The Proper Care And Feeding Of Cribs

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The Proper Care And Feeding Of Cribs; Crib Patterns
Topic Started: Oct 17 2005, 06:49 AM (263 Views)
Donald	Oct 17 2005, 06:49 AM Post #1
Elite member Posts: 454 Group: Trusted Member Member #8 Joined: September 2, 2005	Some notes on crib use (that probably everyone except ME already understood!) Big thanks to rot13 for pointing in the correct direction on this. I know how to search for patterns in standard mono subs. But I was going about it all wrong for Vigenere and Playfairs. A "crib" is a sample of known plaintext that can help you decrypt a message. When using a crib on a Vigenere, you look for letters that will repeat because of the period. For example, say your Vigenere had a period of 8. A good crib for that period would be: "one ring to rule them all" Code: `12345678 <-Vigenere period is 8 oneringt <-this is our crib orulethe mall` Now notice the pattern. The "O"s in position 1 and 9 will both be encrypted by the same Vigenere key letter. That means they will BOTH encipher to the same letter. Also, the "L"s in positions 12 and 20 will both be enciphered by the same vigenere key letter and will be identical in the crypt text. Please note that it doesn't matter where this sequence starts within the Vigenere key, as long as the period is 8, the two "O"s will be encrypted with the same key letter, as will the two "L"s. It's the period, not the position within the key that matters. So, we have a very distinct pattern to look for in our crypt text. Code: `oneringtorulethemall 123456789123456789 A???????A??B???????B (A could equal B here, that's not important)` If we find identical letters at this displacement anywhere in our crypt text, there is a very good chance we have located our crib. Now playfair cribs are a little bit more complicated. Since a playfair divides everything into digram pairs, you have two possible configurations for each crib. Say our crib was: I LIKE WHITE KITTENS It could be segmented as: Code: `il ik ew hi te ki tx te ns <-in playfair we seperate double letters with an x or -i li ke wh it ek it te ns` Now either way, we have VERY nice playfair patterns. In the first version we have repitition of the digram TE. BUT we also have the reverse pair IK, KI! Remember that in a playfair, any digraph and its reverse (eg AB and BA) will decrypt to the same letter pattern in the plaintext (eg RE and ER). So in our crypt we would be looking for something like: Code: `?? AB ?? ?? CD BA ?? CD ??` In the second version we have an IT repitition, and a KE, EK reversal. If thats the way it happened to line up we look for: Code: `?? ?? AB ?? CD BA CD ?? ??` If "I LIKE WHITE KITTENS" is in our plaintext, we will find one of these two patterns in the crypt text. Donald



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