Caesar Cipher in Python

Introduction

In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code, or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. Same as that the right shift for A is D, B is E, and so on. The method is named after Julius Caesar, who used it in his private correspondence.

And now for this, we are going to write the code. We just break the code into three parts where we mostly concentrate on encoding, decoding, and then making the cleaner interface.

Encoding

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

direction = input("Type 'encode' to encrypt, type 'decode' to decrypt:\n")
text = input("Type your message:\n").lower()
shift = int(input("Type the shift number:\n"))

def encrypt(plain_text, shift_amount):
  cipher_text = ""
  for letters in plain_text:
    position = alphabet.index(letters)
    new_position = position + shift_amount
    new_letter = alphabet[new_position]
    cipher_text += new_letter
  print(cipher_text)

encrypt(plain_text=text, shift_amount=shift)

Here we have written alphabets two times because if Z is the input and shift is 5 then it gives the error as it is the last element in the list. The beauty of the index function is that it only considers the first element in the list. So, we will never get errors while executing this code.

Decoding

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

direction = input("Type 'encode' to encrypt, type 'decode' to decrypt:\n")
text = input("Type your message:\n").lower()
shift = int(input("Type the shift number:\n"))

def encrypt(plain_text, shift_amount):
  cipher_text = ""
  for letter in plain_text:
    position = alphabet.index(letter)
    new_position = position + shift_amount
    cipher_text += alphabet[new_position]
  print(f"The encoded text is {cipher_text}")


def decrypt(cipher_text, shift_amount):
  plain_text = ""
  for letter in cipher_text:
    position = alphabet.index(letter)
    new_position = position - shift_amount
    plain_text += alphabet[new_position]
  print(f"The encoded text is {plain_text}")

if direction == "encode":
  encrypt(plain_text=text, shift_amount=shift)
elif direction == "decode":
  decrypt(cipher_text = text, shift_amount=shift)
else:
  print("Enter crct input.")

The only difference between encryption and decryption is that in encryption it adds shift while in decryption it subtracts.

The last one is that modifying the user interface:

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

def caesar(start_text, shift_amount, cipher_direction):
  end_text = ""
  if cipher_direction == "decode":
    shift_amount *= -1
  for char in start_text:
    if char in alphabet:
      position = alphabet.index(char)
      new_position = position + shift_amount
      end_text += alphabet[new_position]
    else:
      end_text += char

  print(f"Here's the {cipher_direction}d result: {end_text}")

from art import logo
print(logo)

should_continue = True
while should_continue:
  direction = input("Type 'encode' to encrypt, type 'decode' to decrypt:\n")
  text = input("Type your message:\n").lower()
  shift = int(input("Type the shift number:\n"))

  shift = shift % 25
  caesar(start_text=text, shift_amount=shift, cipher_direction=direction)
  result = input("Type 'yes' if you want to continue. Otherwise 'no' \n")
  if result == 'no':
    should_continue = False
    print("Good Byee")

That's it. We have successfully implemented Caesar cipher in Python.

You can check my code and ASCII at this link: replit.com/@BharadwajV/CaesarCipher#main.py