Stuck on cryptology problem(vigenere cipher)

[FYAD]

I was assigned to decrypt this cipher text:

EWPW DRAPO MGTGT EXVAH RPSWI LSFUN RZVSE FOIWJ
NYOU MHSER EFBMB VANHA LOOXI VPBIF FIFMA BFGRH
GARE TLKLF NZQXQ DWGKP UWNWQ QFRZL VROOP UGBQQ
HVKS CDOPW LSGTE JMFSE MLZMD TNDED VVGRO UUMLV
NLHA KWASO ITAPR DTBBG CHDSH TNSFM NGWMF CASWM
WGKD RWJRN UNDVV SFFAE TAGUF HLAUC AETLB MHVAN
WJHU QUQQL SQETD BWGBR APMJW PM

the kasiski test is of no real help; the frequencies are to close together.

highest frequencies of 2 and 3 letter substrings
(higher substrings have almost no discrepancies of frequency):


et: 0.01544402 (132,12,20)
aet: 0.007751938 = 12

kasiski yields 4 as gcd, which suggests 4 as the keylength.

Index of Coincidence is 0.04110484 ; this value is much closer to 0.038 than 0.065, which suggests that the encipherment scheme is polyalphabetic(confirmed by professor to be vigenere).

using the friedman test yields:
(.027*260)/((259*.04110484)-(.038*260+.065)) = 10.01

suggesting that the keylength is 10, which is definitely a better guess than 4(our professor supplied the hint that the keyword was of length 7,8,9 or 10).

going on the guess that the keylength is 10, i split the cipher text into 10 subsequences:

egpzybofrxwosedokdtcwflhqg
wtsvomxmeqqpcjtuwtnajfavqb
pgwsubiatdqudmnuabssraualr
wtiemvvblwfgofdmsbfwnecnsa
delfhapfkgrbpselogmmutawqp
rxsosnbglkzqwedvicnwnaejem
avfiehirfplqlmvnthggdgthtj
pauwrafhnuvhslvladwkvuludw
ohnjelfgzwrvgzghpsmdvfbqbp
mrrnfoiaqnoktmrarhfrshmuwm

with respective ic's of:
0.02461538
0.03384615
0.05538461
0.03692308
0.02769231
0.02461538
0.04
0.06153846
0.02769231
0.06461538

which isn't very much help, as only 3 of the 10 are closer to .065 than .038. supposing that the keylength guess of 10 was incorrect, i split the message into subsequences of 7,8 and 9, and calculated their ic's only to be equally disappointed in the results.

now, going by the results of the friedman test, i stick with a keylength of 10 and use frequency analysis on the results of the split, starting with column ten as its ic was closest to .065 than the others, and i figured it would be easiest.

this netted frequency results of:
r: 0.1923077
m: 0.1538462
a: 0.07692308
f: 0.07692308
h: 0.07692308
n: 0.07692308
o: 0.07692308
i: 0.03846154
k: 0.03846154
q: 0.03846154
s: 0.03846154
t: 0.03846154
u: 0.03846154
w: 0.03846154

suggesting that e mapped to r, and t mapped to m. however, this implies a shift of 13 and 19 respectively, which is inconsistent. i play around with the most commonly used letters(e,t,a,i,n), and i find a shift that looks possible:4.

n to r = 4
i to m = 4
e to a = 4
b to f = 4
d to h = 4
j to n = 4
k to o = 4
a to w = 4

so e mapps to a and presumably the last letter of the keyword is w. trying similar methods on the other subsequences doesn't yield any satisfactory results, and I'm stuck. I've tried asking other people in my class, but none have gotten any farther than i! my professor said this was a somewhat difficult problem and I'm getting fustrated. I'm thinking about writing a brute force program; try every possible combination of ten characters for the keyword - ignoring keywords with repeated letters - and using those keywords to decipher the message and searching the output for the most common digraphs and trigraphs. I'm not exactly sure how feasible this is (26^10 possible combinations?) but I'm going crazy here.

 

[Evalesco]

That does sound like a difficult problem. Maybe you can try some other methods like the hill-climbing method or the genetic algorithm. You could also try to break it up into smaller chunks and solve each chunk separately. If you are writing a brute force program, make sure to check out existing implementations and see if they can help you. Good luck!

 

[thepatientmental]

It seems like you have made a lot of progress in your attempts to decrypt the cipher text, but you have hit a roadblock. It can be frustrating when you have tried different methods and still can't find the solution. I would suggest taking a step back and looking at the problem from a different perspective. Have you tried looking for patterns in the cipher text? Are there any repeated words or phrases that could give you a clue about the keyword? Also, have you considered using a different approach such as frequency analysis on the whole text instead of just individual columns? It may also be helpful to consult with your professor again and see if they can provide any additional hints or tips. And, while a brute force program may be an option, it may be time-consuming and not guaranteed to give you the correct keyword. Keep trying different methods and don't give up. Sometimes the solution comes when you least expect it. Good luck!