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Chapter4Notes

# Chapter4Notes - 4.1 Playfair Cipher You create a grid...

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4.1 Playfair Cipher You create a grid similar to that described in Nihilist cipher. Consider picking a keyword (without repetition) such as "PROBLEMS": P R O B L E M S A C D F G H I/J K N Q T U V W X Y Z The only difference here is that we don't have the numerical labels for the rows and columns. In this cipher, we will encipher letters pairs at a time. Consider the following plaintext: SHE WENT TO THE STORE When we pair up the letters they get grouped as follows: SH EW EN TT OT HE ST OR E But, we are not allowed to encipher any double letters. So, in this case, we will insert an Q into the plaintext. (If Q is a double letter, then insert another infrequent letter, say X.) SH EW EN TQ TO TH ES TO RE To encipher pairs of letters, adhere to the following rules: 1) If the two letters are on the same row of the chart, like "ES", then replace each letter by the letter to the right. (If necessary, wrap around to the left end of the row. So "ES" encrypts to "MA". 2) If the two letters are on the same column of the chart, like, "TH", then replace each letter by the letter below it. (If necessary, wrap around to the top end of the column.) So "TH" encrypts to "YT". 3) If two letters are on a different row and column, like, "SH", then replace each letter by another letter on its same row, but in the column of the other letter. So "SH" encrypts to "AG". Using these rules, here is the encryption of the plaintext above: Plaintext : SH EW EN TQ TO TH ES TO RE Ciphertext: AG MV MK UT QB YT MA QB PM

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For decryption, if two ciphertext letters are on the same row or column, replace them with the two letters to the left or above, respectively. Otherwise, for each letter choose the letter on the same row and the other letter's column for decryption. (So this is the original operation, it is the reverse of itself.) To cryptanalyze Playfair, we first might want to try to determine if a ciphertext is using Playfair. Here are some clues that it is: 1) There must be an even number of characters in the cipher text. 2) The rare consonants (j,k,q,x,z) will appear more frequently in the plaintext. 3) When divided into digraphs, no repeated letters will appear. 4) The frequency distribution of digraphs will approximate that of plaintext. Here are some other unique characteristics of the Playfair cipher: 1) No single letter ever encrypts to itself.
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Chapter4Notes - 4.1 Playfair Cipher You create a grid...

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