Question

1. Monoalphabetic substitution (using the Caesar Cipher tool right) Encipher (convert plaintext into ciphertext): meal times  Decipher (convert ciphertext in to plaintext):  JR PHDQ JUHHQ 2. Polyalphabetic substitution (using the Vigenere Square in the lecture slide) Encipher: fall  Decipher:  VPX TWOKM

Answer 1

def encrypt(text,s):
result = ""
   # transverse the plain text
   for i in range(len(text)):
      char = text[i]
      # Encrypt uppercase characters in plain text
      
      if (char.isupper()):
         result += chr((ord(char) + s-65) % 26 + 65)
      # Encrypt lowercase characters in plain text
      else:
         result += chr((ord(char) + s - 97) % 26 + 97)
      return result
#check the above function
text = "meal times"
s = 4

print "Plain Text : " + text
print "Shift pattern : " + str(s)
print "Cipher: " + encrypt(text,s)

This is the encipher of Monoalphabetic substitution with key 4.

message = "JR PHDQ JUHHQ"
LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

for key in range(len(LETTERS)):
    translated = ''
    for symbol in message:
        if symbol in LETTERS:
            num = LETTERS.find(symbol)
            num = num - key
            if num < 0:
                num = num + len(LETTERS)
            translated = translated + LETTERS[num]
        else:
            translated = translated + symbol
    print('Hacking key #%s: %s' % (key, translated))

This is the solution for deciphering monoalphabetic substitution using each and every key. By observing all the outputs the solution is "GO MEAN GREEN".

2.

For viginere cipher, we should have a key with which we have to encipher the given text. So, Giving the function for viginere cipher in python,

import random
import string
from itertools import cycle

def encrypt(flag , key):
        ciphertext=[]                           
        for i,j in zip(flag,cycle(key)):
                    x=(ord(i)+ord(j))%95
                    x+=ord(' ')
                    ciphertext.append(chr(x))
        return("" . join(ciphertext))

For deciphering, the function is as follows,

def generateKey(string, key): 
    key = list(key) 
    if len(string) == len(key): 
        return(key) 
    else: 
        for i in range(len(string) - 
                       len(key)): 
            key.append(key[i % len(key)]) 
    return("" . join(key)) 

def originalText(cipher_text, key): 
    orig_text = [] 
    for i in range(len(cipher_text)): 
        x = (ord(cipher_text[i]) - 
             ord(key[i]) + 26) % 26
        x += ord('A') 
        orig_text.append(chr(x)) 
    return("" . join(orig_text))

if __name__ == "__main__": 
    string = input()
    keyword = input()
    key = generateKey(string, keyword) 
    cipher_text = cipherText(string,key) 
    print("Ciphertext :", cipher_text) 
    print("Original/Decrypted Text :",  
           originalText(cipher_text, key))