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How to recover a key after reuse of an otp key

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yaniva04
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PostPosted: Sun Nov 23, 2008 12:47 am    Post subject: How to recover a key after reuse of an otp key Reply with quote

Hi!
I understood that if a text was encrypted with the same key more then once , it's sopose to be easy to recover the key , but I still could not figure out how ...

Let say (p1+k)mod26=c1
and (p2+k)mod26=c2
how can i figure out the value of k??

Thanks!

Admin Note: Edited bold commands of post and moved topic to right forum Smile - Tom
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BluePass
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PostPosted: Sun Nov 23, 2008 1:22 am    Post subject: Reply with quote

First of all, this post is not in the right forum. Second of all, there is no need for making all the text bold. I'll let the moderators deal with that part though.

As for the answer to your question, the two options I see are frequency analysis or bruteforcing. Bruteforcing should be the easiest since you only have 25 possibilities to go through in your example.
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Combo
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PostPosted: Thu Nov 27, 2008 10:36 pm    Post subject: Reply with quote

(p1+k)mod26=c1
(p2+k)mod26=c2

so:
(c1-c2) mod26 = (p1+k)mod26 - (p2+k)mod26 = (p1-p2)mod26

that removes the key, exposing the text to other attacks potentially allowing p1 and p2 to be peeled apart.
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