Question

1. Using block diagram, design the architectural layout of the simplified model of a symmetric Cryptosystem.
2. Implement a cryptosystem columnar transposition technique with a 4 x 4 matrix arrangement. Test your code with ‘cryptography’ as the plaintext.

prefer python but it doesn't matter.

Answer 1

Q 1.

Q 2.

Python implementation of columnar transposition technique with a 4 x 4 matrix arrangement

# Python3 implementation of Columnar Transposition 
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 

# Plaintext 
msg = "cryptography"

cipher = encryptMessage(msg) 
print("Encrypted Message: {}". 
                        format(cipher)) 

print("Decryped Message: {}". 
        format(decryptMessage(cipher))) 

Output:

Hope this answers your questions, please leave a upvote if you find this helpful.