hill_cipher.py

import numpy as np
import math
import utils

# decryption function
def decrypt(key_matrix_inv, cipher_text):
    """
    Arguments: key matrix inverse, cipher text
    Returns: plain text
    """
    print("Matrix Inverse is: \n", key_matrix_inv)
    dimensions = len(cipher_text)
    
    # create cipher text matrix (ASCII Values - 65 to get from 0 to X)

    cipher_text_matrix = []
    for i in range(dimensions):
        cipher_text_matrix.append(ord(cipher_text[i]) - 65)
    
    cipher_text_matrix = np.array(cipher_text_matrix)
    print("Cipher Key Matrix: \n", cipher_text_matrix)

    # multiply inverse with cipher text matrix
    result = np.array(np.dot(key_matrix_inv, cipher_text_matrix))

    # BUG
    # print(result[0][1], int(result[0][1]))
    print("Decrypted Matrix\n", result) 

    # create empty string for plain text
    plain_text = ""
    # convert result matrix to plain text by using chr()
    for i in range(dimensions):
        plain_text += chr(int(round(result[0][i], 0) % 26 + 65))

    # return the decrypted plain text
    return plain_text

if __name__ == "__main__":
    # take input from the user
    plain_text = str(input("Plain Text: "))

    # dimensions of the matrix = length(plain text) x length(plain text)
    dimensions = len(plain_text)
    
    # plain text matrix
    plain_text_matrix = []
    
    # creating a column matrix for plain text characters
    for i in range(dimensions):
        plain_text_matrix.append(ord(plain_text[i]) - 65)
    
    plain_text_matrix = np.array(plain_text_matrix)
    
    print("Plain Text Matrix\n", plain_text_matrix)

    print("Enter values for the key: ")

    # take values for the key matrix
    key_matrix = []
    for i in range(dimensions):
        row_ = []
        for j in range(dimensions):
            value = int(input(str(i) + ", " + str(j) + " value: "))                         
            row_.append(value)
        key_matrix.append(row_)

    print("Key Matrix: \n")
        
    # for encryption
    key_matrix = np.array(key_matrix)
    # for decryption
    # key_matrix_inv = (np.linalg.inv(np.matrix(key_matrix)) % 26)
    # key_matrix_inv = utils.multi_inverse(np.linalg.det(key_matrix), 26) * \
    #        np.matrix(key_matrix).getH()
    # print(np.matrix(key_matrix).getH())
    
    # calculate key matrix inverse using modulo multiplicative inverse
    key_matrix_inv = np.linalg.inv(np.matrix(key_matrix)) * \
            np.linalg.det(key_matrix) * \
            utils.multi_inverse(np.linalg.det(key_matrix), 26) % 26
        
    print(key_matrix)

    print("Inverse Key Matrix: \n", key_matrix_inv)
    
    result = key_matrix.dot(plain_text_matrix)
     
    print("Cipher Matrix: \n", result)
    
    cipher_text = ""

    for i in range(dimensions):
        cipher_text += chr(result[i] % 26 + 65)

    print("Cipher Text: \n", cipher_text)

    decrypted_plain_text = decrypt(key_matrix_inv, cipher_text)

    print("Decrypted plain text: ", decrypted_plain_text)