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I've encrypted a couple of 10 letters words from https://www.bestwordlist.com/10letterwords.htm using a key of 10 characters. This is done using Vigenere cipher. Decrypt the cipher and give the key

Ciphertext: XRPEFGMHNL XDBEWUMHNL

How we can decipher when we have only 2n size ciphertext where n is the key size

Can you please explain how it can be done? Frequency analysis is not effective in this case as the ciphertext is really short...

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  • $\begingroup$ Possible duplicate of How can a Vigenère cipher be broken if the message is short? $\endgroup$ – kelalaka Nov 4 '18 at 10:35
  • $\begingroup$ I did come across that question, however, it did not help me solve the question $\endgroup$ – Asad Masroor Nov 4 '18 at 10:37
  • $\begingroup$ By the way, in this way you asked in the body, is off-topic here. You should as, how we can decipher when we have only $2n$ size ciphertext where $n$ is the key size. $\endgroup$ – kelalaka Nov 4 '18 at 10:47
  • $\begingroup$ I'll change it to make it clearer $\endgroup$ – Asad Masroor Nov 4 '18 at 10:50
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You used the same key for both words so we can start with the obvious: both words start with the same letter and end with the same four letters. We can process your initial word list, picking out sets of words that match this pattern. Then just brute force each possible pair within each set until we find one or more pairs. Work back from the first possible word of a pair to the key, and then forward to the encryption of the second possible word. Check whether or not it matches "XDBEWUMHNL".

If there is more than one matching pair, then we would need more information.

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