Rail-Fence Cipher

March 31, 2009

The rail-fence cipher is a transposition cipher that rearranges the characters of a clear-text to form the cipher-text. The clear-text is arranged in up-and-down waves like the tops of the pickets on a rail fence; the cipher key is the height of the fence. For instance, the encipherment of “PROGRAMMING PRAXIS” with a key of 4 is shown below, using an underscore to make the space character visible:

P R O G R A M M I N G _ P R A X I S
P           M           P            = P M P
  R       A   M       _   R       S  = R A M _ R S
    O   R       I   G       A   I    = O R I G A I
      G           N           X      = G N X

The cipher-text is read at the right of the pickets: “PMPRAM RSORIGAIGNX”.

Your task is to write functions that encipher and decipher texts using the rail-fence method. When you are finished, you are welcome to read or run a suggested solution, or to post your solution or discuss the exercise in the comments below.

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