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#1
September 20th, 2012, 10:06 PM
 memo1983
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Crypto Algorithm Question - Vigenere cipher

Hello,

What key which if used to encrypt the ciphertext again would give back the plaintext (key is a weak key)? Define a formula for identifying weak keys for the cipher below (assume |K1| = |K2| = |K3|)

#2
October 3rd, 2012, 12:13 PM
 Decave
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It would be any key where every plaintext letter is shifted by a degree of half the alphabet, namely k = 'nnn...nnnn' where the period t = n. If you have a plaintext m ='agp', key k = 'nnn' and ciphertext c = 'ntc' then applying Enc_k(c) = 'agp' = c' = m because we shift all of the letters by the same degree forward to get the plaintext.

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