Vigenere Cipher

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Vigenere Cipher
Topic Started: Oct 5 2005, 02:20 PM (251 Views)
Colly	Oct 5 2005, 02:20 PM Post #1
Unregistered	I was just wondering if anone can tell me how I could break a Vigenere Cipher without using a program?
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Revelation	Oct 5 2005, 02:30 PM Post #2
Administrator Posts: 386 Group: Admin Member #1 Joined: August 23, 2005	Well, you can use the Kasaki method and then find the shift number by looking at the frequencies. In a plain English text, the e will pop out. btw, you can also register and make a profile, so people can find your posts.
	RRRREJMEEEEEPVKLWENFNVJKEEEEEAOLKAFKLXCFZAASDJXZTTTTTTTLSIOWJXMOKLAFJNNKFNXN RAGRBAQEMHIGDJVDSEOXVIYCELFHWLELJFIENXLRATALSJFSLCYTKLASJDKMHGOVOKAJDNMNUITN RRRRLJVEEEEECLYVYHNVPFTAEEEEEMWLMEIRNGLARWJAKJDFLWNTIERJMIPQWOTZEOCXKNUBNXCN RJIRPOWEANFUSNCZVDVZNMSFEKLOEPZLDKDJWSAAAAAAAOERHJCTNCKFRIMVKSOFOMKMANREWNBN RZUDRGXEEEEENFQIDVLQNCKNEEEEEDGLLLLLLAWIOSNCDARLODMTOEJXMILDFJROTKJSDNLVCZNN


colly	Oct 5 2005, 02:49 PM Post #3
Just registered Posts: 14 Group: New Members Member #17 Joined: October 5, 2005	Thanks, is there any way that I can know for definite that this is a vigenere cipher. Heres a tiny sample of the ciphertext that im looking at: P<&:%EP&*T]%P]% Do vigenere ciphers generally use characters outside of the 26 letter alphabet?



Revelation	Oct 5 2005, 02:54 PM Post #4
Administrator Posts: 386 Group: Admin Member #1 Joined: August 23, 2005	As far as I know, Vigenère is almost never used outside the 26 characters of the alphabet. Though, it is possible. Only the shift would be 255.
	RRRREJMEEEEEPVKLWENFNVJKEEEEEAOLKAFKLXCFZAASDJXZTTTTTTTLSIOWJXMOKLAFJNNKFNXN RAGRBAQEMHIGDJVDSEOXVIYCELFHWLELJFIENXLRATALSJFSLCYTKLASJDKMHGOVOKAJDNMNUITN RRRRLJVEEEEECLYVYHNVPFTAEEEEEMWLMEIRNGLARWJAKJDFLWNTIERJMIPQWOTZEOCXKNUBNXCN RJIRPOWEANFUSNCZVDVZNMSFEKLOEPZLDKDJWSAAAAAAAOERHJCTNCKFRIMVKSOFOMKMANREWNBN RZUDRGXEEEEENFQIDVLQNCKNEEEEEDGLLLLLLAWIOSNCDARLODMTOEJXMILDFJROTKJSDNLVCZNN


colly	Oct 5 2005, 03:01 PM Post #5
Just registered Posts: 14 Group: New Members Member #17 Joined: October 5, 2005	Revelation Oct 5 2005, 02:54 PM As far as I know, Vigenère is almost never used outside the 26 characters of the alphabet. Though, it is possible. Only the shift would be 255. I assume that if Vigenère used a shift of 255 then the key word could not be a word in the English dictionary, so it probably isn't Vigenère. I thought it may have been Affine but when I tried frequency analysis on it it really didn't look like a mono-alphabetic ciphertext. Can you think off a basic encryption method which uses more than 26 characters and is not mono-alphabetic



insecure	Oct 5 2005, 03:23 PM Post #6
Elite member Posts: 381 Group: Trusted Member Member #10 Joined: September 9, 2005	Don't get hung up on the magic word "Vigenere". It could be a polyalphabetic cipher which uses a different alphabet (e.g. one including punctuation, or even one that includes all 256 8-bit combinations of an octet). So look for the Kasiski distance or the IOC, and see what it tells you. If you get a big jump of matching characters (or character pairs) at certain regular intervals, you'd be within your rights to conclude that it is a polyalphabetic cipher, in which case the same techniques which work on Vigenere will work on it too (i.e. stripe it and frequency-analyse the columns). But beware spending too much time on this, unless it is really important to you to recover the plaintext - if someone has just given you what looks like random junk and said "betcha ya can't crack this!", just nod and smile and hand it back, together with a few random numbers. Tell them "you're right, and you can't crack this either, so there". Modern ciphers are resistant to attacks that are based on knowledge of the algorithm used to do the encryption. If you are simply solving a puzzle for fun, ask for a hint. If you have been challenged by someone who thinks their algorithm uncrackable, point out that the security should rest in the key, not the secrecy of the algorithm. And if you have intercepted this ciphertext from an Al-Qaeda radio station, perhaps you should just hand it to the NSA and let them worry about it!



Donald	Oct 5 2005, 04:14 PM Post #7
Elite member Posts: 454 Group: Trusted Member Member #8 Joined: September 2, 2005	Quote: I assume that if Vigenère used a shift of 255 then the key word could not be a word in the English dictionary It could still use a word in the english dictionary. There is no reason the key has to cover the entire range of posibilites. For example, in regular vigenere, you could use a key of ABC if you wanted to. Not a good idea, but you could do it. Donald



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