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Shorte Code
Topic Started: Feb 5 2006, 09:58 PM (1,296 Views)
Boulemans
Just registered
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Hi,

I need to show my math teacher how to crack following code:
XF GTLTD UIUJHFVRA DTC VVGY FKUE YPMRFUV H UYCD IBXCJ
anyone who knows how to begin? I think it isn't so difficult, but, since I am a n00b, I don't know were to start.

My teacher got an extra hint; here it is in white:

The code is in Vinegere, the language is (of course) English
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Revelation
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Sure, if it is a Vigenere, you should check this out. Finding the key length is probably the hardest part. You should try to find the IOC. Here is a tool. :)

Good luck :)
RRRREJMEEEEEPVKLWENFNVJKEEEEEAOLKAFKLXCFZAASDJXZTTTTTTTLSIOWJXMOKLAFJNNKFNXN
RAGRBAQEMHIGDJVDSEOXVIYCELFHWLELJFIENXLRATALSJFSLCYTKLASJDKMHGOVOKAJDNMNUITN
RRRRLJVEEEEECLYVYHNVPFTAEEEEEMWLMEIRNGLARWJAKJDFLWNTIERJMIPQWOTZEOCXKNUBNXCN
RJIRPOWEANFUSNCZVDVZNMSFEKLOEPZLDKDJWSAAAAAAAOERHJCTNCKFRIMVKSOFOMKMANREWNBN
RZUDRGXEEEEENFQIDVLQNCKNEEEEEDGLLLLLLAWIOSNCDARLODMTOEJXMILDFJROTKJSDNLVCZNN
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Donald
NSA worthy
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The problem is that this cipher is VERY short for a Vigenere. It's under the minimum ACA recommended length for even a simple substitution cipher. It's not very far above the point where you can no longer guarantee a unique solution.

The short length will make Kasiski or IOC very difficult to determine I fear. Of course, having the word divisions should help. I made a quick attempt at it and wasn't getting anywhere. I'll do some more looking at it later if I get time. Of course, one of the better cryptographers here may be able to show you how to rip it open in no time.
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rot13
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The word breaks are definitely the thing I needed to break it. I used trial and error, and a bit of guessing. The single-letter word was the first point of attack. I kept trying various key lengths until the other letters seemed at least normal. Then I took a guess for the letters near it in the key. After that it came apart. I'd say it took about 10 minutes.
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Donald
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Quote:
 
I'd say it took about 10 minutes.

EXCELLENT! That was where I started, The two letter word at the begining also looked promising. But I'm not as good at recognizing when "the other letters seem at least normal" yet. :)
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rot13
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Here's how I went about the first part (SPOILER):


I made a guess that H was A, since there are only 2 common 1-letter words. It was between A and I. In Vigenere A -> H when the key is H, I -> H when the key is Z. I figured that H is more likely to be in the keyword than Z, so I went with that. Then I tried various combinations:

Key: H
qy zmemw nbncayokt wmv oozr ydnx rifkyno a nrvw buqvc
XF GTLTD UIUJHFVRA DTC VVGY FKUE YPMRFUV H UYCD IBXCJ

Definitely no good

Key: H.
<span style='font-family:Courier'>
q. z.e.w .b.c.y.k. w.v .o.r .d.x .i.k.n. a .r.w .u.v
XF GTLTD UIUJHFVRA DTC VVGY FKUE YPMRFUV H UYCD IBXCJ


Key: .H doesn't touch the single-letter word - only 1 position in each key length will hit the single letter word

Key: .H.
.y ..e.. n..c..o.. w.. o..r ..n. .i..y.. a ..v. .u..c
XF GTLTD UIUJHFVRA DTC VVGY FKUE YPMRFUV H UYCD IBXCJ

This at least looks possible. The first word could be My or By. The 4-letter word with V could end with E. Trying any of those possibilities yields garbage words, though.

Key: ..H.
.. z...w ...c...k. ..v ...r ...x ...k... a ...w ...v
XF GTLTD UIUJHFVRA DTC VVGY FKUE YPMRFUV H UYCD IBXCJ

Nope.

Key: ....H
.. ..e.. ..n....k. ... o... .d.. ..f.... a .... b
XF GTLTD UIUJHFVRA DTC VVGY FKUE YPMRFUV H UYCD IBXCJ

This looks a little promising. The 4-letter word with D second is a good point of attack because there should be a vowel there. I tried A,E,I and O and got garbage.

Key: ....H.
.. ..e.. ...c..... w.. ...r .... .i..... a .... .u
XF GTLTD UIUJHFVRA DTC VVGY FKUE YPMRFUV H UYCD IBXCJ

This also looks promising. The 3-letter word beginning with W is a good next point. Once I got that word, it really fell apart quickly.
</span>

As you can see, the word breaks really make a difference here.
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Donald
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Thank you rot13, that was an excellent lesson!
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