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#1
December 15th, 2011, 08:42 PM
 dalibor
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Crypto Algorithm Question - Vigenere cipher - running key

Hi, I'm writing some project, and I need to find how to decrypt vigenere cipher with running key, so can anyone describe, or give me some link?

#2
December 25th, 2011, 11:58 AM
 christopherx
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They're pretty much the same algorithm, but in the running key cipher you don't repeat the key, you continue to generate the keystream from the pre-agreed text, but if you're going to decrypt with a running key, why not just encrypt with a running key as well ? It's undoubtedly more secure..

#3
December 27th, 2011, 11:25 AM
 dalibor
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I made a mistake, when I said decryption, I thought to cryptanalysis

#4
December 29th, 2011, 04:44 PM
 christopherx
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Riiiiight. Different story all together

Well firstly, it's a vigenere function, just the key generation is a little bit different so get the basic details down first.

1: A running key uses a set text, at random points in the book.
2: It uses a Vigenere-like function to encrypt the text.

So what you first want to do is try and find out the text used as the running key. Secondly, when using a running key encryption somewhere in the ciphertext is where abouts in this set text there is, otherwise the recipient is screwed for decrypting it. So I would begin by finding out the source of the running key, then testing out some numerical values that you find in the text. If there aren't any, and with it being a vigenere cipher I dont think there will be, you've got to try language analysis.

Think about commonly used letters and words. Vowels are in every word, and most sentences have connectives. Another crucial point is the language the source text is in.

There's a few ideas you can try!

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