Classical and information theory Flashcards
Vigenere’s cipher had been considered unbreakable for centuries, but in the 19th century Kasiski managed to break it.
a) Describe Vigenere’s cipher and its encipherment and decipherment transformations.
b) Describe the Kasiski’s method of cryptanalysis of Vigenere’s cipher.
c) How can we make Vigenere’s cipher resistant to Kasiski’s attack?
Kasiski’ s method of cryptanalysis reduces the cryptanalysis of a polyalphabetic cipher to several problems of cryptanalysis of monoalphabetic ciphers. These monoalphabetic ciphers are shift ciphers.
a) To break a shift cipher, it is enough to find the most frequent letter in the cryptogram and that will be the substitution for the most frequent letter of the original alphabet Why don’t we need to do the same for the rest of the alphabet?
b) Caesar’s cipher is a shift cipher. What is the value of the shift in the Caesar’s cipher?
How can we break the Vigenere’s cipher?
Explain the concepts of confusion and diffusion?
Explain the requirements for an entropy function. Give the definition of Shannon’s entropy function and explain how it satisfies these requirements.
Why isn’t the Vigenere’s cipher perfectly secure?
