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I'm currently studying for a cryptography exam. I've been given ciphertext that has been encrypted by a columnar transposition cipher. I've been given no shift key length or key word, the only thing I know is that only 2 columns have been shifted.

How can I determine the key? Or better yet, is there another way to decipher the ciphertext?

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  • $\begingroup$ I am not looking for someone to decipher, simply to assist in how I would go about this. I find myself going around in circles. $\endgroup$ – EMJ Oct 6 '15 at 7:39
  • $\begingroup$ Are you allowed to just perform plain brute-force? BTW: The #1 technique for this would be sliding window. $\endgroup$ – SEJPM Oct 6 '15 at 12:38
  • $\begingroup$ " the only thing I know is that only 2 columns have been shifted." - So you don't have just a simple columnar transposition cipher but rather some combination of a columnar transposition and a shift cipher? $\endgroup$ – SEJPM Oct 6 '15 at 15:47
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(Although your question is for a long time ago) Transposition ciphers don't change the frequency of letters in their ciphertexts. So, you can implement a frequency attack. For decryption by this you only need the table of frequency of English letters and your ciphertext. (Your encryption scheme here is a Columnar transposition with a key that all its letters are in order except 2 of them.)

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