Caesar Cipher in Python

Hello everyone, in this tutorial you’ll learn about Caesar cipher in Python. If you have learned about cryptography then you should have known this term Caesar cipher. Well if you don’t know what is this then let me explain it to you.

What is Caesar Cipher?

In cryptography, Caesar cipher is one of the simplest and most widely known encryption techniques. It is also known with other names like Caesar’s cipher, the shift cipher, Caesar’s code or Caesar shift. This encryption technique is used to encrypt plain text, so only the person you want can read it. The method is named after Julius Caesar, who used it in his private correspondence.

In this encryption technique, to encrypt our data,  we have to replace each letter in the text by a some other letter at a fixed difference. Let’s say, there is a letter ‘T’ then with a right shift of 1 it will be ‘U’ and with a left shift of 1 it will become ‘S’.  So here, the difference is 1 and the direction will also be same for a text. Either we can use left shift or right, not both in same text.

Let’s understand it with an easy example.

Example

Suppose we have text “the crazy programmer” to be encrypted. Then what we can do is replace each of letter present in the text by a another letter having fixed difference. Lets say we want right shift by 2 then each letter of the above text have to replaced by the letter, positioned second from the letter.

Plaintext: the crazy programmer

Ciphertext: vjg etcba rtqitcoogt

Now user can’t  read this text until he/she have the decrypt key.  Decrypt key is nothing just the knowledge about how we shifted those letters while encrypting it. To decrypt this we have to left shift all the letters by 2.

That was the basic concept of Caesar cipher.  If we see this encryption technique in mathematical way then the formula to get encrypted letter will be:

c = (x + n) mod 26

where, c is place value of encrypted letter,

x is place value of actual letter,

n is the number that shows us how many positions of letters we have to replace.

On other hand, to decrypt each letter we’ll use the formula given below:

c = (x – n) mod 26

Program for Caesar Cipher in Python

def encrypt(string, shift):

  cipher = ''
  for char in string: 
    if char == ' ':
      cipher = cipher + char
    elif  char.isupper():
      cipher = cipher + chr((ord(char) + shift - 65) % 26 + 65)
    else:
      cipher = cipher + chr((ord(char) + shift - 97) % 26 + 97)
  
  return cipher

text = input("enter string: ")
s = int(input("enter shift number: "))
print("original string: ", text)
print("after encryption: ", encrypt(text, s))

Output:

enter string: the crazy programmer
enter shift number: 2
original string: the crazy programmer
after encryption: vjg etcba rtqitcoogt

So in above program we have used the same formula (with some modification) we mentioned above. But in computer science ‘A’ is different from ‘a’ thats why we have to write that formula twice, (for uppercase and lowercase letters).

As you can see in the program we have added and subtracted 65 (for Uppercase) and 97 (for lowercase) in that mathematical formula because the ascii value of ‘A’ is 65 and of ‘a’ is 97. The ord() method is used to get the ascii value of the letters.

Note 1: if you want left shift instead of right then please enter a negative number in ‘enter shift number: ’.

Note 2: the above program will work only for Python 3.x because input() method works different in both Python 2 and 3. To use the above program in Python 2, use raw_input() in place of input() method.

To decrypt this message, we will use the same above program but with a small modification.

cipher = cipher + chr((ord(char) – shift – 65) % 26 + 65)

If you’ve any problem or suggestion related to caesar cipher in python then please let us know in comments.

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