knowledge of the key length help break the code?
    • The ciphertext can be separated into groups, with each group being ciphertext from using the jth element of the key.
    • If the key length is L, then the L groups are analyzed as simple Cesar ciphers.
    • Thus the Vigenère cipher, like the Cesar cipher can be broken using ciphertext only.