The previous post was about encrypting a text with Transposition Cipher but now we will make a program to decrypt the message provided that you have the key.

Source Code :-

# Transposition Cipher Decryption
import math, pyperclip

def main():
    myMessage = input("Enter the encoded text"" ")
    myKey = int(input("Enter the key"" "))

    plaintext = decryptMessage(myKey, myMessage)

# Print with a | (called "pipe" character) after it in case

# there are spaces at the end of the decrypted message.

    print(plaintext + '|')

    pyperclip.copy(plaintext)


def decryptMessage(key, message):

# The Transposition decrypt function will simulate the "columns" and    

#"rows" of the Grid that the plaintext is written on by using a list    

# of strings. First, we need to calculate a few values.

    # The number of "columns" in our transposition grid:    

    numOfColumns = math.ceil(len(message) / key)

 # The number of "rows" in our grid will need:    

    numOfRows = key

    # The number of "shaded boxes" in the last "column" of the grid:    

    numOfShadedBoxes = (numOfColumns * numOfRows) - len(message)

# Each string in plaintext represents a column in the grid.    

    plaintext = [''] * numOfColumns

# The col and row variables point to where in the grid the next   

 # character in the encrypted message will go.    

    col = 0   

    row = 0
    for symbol in message:
        plaintext[col] += symbol
        col += 1 # point to next column
 

       # If there are no more columns OR we're at a shaded box, go back to

        # the first column and the next row.       

        if (col == numOfColumns) or (col == numOfColumns - 1 and row >= numOfRows - numOfShadedBoxes):
            col = 0            row += 1
    return ''.join(plaintext)


# If transpositionDecrypt.py is run (instead of imported as a module) call

#the main() function.if __name__ == '__main__':
    main()

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Result :-

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