Question

Write a Python program that takes the following phrases and encrypts them using a substitution cipher. It should ask for a key and a phrase and then proceed to encrypt that phrase using the given shared key. Make sure that you have the key for the cipher that you use and the output of encrypted phrases.

1. He who fights with monsters should look to it that he himself does not become a monster . And if you gaze long into an abyss , the abyss also gazes into you .

2. There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here , it will instantly disappear and be replaced by something even more bizarre and inexplicable . There is another theory which states that this has already happened .

Answer 1

I have provided the code (well-commented) and screenshots of output and code for indentation issue.

The program runs until a q is entered.

The program asks for phrase and a key.

key must contain all alphabets and length 26. If incorrect key is entered , program prompts for another key.

Input any key. Here key = lshjdifktbwgxernmuozypaqvc

OUTPUT :

Code Screenshot for indentation:

CODE:

def encrypt(phrase,key):
   output_phrase=""
   for char in phrase: #iterate through every character      
      
       # substitute the phrase only if it is an alphabet
       if ord(char) >= ord('a') and ord(char) <= ord('z') :
           # map the alphabet to 0-26 respectively
           # 'a' maps to 0 and 'z' maps to 25
           idx = ord(char) - ord('a')
           #concatenate the character in key to output
           output_phrase = output_phrase + key[idx]
      
       else:
           #concatenate the character as it is to output
           output_phrase = output_phrase + char

   return output_phrase

def main():

   while True:

       phrase = input("Enter the phrase : 'q' to quit ").lower()

       if phrase[0] == 'q' :
           return

       #take input and convert phrase to lower case
       key = input("Please enter the key of length 26 : ")
       while len(key) != 26 : # promt for key until it is valid
           key = input("Please enter correct key of length 26 :")

       print(encrypt(phrase,key)) # function to encrypt

if __name__ == '__main__':
   main()

**Thank You. Please comment for any further clarification**