# Vigenere cipher

The following is a Vigenere cipher:

AYXHK XRGZE RIRCL ONESU RCKFE KRFXS MNSMK MSCMS KVTNE NNIWN SHGWN KZEXP ELXHO WOCRD USRYX EVWOG ONUAL KHGKS FUREU XHKVC APTAV EYLOA PDYLA XTETS UXEBO PIZCT UWCXY TORIF IMUVE YXEGH IRCTU EPVVE IMAZI MUVER SVORG RCOAV OCR

Here it is again, with no letter groups:

AYXHKXRGZERIRCLONESURCKFEKRFXSMNSMKMSCMSKVTNENNIWNSHGWNKZEXPELXHOWOCRDUSRYXEVWOGONUALKHGKSFUREUXHKVCAPTAVEYLOAPDYLAXTETSUXEBOPIZCTUWCXYTORIFIMUVEYXEGHIRCTUEPVVEIMAZI MUVERSVORGRCOAVOCR

(a) Find the length of the keyword using Kasiski's method.

(b) Find the length of the keyword using the index of coincidence.

(c) Decipher the text. (Describe what you are doing as you do this. There will be partial credit even if you can't do it entirely, but it is important to explain clearly what your methods are. One problem, which occurs often, is that two predictions of the keyword length (by using Kasiski's method or the index of coincidence) may differ. In this case, use your judgement.)

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#### Solution Preview

I assume you know how the Viegnere cipher works, and if not you will find the information in the links I attached.

I also attached a program I downloaded, that helps you decipher the code.

Ok, First the message:

AS THE TRAVELER WHO HAS ONCE BEEN FROM HOME IS WISER THAN HE WHO HAS NEVER LEFT HIS OWN DOORSTEP SO AKNOWLEDGE OF ONE OTHER CULTURE SHOULD SHARPEN OUR ABILITY TO SCRUTINIZE MORE STEADILY TO APPRECIATE MORE LOVINGLY OUR OWN.

Now for the method:

Bigram count:

Method described:

http://www.trincoll.edu/depts/cpsc/cryptography/vigenere.html

Method applied (a tool)

http://klein.math.okstate.edu/~wrightd/crypt/tools/Kasiski.html

I looked for bigrams that appear at least 4 times. They were:

VE and RC

VE appear at locations:

26,65,80,99 and the distances respectively: 9,15,19 so the factors are 3,5,19

RC appears at locations: 13,21,152,176. distances: 8,131,24 so the factors are 2,3,131

As you can see the common factor is 3 which strongly suggest a key word length of 3. If you include more bigrams that occur less frequently, the factor 3 appears more than any other factor.

So the Kasinsky method points to 3 as the keyword's length.

b: index of coincidence.

Method described in:

http://raphael.math.uic.edu/~jeremy/crypt/coincidence.html

Program Applied (downloadable) and attached:

http://pajhome.org.uk/crypt/vigenere.html

The basic idea is to shift the message and look for matches.

for ...

#### Solution Summary

The expert examines vigenere cipher functions.