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Vigenere Cipher uses a set of mono-alphabetic substitution rules to encrypt message. For example, with key (3 1 5), an encoder substitutes first letter in plaintext by letter+3, second letter by letter+1, third letter by letter+5. Please describe a known plaintext attacks against Vigenere Cipher

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The classical cipher like Vigenere cipher is more vulnerable to the known plaintext attack . In the known plaintext attack against Vigenere cipher the attacker knows the set of pairs of plaintext-ciphertext. The picked plaintexts are can reveal the structure of the key . The requirement of the attack is encryption algorithm , plaintext and the corresponding ciphertext .

For example if the XOR cipher is used , the key can be obtained by XORing the plaintext and ciphertext (plaintext XOR ciphertext) a vector of m keys are obtained by this , By using the key the original plaintext is reconstructed .

The known plaintext attack can be controlled by following steps :

a) By reducing the use of a word more frequently

b) By reducing the use of any special words

c) By minimizing the validity of the key and by less number of sets of plaintext-ciphertext

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