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When can a transposition cipher be thought of as a substituion cipher?

  1. Oct 24, 2013 #1
    1. The problem statement, all variables and given/known data

    "Compare this cipher (transposition cipher) to a mixed (substitution) cipher and state under what circumstances a
    transposition cipher can be thought of as a mixed cipher."

    2. Relevant equations

    ---

    3. The attempt at a solution

    For the first part of a question I stated that a mixed/substitution cipher changes the letters of the plaintext but not the order whilst a transposition cipher changes the order but not the letters of the plaintext (thus can be analysed through letter frequency count).

    For the second part the only example I can think of is the obvious cipher where nothing changes, but I'm pretty sure that's not what they're looking for. Any clues?

    EDIT: I tried taking a general example and got that they were only equivalent when the shift was zero and every letter was equal. I think I'm on the totally wrong track here. :/

    Thanks!
     
    Last edited: Oct 24, 2013
  2. jcsd
  3. Oct 26, 2013 #2
    Are you familiar with a polygraphic cyphers? These substitute groups of letters so do not as such preserve letter order. How long could the group be?
     
  4. Oct 30, 2013 #3
    So this condition would be a polygraphic cipher where the size of the group of letters that are substituted is the size of the plaintext? That would change both order and letter value, correct?
     
  5. Oct 31, 2013 #4
    Yes potentially, but for this to also be a transposition cypher then letter value would have to be preserved, which is rather contrived. I can't see what else they could be looking for though.
     
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