Question

Create a Columnar Transposition Cipher in Python that encrypts/decrypts the word "Crypt".

Answer 1

Code:

import math

key = "HACK"

# Encryption

def encryptMessage(msg):

    cipher = ""

# track key indices

    k_indx = 0

msg_len = float(len(msg))

    msg_lst = list(msg)

    key_lst = sorted(list(key))

# calculate column of the matrix

    col = len(key)

# calculate maximum row of the matrix

    row = int(math.ceil(msg_len / col))

# add the padding character '_' in empty

    # the empty cell of the matix

    fill_null = int((row * col) - msg_len)

    msg_lst.extend('_' * fill_null)

    # create Matrix and insert message and

    # padding characters row-wise

    matrix = [msg_lst[i: i + col]

              for i in range(0, len(msg_lst), col)]

    # read matrix column-wise using key

    for _ in range(col):

        curr_idx = key.index(key_lst[k_indx])

        cipher += ''.join([row[curr_idx]

                          for row in matrix])

        k_indx += 1

    return cipher

# Decryption

def decryptMessage(cipher):

    msg = ""

    # track key indices

    k_indx = 0

    # track msg indices

    msg_indx = 0

    msg_len = float(len(cipher))

    msg_lst = list(cipher)

    # calculate column of the matrix

    col = len(key)

     

    # calculate maximum row of the matrix

    row = int(math.ceil(msg_len / col))

    # convert key into list and sort

    # alphabetically so we can access

    # each character by its alphabetical position.

    key_lst = sorted(list(key))

    # create an empty matrix to

    # store deciphered message

    dec_cipher = []

    for _ in range(row):

        dec_cipher += [[None] * col]

    # Arrange the matrix column wise according

    # to permutation order by adding into new matrix

    for _ in range(col):

        curr_idx = key.index(key_lst[k_indx])

        for j in range(row):

            dec_cipher[j][curr_idx] = msg_lst[msg_indx]

            msg_indx += 1

        k_indx += 1

    # convert decrypted msg matrix into a string

    try:

        msg = ''.join(sum(dec_cipher, []))

    except TypeError:

        raise TypeError("This program cannot",

                        "handle repeating words.")

    null_count = msg.count('_')

    if null_count > 0:

        return msg[: -null_count]

    return msg

# Driver Code

msg = "Geeks for Geeks"

cipher = encryptMessage(msg)

print("Encrypted Message: {}".

               format(cipher))

print("Decryped Message: {}".

       format(decryptMessage(cipher)))

Code and Output Snapshot: