I have a piece of cipher text, and I need to write a Java program to decrypt it, but I'm unsure how to. I know that you need to do a frequency analysis, and then cross reference with a dictionary list, which is fine when I'm doing it on paper through trial and error, but how can I write a program to do the same thing?
(ps i have'nt included the cipher text cause it's about a page long, but i'll post it if you need me to)
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unfortunately that's the problem.
I don't know how it's been encoded other than it's been done via a substitution cipher. I need to write a Java program that will decrypt the substitution cipher text, but without knowing how it was encrypted.
Here's the cipher text:
UFHLFXHTHKHYFIRJRTFHTIXMFE
have a look at this link they provide some info about frequent letters and stuff
http://www.cs.utexas.edu/u
some things i observed are
char - no of occurrences - pcnt
F-1466-12
U-1109-9
H-923-8
S-856-7
T-816-7
R-789-6
K-730-6
M-622-5
W-601-5
D-393-3
I-359-3
G-351-3
J-326-2
B-288-2
Y-282-2
E-205-1
Q-205-1
X-187-1
O-169-1
L-147-1
P-102-0
A-51-0
V-15-0
C-15-0
Z-14-0
N-0-0
my gues is to replace f with a SPACE and similarly
Std frq - ETAONIRSHLDCUPFMWYBGVJKQXZ
Obs frq - FUHSTRKMWDIGJBYEQXOLPAVCZN
have a method like below and pass on the text
private static String replaceString(String input)
{
String ciphered = "FUHL";
String transformed = " IAM";
for (int i = 0; i < ciphered.length(); i++)
{
input = input.replace(ciphered.cha
}
return input;
}
thanks for the comment, and sorry for the late reply, i´m actually away on holiday, and have only just managed to find a computer with internet access!
This is almost what i need, however the frequency count alone will not decrypt the text fully, you actually need to do digram and trigram checks as some of the letters you listed are not correctly decrypted, and then a full dictionary check to find the spaces. I´ve managed to decrypt the text fully by hand, but am still having trouble writing methods to do these digram/trigram checks, as the method i used for decrypting relies a lot on trial and error.
Again, any help would be appreciated.
Cheers (and i´ll check this more regularly now i´ve found a computer lol)
basically, from the above encrypted text, i need to carry out a linguistic attack, using frequency analysis and to discover possible decryption via word lists, and also use word lists to add spaces into the text.
I've managed to decrypt it via a trial and error process manually, but i'm finding it difficult to write a program to do this, which is the goal. When i did it manually i wrote bits of code to count the number of instances that the 30 most common digrams and the 12 most common trigrams occured in the text, but even then i had to take the results with a 'pinch of salt'.
Do you reckon i'm going the wrong way about writing the program? Is there a better way of doing it?
I have to use frequency analysis and word lists, but possibly digrams and trigrams only work if decrypting by hand due to the small margins in occurences conflicting with each other.
Unfortunately i don't have any algorithms (that's really the bit i need to discover i think), but the basic steps i need to achieve are: an automated linguistic attack via frequency analysis and word lists, and in particular the use of word lists to to automate the detection of possible word decryption and the insertion of spaces into the text.
Thanks for sticking with me on this :-)
I doubt that there is any algorithm that will be 100%, especially without the word breaks.
My first try would be something like:
1. Perform letter frequency count and substitute in based on language frequency counts.
2. Start at the beginning of the text.
2a. Find each word in succession from the word list (I assume provided)
2b. If a word cannot be found "spell check" the text area to find a word from the list that is close. Make that substitution.
Go back to 2.
3. You may want to "declare" a substitution as Final and not let it be changed by future actions, if you do this it would affect your "spell check" function.
--
JimFive
I've actually managed to solve this one with the use of regular expressions and a mixture of lisp and java, I basically used NLP techniques mixed with the java code and a dictionary list for word matching, this gave me a true automatic system (near enough anyway) but thanks for all the comments and ideas.
I'd look at patterns like RTJWRTH = 1234125 and match them to words in your dictionary
trying different word assignments to different patters to find consistent letter assignments
If you don't know where the word breaks are, you can drag the patterns along the text
You can also look at bigram or trigram frequencies
i already said on my previous comment i've done this, don't know how to close the question, but i used digrams and trigrams to get letter frequencies, and then prolog bridged with my java code to use regular expressions to match the words with a word list.
Sorry the question's not closed, but I don't know how to close it, being as i solved it myself.
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by: contactkarthiPosted on 2007-11-15 at 07:49:38ID: 20289802
coupld you just explain with a simple cipher text on how you do it