Chapter5Notes

Chapter5Notes - Chapter 5 Notes Classical Transposition...

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Chapter 5 Notes: Classical Transposition Ciphers Introduction Up until now, we have looked at cryptographic protocols that change letters to other letters to obscure the meaning of a message. But, the same goal can potentially be achieved by simply reordering the letters in a message in such a way to render the message unreadable without knowing how to reorder the letters. (This is basically the definition to transposition; to reorder the letters of a message in some sort of methodical way.) Typically we can solve small anagrams, such as those presented in the Jumble section of the paper, but anagrams of hundreds of letters are more difficult to face. A very simple system was used by the North during the Civil War that involved the transposition of entire words. The following plaintext message: We will attack at dawn with the first division on the right and the second division on the left surprise is important so make all preparations under the cover of darkness nulls fill this up would become We first second important cover will division division so of attack on on make darkness at the the all nulls dawn right left preparations fill with and surprise under this the the is the up. To slightly improve the security of the code above, we could substitute code words for important words like "attack." However, this is about as weak as a transposition code could be. The type of transposition shown above is a polygraphic transposition while one that reorders individual letters is a monographic transposition. A standard transposition is done by writing the elements (letters or words) in order from left to right, top to bottom in a grid format, and then reading them down the columns instead. Roughly speaking, rows and columns are switched. Here is a quick example with the plaintext: are you going to the art museum on Thursday night? Write it out in a grid like this, with a key of 7: A R E Y O U G O I N G T O T H E A R T M U S E U M N T H U R S D A Y N I G H T X X X , (Fill in the X’s to fill up the last row of the grid…)

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Now, to encrypt the message, just read down each column instead of each row. (You can do it in different ways, by reading the columns in a predetermined order, but for this example we’ll just go left to right.) AOHSUIRIEERGENAUSHYGRMDTOTTNAXUOMTYXGTUHNX Now, if you know that there are seven columns, then you can take the total length of the message, 42, and divide it by seven to know that there are six rows. Then, just take the message and write it out by writing down each column in order from left to right. When you are done, you can just read the message by reading each row in order from top to bottom. But, there are a couple other possibilities, like the rail-fence cipher or a triangle from
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This note was uploaded on 07/13/2011 for the course CIS 3362 taught by Professor Staff during the Fall '08 term at University of Central Florida.

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Chapter5Notes - Chapter 5 Notes Classical Transposition...

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