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Copy file name to clipboardExpand all lines: trainingportal/static/lessons/cryptoBreaker/crypto_vignere.md
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The Vignere cipher is a variation of the Caesar cipher. Vignere uses longer keys, which are harder to guess.
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The Vigenère cipher is a variation of the Caesar cipher. Vigenère uses longer keys, which are harder to guess.
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#### Algorithm
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The key contains a sequence of characters which represent shifts. For example `A` would represent `0` shifts being the first letter of the alphabet. `B` would represent `1` shift.
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`ABCD` becomes `ACEG`
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##### Weakness
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The Vignere was considered unbreakable for almost 200 years until the discovery of a method called Kasiski examination.
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The Vigenère was considered unbreakable for almost 200 years until the discovery of a method called Kasiski examination.
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This method takes advantage of the fact that for a large block of text with a fixed length key, common words tend to repeat.
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`what is the name of the store`
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`wict ju tig nboe ph tig suqrf`
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In the case of the Caesar cipher we were able to determine the code for letter `e`, knowing that `e` must be the most common letter in the text. The Vignere cipher can address this problem if the key is sufficiently long.
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In the case of the Caesar cipher we were able to determine the code for letter `e`, knowing that `e` must be the most common letter in the text. The Vigenère cipher can address this problem if the key is sufficiently long.
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In the example we notice the word `tig` appears twice and assuming this word represents `the`, one of the most common English words, we can easily derive the key.
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