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How can I do a brute force (ciphertext only) attack on an CBC-encrypted message?

My question is about a cipher text only attack - I have as input only the initialization vector and one cipher text.

I know the key domain: letters "a-h", and the padding is zero's (don't know if that helps).

Problem is that after XORing (as part of the CBC schema) most of the remaining letters are not part of the domain of "a-h", so I get almost readable text with all the keys I try in the brute-force attack.

How can I know for sure the key I got is the correct one (meaning how do I know when to stop the bruteforce attack - meaning how to know the plain text I got is fully readable)?

What I have tried so far:

I was trying to use a dictionary to check if the words I got in the plain text are words in English and as I said most of the text is readable because of the small domain and the CBC schema which XOR before the encryption function: CipherTextBlock1=E(IV xor PlainTextBlock1, key) => CipherTextBlock2=E(C1 xor PlainTextBlock2, key) and so on...

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  • This sounds a lot like http://crypto.stackexchange.com/questions/2394/how-can-i-do-a-brute-force-ciphertext-only-attack-on-an-cbc-encrypted-message – Ilmari Karonen Apr 24 '12 at 13:37
  • Is it guaranteed that the plaintext length is exactly a multiple of the blocksize? – mikeazo Apr 24 '12 at 17:32
  • Welcome to Cryptography Stack Exchange. I closed your question as a duplicate to the question linked above ... since it is quite similar. (Maybe it was asked by one of your class-mates?) Please have a look if the question there (and the answers) help you. – Paŭlo Ebermann Apr 24 '12 at 20:03
  • No it's not the same question (I already reviewed the above linked before I asked my question), and the methods explained there are not relevant to my question (if you had took the time and read both articles). *. I don't have class-mates :P – gAsk Apr 24 '12 at 22:58
  • @gAsk in that case, can I make a suggestion? Give your question a quick edit - thoroughly demonstrate the relevant bits of your work so far. Walk us through it, with relevant examples. If you make an edit and then reply to us here, we can always re-open the question. Closure is not final. If you're struggling with what to add, feel free to grab us in chat. –  Apr 25 '12 at 08:18

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