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

Last Modified: 2012-06-21
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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