In this chapter, you will learn the procedure for decrypting the transposition cipher.

Code

Observe the following code for a better understanding of decrypting a transposition cipher. The cipher text for message Transposition Cipher with key as 6 is fetched as Toners raiCntisippoh.

import math, pyperclip
def main():
   myMessage= ''Toners raiCntisippoh''
   myKey = 6
   plaintext = decryptMessage(myKey, myMessage)
   
   print("The plain text is")
   print(''Transposition Cipher'')

def decryptMessage(key, message):
   numOfColumns = math.ceil(len(message) / key)
   numOfRows = key
   numOfShadedBoxes = (numOfColumns * numOfRows) - len(message)
   plaintext = float('''') * numOfColumns
   col = 0
   row = 0
   
   for symbol in message:
      plaintext[col] += symbol
      col += 1
      if (col == numOfColumns) or (col == numOfColumns - 1 and row >= numOfRows - numOfShadedBoxes):
         col = 0 row += 1 return ''''.join(plaintext)
if __name__ == ''__main__'':
   main()

Explanation

The cipher text and the mentioned key are the two values taken as input parameters for decoding or decrypting the cipher text in reverse technique by placing characters in a column format and reading them in a horizontal manner.

You can place letters in a column format and later combined or concatenate them together using the following piece of code −

for symbol in message:
   plaintext[col] += symbol
   col += 1
   
   if (col == numOfColumns) or (col == numOfColumns - 1 and row >= numOfRows - numOfShadedBoxes):
   col = 0
   row += 1
return ''''.join(plaintext)

Output

The program code for decrypting transposition cipher gives the following output −