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Could someone please give a walkthrough of how this works: Decrypting a vigenere cipher with a reused one-time pad.

It was never fully answered with the example given in the comment.

e.g

Plaintext 1 - apples

Plaintext 2 - orange

Key : fruits

They are all 6 letters. If 'fruits' was used to encrypt both 'apples' and 'oranges' using vigenere, how could an attacker obtain the plaintexts and key with only knowing the ciphertexts?

Mathew Slingsby
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  • Thanks, can you explain why, and is it the same case for two plaintexts and key all of 10 characters? – Mathew Slingsby Nov 01 '19 at 13:07
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    Note that there was a mistake with that answer. I've corrected it now. After you subtract you will get a relation of plaintext. Then you can use crib-draging. After you found some part of the plaintexts you get the key. you are done. – kelalaka Nov 01 '19 at 13:17
  • Can you explain how please. Say I have two 10 letter plaintexts and one 10 letter key which is OTP. I use that OTP on both plaintexts two obtain 2 10 letter ciphertexts. I do c1 - c2 to obtain p1-p2. From p1-p2 how do i get the key and/or the actual plaintext values. Appreciate the help, thank you – Mathew Slingsby Nov 01 '19 at 13:20
  • Did you look into crib dragging already? It's in the comment, but not in your response. – Maarten Bodewes Nov 01 '19 at 14:56
  • Yeah I dont understand it and wanted an exmaple – Mathew Slingsby Nov 01 '19 at 15:51
  • This question Little problem with Vernam Cipher contains an example with Vernam. The idea is similar and only the relation differs. Your case is so small that when you guess and it is correct the key is revealed. – kelalaka Nov 01 '19 at 17:30

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