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Viewing Single Post From: Brute Forcing The Adfgvx Cipher
jdege Apr 28 2008, 09:41 PM
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Donald
Apr 28 2008, 12:58 PM
"Jdedge"
 
converting it back to single characters using any arbitrary Polybius square would yield a result that was only a simple substitution away from the plaintext.

AH! Ok, it took me a bit, but I think I see your point.
If the sequence was the standard
step 1: fractionate,
step 2: transpose

Then we just try various transpositions until our digram frequency looks like a mono substitution frequency. And that undoes step 2. From there, any polybius square will convert the remaining crypt text into a mono alphabetic substitution, which is easy to solve. Brilliant! And NICE!!!!

BUT, If the sequence is:
step 1: fractionate,
step 2: transpose
step 3: unfractionate

THIS still baffles me. How do you undo step 3?


Can I get by with "ADFGVX doesn't unfractionate, do it doesn't matter"?

Didn't think so.

Now if we're doing Bifid, with an even key-length, it doesn't matter, either, because then the two halves of each letter map to the two halfs of another letter:

Code:
 

   P1  P2  P3  P4
   L1  L2  L3  L4
   R1  R2  R3  R4

  L1:L2  L3:L4 R1:R2 R3:R4


Notice how the first letter of the ciphertext is constructed from the left halves of the first and second letters of the plaintext, and how the third letter of the ciphertext is constructed from the right halves of the first and second letters of the plaintext.

In other words, with a key-length of four, the first and second letters of the plaintext entirely determine the first and third letters of the ciphertext, and the second and fourth letters of the plaintext entirely determine the second and fourth letters of the ciphertext.

It can be solved as an ordinary digram substitution.

With an odd key-length, this doesn't work. How to solve odd-length Bifids? I've a couple of explanations around here, but I've not studied them. (And the explanations aren't as straightforward as the even-length Bifid, so my fast skims haven't yielded anything useful.)

As for the generic problem, with an unknown transposition? Not a clue.
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.
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