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Statistically Normalized Transposition Cipher

Fix transposition cipher's most obvious flaw
  [vote for,

One of the biggest flaws in enciphering text with a transposition cipher is that all of the letters of the plaintext appear in the ciphertext, with the exact same frequencies.

This idea is a method of adding padding to the plaintext, such that the letter frequencies of the plaintext are not known to the attacker.

Step zero: Before sending any plaintexts, the person who will be doing the encryption should do a simple analysis of a corpus of text in the same language as the messages will be in, and determine the relative frequencies of the symbols. This set of relative frequencies is called the frequency model. This step is done once ever, not before every message.

Step one: Alter the plaintext in whatever manner is necessary so that the proper recipient can determine where it ends.

Step two: Determine the relative frequencies of the symbols in the plaintext.

Step three: Identify those symbols which the plaintext is deficient in, compared to the frequency model.

Step four: Append to the plaintext those symbols that the plaintext doesn't have enough of, until the relative frequencies of symbols of the plaintext is sufficiently close to the frequency model.

Step five: Encipher using some form of transposition cipher.

Because all plaintext messages were padded to closely match the frequency model, all ciphertext messages will also match the frequency model. Thus, all ciphertexts will look very much like one another.

goldbb, Feb 18 2011

Dynamic Transposition http://www.ciphersb...m/ARTS/DYNTRAN2.HTM
This guy was talking about similar ideas a few years ago. [Wrongfellow, Feb 18 2011]

Teach yourself cypher cryptanalysis http://www.schneier...per-self-study.html
An excellent guide on how to teach yourself how to 'break' cyphers. [DrBob, Feb 18 2011]

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       This sounds similar to Terry Ritter's "Dynamic Transposition" (link).   

       I couldn't find a concise explanation of it, but here's the basic idea as I remember it (which might be inaccurate, bearing in mind this was about ten years ago):   

       Format your message as a string of N bits. Append the inverse string - that is, replace 0 with 1 and 1 with 0 - to give a string of length 2N which has exactly N ones and N zeros.   

       Choose a random permutation of 2N items and apply it to the string, resulting in an encrypted message.   

       The idea is that any given encrypted message could be the encryption of any possible plaintext, under some key, giving it the same "perfect secrecy" property as OTP.
Wrongfellow, Feb 18 2011


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