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For homework, I have a ciphertext which is 582 chars long. The OTP used was reused over and over. The OTP is 50 chars long.

How exactly can I now get the key and the cleartext?

closed as off-topic by ThoriumBR, Neil Smithline, Steffen Ullrich, Polynomial, Ohnana May 10 '16 at 17:33

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  • "Questions asking us to break the security of a specific system for you are off-topic unless they demonstrate an understanding of the concepts involved and clearly identify a specific problem." – Polynomial, Ohnana
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  • Are we to assume that each character of the key was XOR'ed with each character of the message? – schroeder May 9 '16 at 21:09
  • I also tried to XOR two 50 chars long strings from the ciphertext and then cribdrag that but i don't get to where i want. – Thomas May 9 '16 at 21:30
  • Knowing that it's a contrived situation designed to help you learn makes a huge difference in how we answer you. – schroeder May 9 '16 at 21:44
  • Is what you have posted all of the info you have? I feel like there is a piece missing. – schroeder May 9 '16 at 21:49
5

It's not a one time pad, it's a Vigenère cipher.

You'd break this by organising into 50 character chunks so the aligned characters have been XOR'd with the same key character. You then perform a frequency analysis on this result to get the most likely key value. Repeat for all 50 characters.

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