What is the best way to go about using Cryptanalysis to decrypt a message?

I have a message that I need to decrypt (for learning purposes). We are given the Cyphertext and need to derive the plaintext and key used. I am pretty sure the encryption method is simply substitution of some letter, say "A" for "I", "B" for "U" etc. I am having a tough time getting far with the decryption. I have done a frequency count on the cypher and found that there are 3 letters that are used substantially more often then the others (13 and 12 times to nominal of about 4). I know this means that the plaintext letter is most likely one of the commonly used English letters "e t a o". After fooling around with the text trying to decrypt it after replacing these Letters with the real ones - I am not getting very far (I know - im a crappy crypanalyist). I am wondering if there are any tricks to the trade that someone might be able to fill me in on, or maybe some tools that can guess words based on a few known letters and a few unknown letters. Any suggestions are welcome.
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Gssc1414Asked:
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letharionCommented:
Hmm, I have a book on the subject at home I try to have a look at later. But until I do, if it's a simple substituion, are the spaces left intact? If they are, try to locate the most common words, such as "I", "and",  "a", "are" and other words that tend to repeat over a text. If you can locate such words, you can make pretty good guesses about which a few letters.

Btw, why do you think it's a simple substitution? How many different letters are there?
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letharionCommented:
You could also just post the entire message. I won't just solve it for you (Both because I'm lazy and it wouldn't really be much learning on your part) but I might be able to give you more hints :)
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Infinity08Commented:
There are some nice tools to help you in this. Here's one for a frequency analysis :

        http://www.richkni.co.uk/php/crypta/freq.php

Note that that page also lists the frequencies of the letters in the English language. As well as the most common 2-letter and 3-letter combinations.

If the frequency analysis suggests a substitution cipher (ie. the frequencies obtained correspond roughly to those of the English language), the next step is starting to guess letters based on their frequencies.

The same site has a tool that helps with that :

        http://www.richkni.co.uk/php/crypta/letreplace.php

You guess some letters, look at the result, and see if you spot patterns (either impossible letter combinations, or words you spot, or ...). You continue guessing, and adapting your guesses to the result until you find the decrypted text.


Note that of course there are tools that can do this for you (and often do a pretty good job too), but the point of an exercise like this is to learn from doing it yourself. It's really interesting, and you'll have a nice feeling of satisfaction once you find the solution :)
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Gssc1414Author Commented:
Thanks for all the responses. I have been basically doing the methods that have been listed, but because this is my first time decoding a message, I haven't gotten very far. Currently I am pasting the encrypted message in word (all lower case), then based on frequency count I am replacing the lower case letters with upper case guesses of what letter should actually be there. I guess I am on the right track, just have to spend more time on it and see if i can't get it. Below is the encrypted text - please note I am NOT looking for answers, merely looking for a little guidance and maybe some suggestions as to a systematic approach to solving the problem.

Encypted text: Wrvl jrvspflyo dyl glzeq jdiilk psre fr slymryv vdeb elx mpejfzreo zejipkzeq fwl jreopvsfzre rm wrvlxryn mryvlyib ldfle gb fwl krq
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Infinity08Commented:
Well, I can confirm that it's a simple substitution cipher, so you are on the good way ;)

Instead of using Word, consider using the second link I posted earlier ... It will save you a lot of time replacing characters, so you can concentrate on actually cracking the code.

We can work through this together if you want. What are the first letters you'd substitute based on the frequency analysis ?
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d-glitchCommented:
You have four 3-letter words:   dyl   elx   fwl(2)   krq  

Three of them are candidates for THE   FOR   AND
krq at the end of the sentence has to be something else.
   
You have three 2-letter words:  fr   rm   gb
Try  IN  OF  TO

Try    fwl => THE     fr => TO   rm=> OF
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Infinity08Commented:
You're giving away an awful lot there, d-glitch. That would spoil the fun for me for sure, if it was me asking the question.
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d-glitchCommented:
I'm not trying to spoil any one's fun.
But the techniques mentioned earlier (frequencies of single letters and two letter
combinations) are not the place to start for newpaper puzzle type cryptograms.

Small word substitutions are much more effective.

I have no idea if any of my guesses are correct.
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awking00Commented:
Gssc1414,
It should be a fun course. At least your teacher is showing a sense of humor. d-glitch has given you a substantial start. Once you begin your substitutions, why don't you post where you are and where you're stuck and we can provide some hints about what you might do next, without telling what substitutions to make.
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Infinity08Commented:
>> But the techniques mentioned earlier (frequencies of single letters and two letter combinations) are not the place to start for newpaper puzzle type cryptograms.

Frequency analysis can greatly help in making informed decisions on which small words to choose :) And a frequency analysis is always the best starting point, if only to find out if you're dealing with a substitution cipher or not.
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Gssc1414Author Commented:
Hey everyone,

Well that makes me feel good, because I was doing the method that d-glitch has presented. I was focusing on the small words. I did come to the conclusion that fwl was probably "THE" and that would force fr to be "TO". I basically made these substitutions, but then got stuck as nothing I seemed to do would make any sense. I am going to try out the tool that Infinity08 suggested - I will get back with you guys with the results...

Thanks again for all the help and suggestions (it's nice to see that what you were doing is actually suggested by experts - so im at least on the right track)
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awking00Commented:
If you've substituted those letters, you should now see that the first word looks like HOvE. How many words can that be? Home, hole, hone, hope, and hose come to mind. What are the likely candidates to be the beginning word of a sentence? Why not experiment and see what other words begin to "shape up" or not?
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Infinity08Commented:
>> I basically made these substitutions, but then got stuck as nothing I seemed to do would make any sense.

Once you have substituted a few letters, you can start looking for patterns.
At the same time, keep the frequency table in the back of your head. And let it help you in choosing the right substitutions.
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aikimarkCommented:
if that doesn't produce satisfactory results, consider
fwl = AND
fr   = AT
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awking00Commented:
Any bets on this being a University of Phoenix course? How many have the solution?
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Infinity08Commented:
>> Any bets on this being a University of Phoenix course?

lol. Now that's a hint heh.
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Gssc1414Author Commented:
Well, I figured it out. I basically used all the methods listed above (most of which I was already doing) and finally got the right combination that enabled the decryption of the message.

Thanks for your help and suggestions.
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WaterStreetCommented:
Infinity08,

Nice links for the frequency analysis.  My kind of thing - thanks
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secretmachineCommented:
Hah, I recently had to decipher this. It was so frustrating until I made the connection between one particular repeating pattern. Geez... I think next time will go much more smoothly.
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